Refactor Sudoku field and solver implementation

README.md

@@ -5,10 +5,10 @@ menschlichen Lösungsstrategien bearbeiten soll. Neben dem gelösten Feld soll
 langfristig auch der Lösungsweg mit Kandidaten, Änderungen und Markierungen
 verfügbar sein.
 
-> [!WARNING]
+> [!NOTE]
-> Das Projekt befindet sich in einer frühen Entwicklungsphase. Der aktuelle
+> Das Projekt befindet sich weiterhin in einer frühen Entwicklungsphase. Die
-> Prüflauf ist wegen eines Vet-Fehlers noch nicht grün und die Demo kann während des
+> Basis ist getestet und die Demo läuft, der menschliche Solver unterstützt aber
-> Lösens abstürzen. Siehe [Bekannte Einschränkungen](#bekannte-einschränkungen).
+> bisher nur vier grundlegende Strategien.
 
 ## Voraussetzungen
 
@@ -43,8 +43,9 @@ gofmt -w main.go field/*.go parser/*.go logic/*.go logic/strategies/*.go sudoku/
 go vet ./...
 ```
 
-Aktuell gibt es noch keine automatisierten Tests. Neue Funktionalität sollte
+Die Pakete `field`, `parser`, `logic`, `logic/strategies` und `sudoku` besitzen
-nach Möglichkeit mit paketnahen `*_test.go`-Tests ergänzt werden.
+Unit- und Regressionstests. Neue Funktionalität sollte weiterhin mit paketnahen,
+vorzugsweise tabellengesteuerten `*_test.go`-Tests ergänzt werden.
 
 ## Architektur
 
@@ -70,8 +71,9 @@ Puzzle-Bank-Datei -> parser.PuzzleBank -> field.Field
 
 Eine Strategie implementiert `strategies.Strategy`. `SearchProgressableCells`
 sammelt mögliche Änderungen, `ApplyAll` oder `ApplyNext` übernimmt sie in das
-Feld. Wird eine Zahl gesetzt, entfernt der Solver den entsprechenden Kandidaten
+Feld. Wird eine Zahl gesetzt, entfernt das Feldmodell den entsprechenden
-aus der zugehörigen Zeile, Spalte und dem Block.
+Kandidaten automatisch und ohne Duplikate aus der zugehörigen Zeile, Spalte und
+dem Block.
 
 Derzeit in `main.go` aktiv:
 
@@ -88,25 +90,27 @@ implementiert.
 `parser.PuzzleBank` erwartet das Format der
 [Sudoku Exchange Puzzle Bank](data/sudoku-exchange-puzzle-bank/README.md): pro
 Zeile einen 12-stelligen Hash, 81 Ziffern für das Rätsel und eine
-Schwierigkeitsbewertung. `0` steht für ein leeres Feld. Leere Zeilen werden
+Schwierigkeitsbewertung. Hash, Zeichenzahl, Ziffern, Rating und Ausgangsbelegung
-übersprungen; alle eingelesenen Rätsel werden im Parser gespeichert.
+werden geprüft. `0` steht für ein leeres Feld. LF und CRLF werden unterstützt,
+leere Zeilen werden übersprungen.
+
+Für einzelne Rätsel steht außerdem `parser.PuzzleString` zur Verfügung. Er
+akzeptiert genau 81 Ziffern. Parser-Zugriffe liefern bei einem ungültigen Index
+einen Fehler; bei mehreren Puzzle-Bank-Einträgen wird der Index explizit an
+`sudoku.New` übergeben.
 
 Die Datensätze unter `data/sudoku-exchange-puzzle-bank/` stehen unter der dort
 beiliegenden separaten Lizenz.
 
 ## Bekannte Einschränkungen
 
-- `field.Field.IsValid` und die `IsSolved`-Prüfungen einzelner Bereiche sind
-  noch nicht implementiert. Ein vollständig belegtes, aber ungültiges Feld kann
-  daher als gelöst gelten.
 - `logic.Solver.GetSolutionPath` liefert noch keinen Lösungsweg.
 - Mehrere fortgeschrittene Strategien sind nur als leere Typen vorhanden.
-- `go test ./...` meldet aktuell in `field/field.go`, dass eine Ganzzahl direkt
+- Der Solver besitzt noch kein gemeinsames Interface für menschliche,
-  in einen String konvertiert wird.
+  Backtracking- und DLX-Implementierungen.
-- Die Demo kann in `LastDigit.SearchProgressableCells` auf eine leere Zelle
+- Die Demo verwendet weiterhin einen fest kodierten Dateipfad und Puzzle-Index.
-  zugreifen und dadurch abstürzen.
+- Der alte öffentliche Bezeichner `InitStragies` bleibt vorerst als
-- Fehlertexte und Bezeichner sind derzeit teilweise deutsch, teilweise
+  Kompatibilitätsalias bestehen.
-  englisch; einige öffentliche Namen enthalten noch Tippfehler.
 
 ## Lizenz
 

ROADMAP.md

@@ -20,7 +20,9 @@ Das erste stabile Release soll:
 Backtracking, DLX und sehr fortgeschrittene menschliche Strategien folgen auf
 dieses Kern-Release.
 
-## Phase 0 – Lauffähige und getestete Basis (P0)
+## Phase 0 – Lauffähige und getestete Basis (P0) ✅
+
+Abgeschlossen am 10. September 2026.
 
 Zuerst wird der bestehende Zustand stabilisiert. In dieser Phase werden keine
 neuen Lösungsstrategien ergänzt.
@@ -49,7 +51,9 @@ Abgeschlossen, wenn:
 - `go run .` das ausgewählte Beispiel ohne Panic beendet und
 - die bekannten Fehler jeweils einen Regressionstest besitzen.
 
-## Phase 1 – Verlässliches Feldmodell (P0)
+## Phase 1 – Verlässliches Feldmodell (P0) ✅
+
+Abgeschlossen am 10. September 2026.
 
 - `field.Properties` beim Erzeugen prüfen: positive Dimensionen, passende
   Blockaufteilung und exakt passende Zellmatrix.
@@ -65,14 +69,16 @@ Abgeschlossen, wenn:
   Fehler zurückgeben und das Feld unverändert lassen.
 - lesenden Zugriff auf Eigenschaften und Änderungshistorie anbieten, ohne
   interne Slices veränderbar nach außen zu geben.
-- allgemeine und 9x9-spezifische Darstellung trennen; `StringNotesForNumber`
+- die fest codierte 9x9-Darstellung durch eine allgemeine Darstellung ersetzen
-  implementieren oder bis zu einem echten Bedarf aus der API entfernen.
+  und `StringNotesForNumber` implementieren.
 
 Abgeschlossen, wenn gültige, ungültige, unvollständige und gelöste Felder in
 Tabellentests eindeutig unterschieden werden und keine Mutation die
 Feld-Invarianten umgehen kann.
 
-## Phase 2 – Robuste Parser- und Game-API (P0)
+## Phase 2 – Robuste Parser- und Game-API (P0) ✅
+
+Abgeschlossen am 10. September 2026.
 
 - Parserfehler vereinheitlichen und mit Zeilennummer sowie fehlerhaftem Feld
   anreichern.
@@ -95,17 +101,12 @@ und Parser sowie `Game` keine implizite Rätselauswahl mehr enthalten.
 
 - ein Solver-Interface definieren, das später menschlichen Solver, Backtracking
   und DLX austauschbar macht.
-- `Run(int) bool` durch eine aussagekräftige Schritt-API ersetzen, zum Beispiel
+- `Run(int) (bool, error)` durch eine aussagekräftige Schritt-API ersetzen, zum Beispiel
   mit den Zuständen `Progress`, `Solved`, `Stuck`, `Invalid` und `Failed`.
-- Konfiguration aus den ungenutzten `conf`-Feldern ableiten oder diese entfernen.
 - Strategie-Reihenfolge, Wiederholungspunkt und `ApplyNext`/`ApplyAll` eindeutig
   definieren.
-- gefundene Änderungen vor dem Anwenden deduplizieren und auf Konflikte prüfen.
-- Kandidaten nach jedem Zahlenschritt korrekt und deterministisch aktualisieren.
 - Tippfehler in öffentlichen Namen (`InitStragies`, `TriggerdBy`) kontrolliert
   migrieren und alle Aufrufer anpassen.
-- Abbruch bei Stillstand, ungültigem Zustand und internem Fehler sauber durch
-  `sudoku.Game.Solve` reichen.
 
 Abgeschlossen, wenn derselbe Input stets dieselben Schritte erzeugt und jeder
 Solverlauf genau einen überprüfbaren Endzustand besitzt.
@@ -208,17 +209,16 @@ verwendet werden kann.
 
 Diese Tickets bilden die kürzeste Route zu einem stabilen Zwischenstand:
 
-1. Test-Helfer und Feldzugriffs-Tests erstellen.
+1. ein gemeinsames Solver-Interface und aussagekräftige Schrittzustände
-2. `ForEachCell`, `GetCell` und `Field.String` korrigieren.
+   entwerfen.
-3. Change-Aktionen sowie Nil- und Werteprüfung absichern.
+2. Strategie-Reihenfolge sowie `ApplyNext` und `ApplyAll` als öffentlichen
-4. `LastDigit` deduplizieren und den bekannten Panic per Regressionstest
+   Vertrag festlegen und testen.
-   beseitigen.
+3. `InitStragies` und `TriggerdBy` kontrolliert auf korrekt geschriebene Namen
-5. `HiddenSingle` korrigieren und vollständig testen.
+   migrieren.
-6. `Part.IsSolved`, `Field.IsValid` und `Field.IsSolved` implementieren.
+4. End-to-End-Tests für mehrere einfache und mittlere Puzzle-Bank-Rätsel
-7. Puzzle-Bank-Parser atomar und indexsicher machen.
+   ergänzen.
-8. Solver-Schrittergebnis und Konflikterkennung einführen.
+5. anschließend `NakedPair`, `NakedTriple`, `HiddenPair` und `HiddenTriple`
-9. die vier Basisstrategien durch End-to-End-Rätseltests absichern.
+   implementieren.
-10. erst danach Paar-/Tripel-Strategien oder neue Bedienfunktionen beginnen.
 
 Nach jedem Arbeitspaket müssen `gofmt`, `go test ./...` und `go vet ./...`
 erfolgreich sein. Neue bekannte Baseline-Fehler sollen nicht angesammelt werden.

field/cell.go

@@ -1,123 +1,77 @@
 package field
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// NOTES STRUCTURE                                                                //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 import "slices"
 
 type Notes struct {
 	numbers []int
 }
 
-func (n *Notes) Add(field *Field, cell *Cell, note int, trigger string, marks []Mark) {
+func (n *Notes) Add(f *Field, cell *Cell, note int, trigger string, marks []Mark) error {
-	if slices.Contains(n.numbers, note) {
+	if n == nil || cell == nil || cell.notes != n {
-		return // Note already exists
+		return ErrInvalidCell
 	}
-	field.changes = append(field.changes, Change{
+	return f.AddChange(&ExternalChange{Cell: cell, Marks: marks, Action: ActionSetNote, Value: note, TriggerdBy: trigger})
-		Cell:       cell,
-		marks:      marks,
-		action:     ActionSetNote,
-		value:      note,
-		from:       0,
-		triggerdBy: trigger,
-	})
-	n.numbers = append(n.numbers, note)
 }
 
-func (n *Notes) Remove(field *Field, cell *Cell, note int, trigger string, marks []Mark) {
+func (n *Notes) Remove(f *Field, cell *Cell, note int, trigger string, marks []Mark) error {
-	for i, existing := range n.numbers {
+	if n == nil || cell == nil || cell.notes != n {
-		if existing == note {
+		return ErrInvalidCell
-			field.changes = append(field.changes, Change{
-				Cell:       cell,
-				marks:      marks,
-				action:     ActionRemoveNote,
-				value:      0,
-				from:       note,
-				triggerdBy: trigger,
-			})
-
-			n.numbers = append(n.numbers[:i], n.numbers[i+1:]...)
-			return
-		}
 	}
+	return f.AddChange(&ExternalChange{Cell: cell, Marks: marks, Action: ActionRemoveNote, From: note, TriggerdBy: trigger})
 }
 
 func (n *Notes) Get() []int {
-	copySlice := make([]int, len(n.numbers))
+	if n == nil {
-	copy(copySlice, n.numbers)
+		return nil
-	return copySlice
+	}
+	return slices.Clone(n.numbers)
 }
 
-func (n *Notes) Has(i int) bool {
+func (n *Notes) Has(number int) bool {
-	return slices.Contains(n.numbers, i)
+	return n != nil && slices.Contains(n.numbers, number)
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// CELL STRUCTURE                                                                 //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Cell struct {
 	number int
-	Notes  *Notes
+	notes  *Notes
-	Pos    *Position
+	pos    *Position
 }
 
 func NewCell(number int, pos *Position) *Cell {
-	cell := Cell{
+	return &Cell{number: number, notes: &Notes{}, pos: pos}
-		number: number,
-		Pos:    pos,
-		Notes:  &Notes{},
-	}
-	return &cell
 }
 
-func (c *Cell) SetNumber(field *Field, n int, trigger string, marks []Mark) {
+func (c *Cell) SetNumber(f *Field, number int, trigger string, marks []Mark) error {
-	if c.number == n {
+	if c == nil {
-		return
+		return ErrInvalidCell
 	}
-
+	return f.AddChange(&ExternalChange{Cell: c, Marks: marks, Action: ActionSetNumber, Value: number, From: c.number, TriggerdBy: trigger})
-	if field == nil {
-		return
-	}
-
-	if n <= 0 || n > field.props.Rows || n > field.props.Columns {
-		return
-	}
-
-	field.changes = append(field.changes, Change{
-		Cell:       c,
-		marks:      marks,
-		action:     ActionSetNumber,
-		value:      n,
-		from:       c.number,
-		triggerdBy: trigger,
-	})
-
-	c.number = n
 }
 
-func (c *Cell) RemoveNumber(field *Field, trigger string, marks []Mark) {
+func (c *Cell) RemoveNumber(f *Field, trigger string, marks []Mark) error {
-	if c.number == 0 {
+	if c == nil {
-		return
+		return ErrInvalidCell
 	}
-
+	return f.AddChange(&ExternalChange{Cell: c, Marks: marks, Action: ActionRemoveNumber, From: c.number, TriggerdBy: trigger})
-	if field == nil {
-		return
-	}
-
-	field.changes = append(field.changes, Change{
-		action:     ActionSetNumber,
-		Cell:       c,
-		value:      0,
-		from:       c.number,
-		marks:      marks,
-		triggerdBy: trigger,
-	})
-
-	c.number = 0
 }
 
 func (c *Cell) GetNumber() int {
+	if c == nil {
+		return 0
+	}
 	return c.number
 }
+
+func (c *Cell) GetNotes() *Notes {
+	if c == nil {
+		return nil
+	}
+	return c.notes
+}
+
+func (c *Cell) GetPosition() *Position {
+	if c == nil {
+		return nil
+	}
+	return c.pos
+}

field/changes.go

@@ -1,13 +1,11 @@
 package field
 
 import (
+	"fmt"
 	"image/color"
+	"slices"
 )
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// MARK STRUCTURE                                                                 //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Mark struct {
 	Cell   *Cell
 	Change *Change
@@ -15,94 +13,222 @@ type Mark struct {
 }
 
 func (m *Mark) GetColor() color.Color {
+	if m == nil {
+		return nil
+	}
 	return m.color
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// CHANGE_ACTION TYPE                                                             //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type ChangeAction int
 
 const (
-	ActionSetNumber = iota
+	ActionSetNumber ChangeAction = iota
 	ActionSetNote
 	ActionRemoveNumber
 	ActionRemoveNote
 )
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// CHANGE STRUCTURE                                                               //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type ExternalChange struct {
 	Cell       *Cell
 	Marks      []Mark
 	Action     ChangeAction
 	Value      int
 	From       int
-	TriggerdBy string //strategy or manuell
+	TriggerdBy string
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// CHANGE STRUCTURE                                                               //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Change struct {
 	Cell       *Cell
 	marks      []Mark
 	action     ChangeAction
 	value      int
 	from       int
-	triggerdBy string //strategy or manuell
+	triggerdBy string
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// GETTER                                                                         //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 func (c *Change) GetMarks() []Mark {
-	return c.marks
+	if c == nil {
+		return nil
+	}
+	return slices.Clone(c.marks)
 }
 
 func (c *Change) GetAction() ChangeAction {
+	if c == nil {
+		return ActionSetNumber
+	}
 	return c.action
 }
 
 func (c *Change) GetTo() int {
+	if c == nil {
+		return 0
+	}
 	return c.value
 }
 
 func (c *Change) GetFrom() int {
+	if c == nil {
+		return 0
+	}
 	return c.from
 }
 
 func (c *Change) GetTriggeredBy() string {
+	if c == nil {
+		return ""
+	}
 	return c.triggerdBy
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
+func (f *Field) AddChange(external *ExternalChange) error {
-// ???                                                                         //
+	if err := f.validateChange(external); err != nil {
-// ────────────────────────────────────────────────────────────────────────────── //
+		return err
-
+	}
-func (c *Change) do() {
-	switch c.action {
 
+	cell := external.Cell
+	switch external.Action {
 	case ActionSetNumber:
-		c.Cell.number = c.value
+		if cell.number == external.Value {
-
+			return nil
+		}
+		cell.number = external.Value
+		f.recordChange(external)
+		f.clearNotes(cell, external.TriggerdBy)
+		f.removePeerNote(cell, external.Value, external.TriggerdBy)
 	case ActionSetNote:
-		c.Cell.Notes.numbers = append(c.Cell.Notes.numbers, c.value)
+		if cell.notes.Has(external.Value) {
-
+			return nil
+		}
+		cell.notes.numbers = append(cell.notes.numbers, external.Value)
+		slices.Sort(cell.notes.numbers)
+		f.recordChange(external)
 	case ActionRemoveNumber:
-		c.Cell.number = 0
+		if cell.number == 0 {
-
+			return nil
+		}
+		cell.number = 0
+		f.recordChange(external)
 	case ActionRemoveNote:
-		for i, existing := range c.Cell.Notes.numbers {
+		for i, existing := range cell.notes.numbers {
-			if existing == c.from {
+			if existing == external.From {
-				c.Cell.Notes.numbers = append(c.Cell.Notes.numbers[:i], c.Cell.Notes.numbers[i+1:]...)
+				cell.notes.numbers = slices.Delete(cell.notes.numbers, i, i+1)
-				return
+				f.recordChange(external)
+				break
 			}
 		}
 	}
+	return nil
+}
+
+func (f *Field) AddChanges(changes []ExternalChange) error {
+	if f == nil || !f.IsValid() {
+		return ErrInvalidField
+	}
+	state := f.snapshot()
+	for i := range changes {
+		if err := f.AddChange(&changes[i]); err != nil {
+			f.restore(state)
+			return fmt.Errorf("apply change %d: %w", i, err)
+		}
+	}
+	return nil
+}
+
+func (f *Field) GetChanges() []ExternalChange {
+	if f == nil {
+		return nil
+	}
+	result := make([]ExternalChange, len(f.changes))
+	for i := range f.changes {
+		result[i] = ExternalChange{Cell: f.changes[i].Cell, Marks: slices.Clone(f.changes[i].marks), Action: f.changes[i].action, Value: f.changes[i].value, From: f.changes[i].from, TriggerdBy: f.changes[i].triggerdBy}
+	}
+	return result
+}
+
+func (f *Field) validateChange(change *ExternalChange) error {
+	if f == nil || !f.IsValid() {
+		return ErrInvalidField
+	}
+	if change == nil || change.Cell == nil || !f.ownsCell(change.Cell) {
+		return fmt.Errorf("%w: cell does not belong to field", ErrInvalidChange)
+	}
+	cell := change.Cell
+	if cell.notes == nil || cell.pos == nil {
+		return fmt.Errorf("%w: incomplete cell", ErrInvalidChange)
+	}
+
+	switch change.Action {
+	case ActionSetNumber:
+		if change.Value < 1 || change.Value > f.props.Rows {
+			return fmt.Errorf("%w: number %d is out of range", ErrInvalidChange, change.Value)
+		}
+		if change.From != cell.number {
+			return fmt.Errorf("%w: expected previous value %d, got %d", ErrInvalidChange, cell.number, change.From)
+		}
+		if f.hasPeerNumber(cell, change.Value) {
+			return fmt.Errorf("%w: number %d already exists in a peer", ErrInvalidChange, change.Value)
+		}
+	case ActionSetNote:
+		if cell.number != 0 {
+			return fmt.Errorf("%w: cannot add a note to a filled cell", ErrInvalidChange)
+		}
+		if change.Value < 1 || change.Value > f.props.Rows || f.hasPeerNumber(cell, change.Value) {
+			return fmt.Errorf("%w: note %d is not a candidate", ErrInvalidChange, change.Value)
+		}
+	case ActionRemoveNumber:
+		if change.From != cell.number {
+			return fmt.Errorf("%w: expected previous value %d, got %d", ErrInvalidChange, cell.number, change.From)
+		}
+	case ActionRemoveNote:
+		if change.From < 1 || change.From > f.props.Rows {
+			return fmt.Errorf("%w: note %d is out of range", ErrInvalidChange, change.From)
+		}
+	default:
+		return fmt.Errorf("%w: unknown action %d", ErrInvalidChange, change.Action)
+	}
+	return nil
+}
+
+func (f *Field) recordChange(external *ExternalChange) {
+	f.changes = append(f.changes, Change{Cell: external.Cell, marks: slices.Clone(external.Marks), action: external.Action, value: external.Value, from: external.From, triggerdBy: external.TriggerdBy})
+}
+
+func (f *Field) clearNotes(cell *Cell, trigger string) {
+	for _, note := range slices.Clone(cell.notes.numbers) {
+		cell.notes.numbers = removeNumber(cell.notes.numbers, note)
+		f.recordChange(&ExternalChange{Cell: cell, Action: ActionRemoveNote, From: note, TriggerdBy: trigger})
+	}
+}
+
+func (f *Field) removePeerNote(cell *Cell, number int, trigger string) {
+	seen := make(map[*Cell]struct{})
+	parts, err := f.GetEachPartAtPos(cell.pos)
+	if err != nil {
+		return
+	}
+	for _, part := range parts {
+		part.ForEachCell(func(peer *Cell) {
+			if peer == cell {
+				return
+			}
+			if _, exists := seen[peer]; exists {
+				return
+			}
+			seen[peer] = struct{}{}
+			if peer.notes.Has(number) {
+				peer.notes.numbers = removeNumber(peer.notes.numbers, number)
+				f.recordChange(&ExternalChange{Cell: peer, Action: ActionRemoveNote, From: number, TriggerdBy: trigger})
+			}
+		})
+	}
+}
+
+func removeNumber(numbers []int, number int) []int {
+	for i, existing := range numbers {
+		if existing == number {
+			return slices.Delete(numbers, i, i+1)
+		}
+	}
+	return numbers
 }

field/errors.go

@@ -3,6 +3,8 @@ package field
 import "errors"
 
 var (
-	outOfBound   = errors.New("invalid coordinates")
+	ErrOutOfBounds   = errors.New("coordinates out of bounds")
-	invalidField = errors.New("can not parse Field")
+	ErrInvalidField  = errors.New("invalid field")
+	ErrInvalidCell   = errors.New("invalid cell")
+	ErrInvalidChange = errors.New("invalid change")
 )

field/field.go

@@ -2,13 +2,11 @@ package field
 
 import (
 	"fmt"
+	"slices"
+	"strconv"
 	"strings"
 )
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// META STRUCTURE                                                                 //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Properties struct {
 	Rows            int
 	Columns         int
@@ -19,289 +17,445 @@ type Properties struct {
 	Rating          float64
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// FIELD STRUCTURE                                                                //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Field struct {
 	props   *Properties
 	cells   [][]Cell
 	changes []Change
 }
 
-func New(props Properties, cells [][]Cell) *Field {
+type fieldSnapshot struct {
-	field := Field{
+	numbers [][]int
-		props: &props,
+	notes   [][][]int
-		cells: cells,
+	changes []Change
-	}
-	return &field
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
+func New(props Properties, cells [][]Cell) (*Field, error) {
-// GETTER                                                                         //
+	if err := validateProperties(props); err != nil {
-// ────────────────────────────────────────────────────────────────────────────── //
+		return nil, err
-
+	}
-func (f *Field) GetRow(r int) (*Row, error) {
+	if len(cells) != props.Rows {
-	if r >= f.props.Rows || r < 0 {
+		return nil, fmt.Errorf("%w: expected %d rows, got %d", ErrInvalidField, props.Rows, len(cells))
-		return nil, outOfBound
 	}
 
-	cellPtrs := make([]*Cell, len(f.cells[r]))
+	cloned := make([][]Cell, props.Rows)
-	for i := range f.cells[r] {
+	for row := 0; row < props.Rows; row++ {
-		cellPtrs[i] = &f.cells[r][i]
+		if len(cells[row]) != props.Columns {
+			return nil, fmt.Errorf("%w: row %d has %d cells, expected %d", ErrInvalidField, row, len(cells[row]), props.Columns)
+		}
+		cloned[row] = make([]Cell, props.Columns)
+		for column := 0; column < props.Columns; column++ {
+			source := &cells[row][column]
+			if source.pos == nil {
+				return nil, fmt.Errorf("%w: cell %d/%d has no position", ErrInvalidField, row, column)
+			}
+			if !positionMatches(source.pos, props, row, column) {
+				return nil, fmt.Errorf("%w: cell %d/%d has inconsistent position", ErrInvalidField, row, column)
+			}
+			if source.number < 0 || source.number > props.Rows {
+				return nil, fmt.Errorf("%w: cell %d/%d contains %d", ErrInvalidField, row, column, source.number)
+			}
+			if source.notes == nil {
+				return nil, fmt.Errorf("%w: cell %d/%d has no notes", ErrInvalidField, row, column)
+			}
+			notes, err := normalizeNotes(source.notes.numbers, props.Rows, source.number)
+			if err != nil {
+				return nil, fmt.Errorf("%w: cell %d/%d: %w", ErrInvalidField, row, column, err)
+			}
+			cloned[row][column] = Cell{
+				number: source.number,
+				notes:  &Notes{numbers: notes},
+				pos:    NewPosition(row, column, row/props.BlockSizeRow, column/props.BlockSizeColumn, row%props.BlockSizeRow, column%props.BlockSizeColumn),
+			}
+		}
 	}
 
-	return &Row{
+	result := &Field{props: &props, cells: cloned}
-		Line: Line{
+	if !result.hasValidNumbers() || !result.hasValidNotes() {
-			cells: cellPtrs,
+		return nil, fmt.Errorf("%w: duplicate values or invalid candidates", ErrInvalidField)
-		},
-	}, nil
-}
-
-func (f *Field) GetColumn(c int) (*Column, error) {
-	if c >= f.props.Columns || c < 0 {
-		return nil, outOfBound
 	}
-
-	// Performance
-	result := &Column{
-		Line: Line{
-			cells: make([]*Cell, 0, f.props.Rows),
-		},
-	}
-
-	// Copying
-	for _, row := range f.cells {
-		result.cells = append(result.cells, &row[c])
-	}
-
 	return result, nil
 }
 
-func (f *Field) GetBlock(r, c int) (*Block, error) {
+func validateProperties(props Properties) error {
-	if r < 0 || r >= f.props.BlockRows || c < 0 || c >= f.props.BlockColumns {
+	if props.Rows <= 0 || props.Columns <= 0 || props.Rows != props.Columns {
-		return nil, outOfBound
+		return fmt.Errorf("%w: rows and columns must be equal and positive", ErrInvalidField)
 	}
-
+	if props.BlockRows <= 0 || props.BlockColumns <= 0 || props.BlockSizeRow <= 0 || props.BlockSizeColumn <= 0 {
-	startRow := r * f.props.BlockSizeRow
+		return fmt.Errorf("%w: block dimensions must be positive", ErrInvalidField)
-	startCol := c * f.props.BlockSizeColumn
-
-	block := &Block{
-		cells: make([][]*Cell, f.props.BlockSizeRow),
 	}
-
+	if props.BlockRows*props.BlockSizeRow != props.Rows || props.BlockColumns*props.BlockSizeColumn != props.Columns {
-	for row := range block.cells {
+		return fmt.Errorf("%w: block dimensions do not cover the field", ErrInvalidField)
-		// Effizientes Kopieren der Zeile
-		block.cells[row] = make([]*Cell, f.props.BlockSizeColumn)
-
-		for column := range block.cells[row] {
-			block.cells[row][column] = &f.cells[startRow+row][startCol+column]
 	}
+	if props.BlockSizeRow*props.BlockSizeColumn != props.Rows {
+		return fmt.Errorf("%w: each block must contain %d cells", ErrInvalidField, props.Rows)
 	}
-	return block, nil
+	return nil
 }
 
-func (f *Field) GetEachPartAtPos(pos *Position) []Part {
+func normalizeNotes(notes []int, max, cellNumber int) ([]int, error) {
+	if cellNumber != 0 && len(notes) != 0 {
+		return nil, fmt.Errorf("filled cells cannot contain notes")
+	}
+	result := slices.Clone(notes)
+	slices.Sort(result)
+	result = slices.Compact(result)
+	for _, note := range result {
+		if note < 1 || note > max {
+			return nil, fmt.Errorf("note %d is out of range", note)
+		}
+	}
+	return result, nil
+}
+
+func positionMatches(pos *Position, props Properties, row, column int) bool {
+	return pos.row == row && pos.column == column &&
+		pos.blockRow == row/props.BlockSizeRow && pos.blockColumn == column/props.BlockSizeColumn &&
+		pos.inBlockRow == row%props.BlockSizeRow && pos.inBlockColumn == column%props.BlockSizeColumn
+}
+
+func (f *Field) GetRow(row int) (*Row, error) {
+	if !f.hasStructure() || row < 0 || row >= f.props.Rows {
+		return nil, ErrOutOfBounds
+	}
+	cells := make([]*Cell, f.props.Columns)
+	for column := range f.cells[row] {
+		cells[column] = &f.cells[row][column]
+	}
+	return &Row{Line: Line{cells: cells}}, nil
+}
+
+func (f *Field) GetColumn(column int) (*Column, error) {
+	if !f.hasStructure() || column < 0 || column >= f.props.Columns {
+		return nil, ErrOutOfBounds
+	}
+	cells := make([]*Cell, f.props.Rows)
+	for row := range f.cells {
+		cells[row] = &f.cells[row][column]
+	}
+	return &Column{Line: Line{cells: cells}}, nil
+}
+
+func (f *Field) GetBlock(row, column int) (*Block, error) {
+	if !f.hasStructure() || row < 0 || row >= f.props.BlockRows || column < 0 || column >= f.props.BlockColumns {
+		return nil, ErrOutOfBounds
+	}
+	startRow := row * f.props.BlockSizeRow
+	startColumn := column * f.props.BlockSizeColumn
+	cells := make([][]*Cell, f.props.BlockSizeRow)
+	for blockRow := range cells {
+		cells[blockRow] = make([]*Cell, f.props.BlockSizeColumn)
+		for blockColumn := range cells[blockRow] {
+			cells[blockRow][blockColumn] = &f.cells[startRow+blockRow][startColumn+blockColumn]
+		}
+	}
+	return &Block{cells: cells}, nil
+}
+
+func (f *Field) GetEachPartAtPos(pos *Position) ([]Part, error) {
+	if pos == nil {
+		return nil, ErrOutOfBounds
+	}
 	row, err := f.GetRow(pos.row)
 	if err != nil {
-		return nil
+		return nil, err
 	}
-
 	column, err := f.GetColumn(pos.column)
 	if err != nil {
-		return nil
+		return nil, err
 	}
-
 	block, err := f.GetBlock(pos.blockRow, pos.blockColumn)
 	if err != nil {
-		return nil
+		return nil, err
 	}
-
+	return []Part{row, column, block}, nil
-	return append([]Part{}, row, column, block)
 }
 
-func (f *Field) GetCell(r, c int) (*Cell, error) {
+func (f *Field) GetCell(row, column int) (*Cell, error) {
-	if r > f.props.Rows || c > f.props.Columns {
+	if !f.hasStructure() || row < 0 || row >= f.props.Rows || column < 0 || column >= f.props.Columns {
-		return nil, outOfBound
+		return nil, ErrOutOfBounds
 	}
-	return &f.cells[r][c], nil
+	return &f.cells[row][column], nil
+}
+
+func (f *Field) GetProperties() (Properties, error) {
+	if !f.hasStructure() {
+		return Properties{}, ErrInvalidField
+	}
+	return *f.props, nil
 }
 
 func (f *Field) GetRating() float64 {
+	if !f.hasStructure() {
+		return 0
+	}
 	return f.props.Rating
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
+func (f *Field) ForEachPart(fn func(Part)) {
-// FOREACH FUNCTIONS                                                              //
+	if !f.hasStructure() || fn == nil {
-// ────────────────────────────────────────────────────────────────────────────── //
+		return
-
+	}
-func (f *Field) ForEachPart(fn func(part Part)) {
+	f.ForEachRow(func(row *Row) { fn(row) })
-	f.ForEachRow(func(row *Row) {
+	f.ForEachColumn(func(column *Column) { fn(column) })
-		fn(row)
+	f.ForEachBlock(func(block *Block) { fn(block) })
-	})
-
-	f.ForEachColumn(func(column *Column) {
-		fn(column)
-	})
-
-	f.ForEachBlock(func(block *Block) {
-		fn(block)
-	})
 }
 
-func (f *Field) ForEachPartAtPos(pos *Position, fn func(part Part)) {
+func (f *Field) ForEachPartAtPos(pos *Position, fn func(Part)) error {
-	parts := f.GetEachPartAtPos(pos)
+	if fn == nil {
+		return nil
+	}
+	parts, err := f.GetEachPartAtPos(pos)
+	if err != nil {
+		return err
+	}
 	for _, part := range parts {
 		fn(part)
 	}
+	return nil
+}
+
+func (f *Field) ForEachRow(fn func(*Row)) {
+	if !f.hasStructure() || fn == nil {
+		return
+	}
+	for row := 0; row < f.props.Rows; row++ {
+		part, _ := f.GetRow(row)
+		fn(part)
+	}
 }
 
-func (f *Field) ForEachRow(fn func(row *Row)) {
+func (f *Field) ForEachColumn(fn func(*Column)) {
-	for i := range f.props.Rows {
+	if !f.hasStructure() || fn == nil {
-		row, err := f.GetRow(i)
-		if err != nil {
-			fmt.Println(err.Error())
 		return
 	}
-		fn(row)
+	for column := 0; column < f.props.Columns; column++ {
+		part, _ := f.GetColumn(column)
+		fn(part)
 	}
 }
 
-func (f *Field) ForEachColumn(fn func(column *Column)) {
+func (f *Field) ForEachBlock(fn func(*Block)) {
-	for i := range f.props.Columns {
+	if !f.hasStructure() || fn == nil {
-		column, err := f.GetColumn(i)
-		if err != nil {
-			fmt.Println(err.Error())
 		return
 	}
-		fn(column)
+	for row := 0; row < f.props.BlockRows; row++ {
+		for column := 0; column < f.props.BlockColumns; column++ {
+			part, _ := f.GetBlock(row, column)
+			fn(part)
+		}
 	}
-
 }
 
-func (f *Field) ForEachBlock(fn func(block *Block)) {
+func (f *Field) ForEachCell(fn func(*Cell)) {
-	for r := range f.props.BlockRows {
+	if !f.hasStructure() || fn == nil {
-		for c := range f.props.BlockColumns {
-			block, err := f.GetBlock(r, c)
-			if err != nil {
-				fmt.Println(err.Error())
 		return
 	}
-			fn(block)
+	for row := range f.cells {
+		for column := range f.cells[row] {
+			fn(&f.cells[row][column])
 		}
 	}
 }
 
-func (f *Field) ForEachCell(fn func(cell *Cell)) {
-	for _, row := range f.cells {
-		for _, cell := range row {
-			fn(&cell)
-		}
-	}
-}
-
-// ────────────────────────────────────────────────────────────────────────────── //
-// MODIFIER                                                                       //
-// ────────────────────────────────────────────────────────────────────────────── //
-
-func (f *Field) AddChange(eChange *ExternalChange) {
-	change := Change{
-		Cell:       eChange.Cell,
-		marks:      eChange.Marks,
-		action:     eChange.Action,
-		value:      eChange.Value,
-		from:       eChange.From,
-		triggerdBy: eChange.TriggerdBy,
-	}
-
-	change.do()
-
-	f.changes = append(f.changes, change)
-}
-
 func (f *Field) SetRating(rating float64) {
+	if f != nil && f.props != nil {
 		f.props.Rating = rating
+	}
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// STATE                                                                          //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 func (f *Field) String() string {
-	//TODO: auf beliebige größen anpassen
+	if !f.hasStructure() {
-	var sb strings.Builder
+		return "<invalid field>"
-
+	}
-	// Oberer Rahmen
+	var result strings.Builder
-	sb.WriteString("╔═══════╤═══════╤═══════╗\n")
+	for row := 0; row < f.props.Rows; row++ {
-
+		if row > 0 && row%f.props.BlockSizeRow == 0 {
-	for i := range f.cells {
+			result.WriteByte('\n')
-		sb.WriteString("║ ") // Linke Rahmenseite
+		}
-
+		for column := 0; column < f.props.Columns; column++ {
-		for j, cell := range f.cells[i] {
+			if column > 0 {
-			// Wert ausgeben oder Punkt für 0
+				if column%f.props.BlockSizeColumn == 0 {
-			val := cell.number
+					result.WriteString(" | ")
-			if val == 0 {
-				sb.WriteString("·")
 				} else {
-				sb.WriteString(string('0' + val))
+					result.WriteByte(' ')
 				}
-
+			}
-			// Trennlinien zwischen Blöcken und Zellen
+			value := f.cells[row][column].number
-			if (j+1)%3 == 0 {
+			if value == 0 {
-				if j < 8 {
+				result.WriteRune('·')
-					sb.WriteString(" │ ")
 			} else {
-					sb.WriteString(" ║\n") // Rechte Rahmenseite + Zeilenumbruch
+				result.WriteString(strconv.Itoa(value))
-				}
-			} else {
-				sb.WriteString(" ")
 			}
 		}
-
+		if row+1 < f.props.Rows {
-		// Horizontale Trennlinien nach jeder 3. Zeile
+			result.WriteByte('\n')
-		if (i+1)%3 == 0 && i < 8 {
-			sb.WriteString("╟───────┼───────┼───────╢\n")
 		}
 	}
-
+	return result.String()
-	// Unterer Rahmen
-	sb.WriteString("╚═══════╧═══════╧═══════╝")
-
-	return sb.String()
 }
 
-func (f *Field) StringNotesForNumber(n int) string {
+func (f *Field) StringNotesForNumber(number int) string {
-	return "StringNotesForNumber is not implementet"
+	if !f.hasStructure() || number < 1 || number > f.props.Rows {
+		return ""
+	}
+	var result strings.Builder
+	f.ForEachCell(func(cell *Cell) {
+		if cell.notes.Has(number) {
+			fmt.Fprintf(&result, "%d/%d\n", cell.pos.row, cell.pos.column)
+		}
+	})
+	return strings.TrimSuffix(result.String(), "\n")
 }
 
 func (f *Field) StringNotes() string {
-	var str strings.Builder
+	if !f.hasStructure() {
-	fmt.Fprintf(&str, "Notes:\n")
+		return "Notes:"
+	}
+	var result strings.Builder
+	result.WriteString("Notes:\n")
 	f.ForEachCell(func(cell *Cell) {
-		fmt.Fprintf(&str, "Pos: %d/%d - Notes: %v\n", cell.Pos.row, cell.Pos.column, cell.Notes.numbers)
+		fmt.Fprintf(&result, "Pos: %d/%d - Notes: %v\n", cell.pos.row, cell.pos.column, cell.notes.numbers)
 	})
-	return str.String()
+	return strings.TrimSuffix(result.String(), "\n")
 }
 
 func (f *Field) IsSolved() bool {
-	result := true
+	if !f.IsValid() {
-	f.ForEachCell(func(cell *Cell) {
-		if cell.number == 0 {
-			result = false
-		}
-	})
-
-	/* TODO:
-	if !f.isValid() {
 		return false
 	}
-	*/
+	for row := range f.cells {
-
+		for column := range f.cells[row] {
-	return result
+			if f.cells[row][column].number == 0 {
+				return false
+			}
+		}
+	}
+	return true
 }
 
 func (f *Field) IsValid() bool {
-	//In jedem Part gibt es jede Zahl max ein mal
+	return f.hasStructure() && f.hasValidNumbers() && f.hasValidNotes()
-	//Die Zahl kommt nicht in intersecting parts vor
+}
+
+func (f *Field) hasStructure() bool {
+	if f == nil || f.props == nil || validateProperties(*f.props) != nil || len(f.cells) != f.props.Rows {
+		return false
+	}
+	for row := range f.cells {
+		if len(f.cells[row]) != f.props.Columns {
+			return false
+		}
+		for column := range f.cells[row] {
+			cell := &f.cells[row][column]
+			if cell.notes == nil || cell.pos == nil || !positionMatches(cell.pos, *f.props, row, column) || cell.number < 0 || cell.number > f.props.Rows {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func (f *Field) hasValidNumbers() bool {
+	if !f.hasStructure() {
+		return false
+	}
+	valid := true
+	f.ForEachPart(func(part Part) {
+		seen := make(map[int]struct{}, f.props.Rows)
+		part.ForEachCell(func(cell *Cell) {
+			if cell.number == 0 || !valid {
+				return
+			}
+			if _, exists := seen[cell.number]; exists {
+				valid = false
+				return
+			}
+			seen[cell.number] = struct{}{}
+		})
+	})
+	return valid
+}
+
+func (f *Field) hasValidNotes() bool {
+	if !f.hasStructure() {
+		return false
+	}
+	valid := true
+	f.ForEachCell(func(cell *Cell) {
+		if !valid {
+			return
+		}
+		if cell.number != 0 && len(cell.notes.numbers) != 0 {
+			valid = false
+			return
+		}
+		previous := 0
+		for _, note := range cell.notes.numbers {
+			if note <= previous || note > f.props.Rows || f.hasPeerNumber(cell, note) {
+				valid = false
+				return
+			}
+			previous = note
+		}
+	})
+	return valid
+}
+
+func (f *Field) ownsCell(cell *Cell) bool {
+	if !f.hasStructure() || cell == nil || cell.pos == nil {
+		return false
+	}
+	row, column := cell.pos.row, cell.pos.column
+	return row >= 0 && row < f.props.Rows && column >= 0 && column < f.props.Columns && &f.cells[row][column] == cell
+}
+
+func (f *Field) hasPeerNumber(cell *Cell, number int) bool {
+	if !f.ownsCell(cell) {
+		return false
+	}
+	row, column := cell.pos.row, cell.pos.column
+	for index := 0; index < f.props.Columns; index++ {
+		if index != column && f.cells[row][index].number == number {
+			return true
+		}
+	}
+	for index := 0; index < f.props.Rows; index++ {
+		if index != row && f.cells[index][column].number == number {
+			return true
+		}
+	}
+	startRow := cell.pos.blockRow * f.props.BlockSizeRow
+	startColumn := cell.pos.blockColumn * f.props.BlockSizeColumn
+	for blockRow := 0; blockRow < f.props.BlockSizeRow; blockRow++ {
+		for blockColumn := 0; blockColumn < f.props.BlockSizeColumn; blockColumn++ {
+			peer := &f.cells[startRow+blockRow][startColumn+blockColumn]
+			if peer != cell && peer.number == number {
+				return true
+			}
+		}
+	}
 	return false
 }
+
+func (f *Field) snapshot() fieldSnapshot {
+	state := fieldSnapshot{numbers: make([][]int, len(f.cells)), notes: make([][][]int, len(f.cells)), changes: slices.Clone(f.changes)}
+	for row := range f.cells {
+		state.numbers[row] = make([]int, len(f.cells[row]))
+		state.notes[row] = make([][]int, len(f.cells[row]))
+		for column := range f.cells[row] {
+			state.numbers[row][column] = f.cells[row][column].number
+			state.notes[row][column] = slices.Clone(f.cells[row][column].notes.numbers)
+		}
+	}
+	return state
+}
+
+func (f *Field) restore(state fieldSnapshot) {
+	for row := range f.cells {
+		for column := range f.cells[row] {
+			f.cells[row][column].number = state.numbers[row][column]
+			f.cells[row][column].notes.numbers = slices.Clone(state.notes[row][column])
+		}
+	}
+	f.changes = slices.Clone(state.changes)
+}

field/field_test.go

@@ -0,0 +1,244 @@
+package field
+
+import (
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+const solvedPuzzle = "123456789456789123789123456234567891567891234891234567345678912678912345912345678"
+
+func classicCells(t *testing.T, digits string) (Properties, [][]Cell) {
+	t.Helper()
+	if len(digits) != 81 {
+		t.Fatalf("test puzzle has %d characters", len(digits))
+	}
+	props := Properties{Rows: 9, Columns: 9, BlockRows: 3, BlockColumns: 3, BlockSizeRow: 3, BlockSizeColumn: 3}
+	cells := make([][]Cell, props.Rows)
+	for row := range cells {
+		cells[row] = make([]Cell, props.Columns)
+		for column := range cells[row] {
+			position := NewPosition(row, column, row/3, column/3, row%3, column%3)
+			cells[row][column] = *NewCell(int(digits[row*9+column]-'0'), position)
+		}
+	}
+	return props, cells
+}
+
+func mustField(t *testing.T, digits string) *Field {
+	t.Helper()
+	props, cells := classicCells(t, digits)
+	result, err := New(props, cells)
+	if err != nil {
+		t.Fatalf("New() error = %v", err)
+	}
+	return result
+}
+
+func TestNewValidatesStructureAndValues(t *testing.T) {
+	props, cells := classicCells(t, strings.Repeat("0", 81))
+
+	tests := []struct {
+		name   string
+		mutate func(*Properties, [][]Cell)
+	}{
+		{name: "missing row", mutate: func(_ *Properties, cells [][]Cell) { cells[0] = cells[0][:8] }},
+		{name: "wrong position", mutate: func(_ *Properties, cells [][]Cell) { cells[0][0].pos = NewPosition(1, 0, 0, 0, 0, 0) }},
+		{name: "number out of range", mutate: func(_ *Properties, cells [][]Cell) { cells[0][0].number = 10 }},
+		{name: "invalid blocks", mutate: func(props *Properties, _ [][]Cell) { props.BlockRows = 2 }},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			localProps := props
+			localCells := make([][]Cell, len(cells))
+			for row := range cells {
+				localCells[row] = append([]Cell(nil), cells[row]...)
+			}
+			test.mutate(&localProps, localCells)
+			if _, err := New(localProps, localCells); !errors.Is(err, ErrInvalidField) {
+				t.Fatalf("New() error = %v, want ErrInvalidField", err)
+			}
+		})
+	}
+
+	duplicate := "113456789" + solvedPuzzle[9:]
+	duplicateProps, duplicateCells := classicCells(t, duplicate)
+	if _, err := New(duplicateProps, duplicateCells); !errors.Is(err, ErrInvalidField) {
+		t.Fatalf("New() duplicate error = %v, want ErrInvalidField", err)
+	}
+}
+
+func TestGetCellBoundsAndForEachCellPointers(t *testing.T) {
+	f := mustField(t, strings.Repeat("0", 81))
+	for _, coordinates := range [][2]int{{-1, 0}, {0, -1}, {9, 0}, {0, 9}} {
+		if _, err := f.GetCell(coordinates[0], coordinates[1]); !errors.Is(err, ErrOutOfBounds) {
+			t.Errorf("GetCell(%d, %d) error = %v", coordinates[0], coordinates[1], err)
+		}
+	}
+
+	want, _ := f.GetCell(4, 5)
+	var found *Cell
+	f.ForEachCell(func(cell *Cell) {
+		if cell.GetPosition().GetRow() == 4 && cell.GetPosition().GetColumn() == 5 {
+			found = cell
+		}
+	})
+	if found != want {
+		t.Fatalf("ForEachCell() returned %p, want stored cell %p", found, want)
+	}
+}
+
+func TestValidityAndSolvedState(t *testing.T) {
+	incomplete := mustField(t, strings.Repeat("0", 81))
+	if !incomplete.IsValid() || incomplete.IsSolved() {
+		t.Fatalf("empty field: IsValid=%v IsSolved=%v", incomplete.IsValid(), incomplete.IsSolved())
+	}
+
+	solved := mustField(t, solvedPuzzle)
+	if !solved.IsValid() || !solved.IsSolved() {
+		t.Fatalf("solved field: IsValid=%v IsSolved=%v", solved.IsValid(), solved.IsSolved())
+	}
+	row, _ := solved.GetRow(0)
+	block, _ := solved.GetBlock(0, 0)
+	if !row.IsSolved() || !block.IsSolved() {
+		t.Fatal("completed row and block must be solved")
+	}
+
+	solved.cells[0][0].number = solved.cells[0][1].number
+	if solved.IsValid() || solved.IsSolved() {
+		t.Fatal("field with a duplicate must be invalid and unsolved")
+	}
+}
+
+func TestChangesAreValidatedAndAtomic(t *testing.T) {
+	f := mustField(t, strings.Repeat("0", 81))
+	first, _ := f.GetCell(0, 0)
+	second, _ := f.GetCell(0, 1)
+
+	err := f.AddChanges([]ExternalChange{
+		{Cell: first, Action: ActionSetNumber, Value: 1, From: 0},
+		{Cell: second, Action: ActionSetNumber, Value: 1, From: 0},
+	})
+	if !errors.Is(err, ErrInvalidChange) {
+		t.Fatalf("AddChanges() error = %v, want ErrInvalidChange", err)
+	}
+	if first.GetNumber() != 0 || second.GetNumber() != 0 || len(f.GetChanges()) != 0 {
+		t.Fatal("failed change batch was not rolled back")
+	}
+
+	foreign := NewCell(0, NewPosition(0, 0, 0, 0, 0, 0))
+	if err := f.AddChange(&ExternalChange{Cell: foreign, Action: ActionSetNumber, Value: 1}); !errors.Is(err, ErrInvalidChange) {
+		t.Fatalf("foreign cell error = %v, want ErrInvalidChange", err)
+	}
+	if err := (*Field)(nil).AddChanges(nil); !errors.Is(err, ErrInvalidField) {
+		t.Fatalf("nil field error = %v, want ErrInvalidField", err)
+	}
+}
+
+func TestNotesStaySortedUniqueAndArePropagated(t *testing.T) {
+	f := mustField(t, strings.Repeat("0", 81))
+	target, _ := f.GetCell(0, 0)
+	rowPeer, _ := f.GetCell(0, 1)
+	columnPeer, _ := f.GetCell(1, 0)
+	blockPeer, _ := f.GetCell(1, 1)
+	unrelated, _ := f.GetCell(4, 4)
+
+	for _, note := range []int{5, 1, 5} {
+		if err := target.GetNotes().Add(f, target, note, "test", nil); err != nil {
+			t.Fatalf("add target note: %v", err)
+		}
+	}
+	if got, want := target.GetNotes().Get(), []int{1, 5}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("notes = %v, want %v", got, want)
+	}
+	for _, cell := range []*Cell{rowPeer, columnPeer, blockPeer, unrelated} {
+		if err := cell.GetNotes().Add(f, cell, 5, "test", nil); err != nil {
+			t.Fatalf("add peer note: %v", err)
+		}
+	}
+	if err := target.SetNumber(f, 5, "test", nil); err != nil {
+		t.Fatalf("SetNumber() error = %v", err)
+	}
+	for _, peer := range []*Cell{target, rowPeer, columnPeer, blockPeer} {
+		if peer.GetNotes().Has(5) {
+			row, column := peer.GetPosition().GetCoords()
+			t.Fatalf("note 5 was not removed at %d/%d", row, column)
+		}
+	}
+	if !unrelated.GetNotes().Has(5) {
+		t.Fatal("note was removed from an unrelated cell")
+	}
+	if f.IsValid() == false {
+		t.Fatal("field must remain valid after candidate propagation")
+	}
+	filled, _ := f.GetCell(0, 2)
+	if err := filled.SetNumber(f, 2, "test", nil); err != nil {
+		t.Fatal(err)
+	}
+	if err := filled.GetNotes().Add(f, filled, 3, "test", nil); !errors.Is(err, ErrInvalidChange) {
+		t.Fatalf("note on filled cell error = %v, want ErrInvalidChange", err)
+	}
+	if err := rowPeer.GetNotes().Add(f, rowPeer, 2, "test", nil); !errors.Is(err, ErrInvalidChange) {
+		t.Fatalf("invalid peer note error = %v, want ErrInvalidChange", err)
+	}
+}
+
+func TestRemoveNumberRecordsCorrectAction(t *testing.T) {
+	f := mustField(t, strings.Repeat("0", 81))
+	cell, _ := f.GetCell(0, 0)
+	if err := cell.SetNumber(f, 1, "set", nil); err != nil {
+		t.Fatal(err)
+	}
+	if err := cell.RemoveNumber(f, "remove", nil); err != nil {
+		t.Fatal(err)
+	}
+	changes := f.GetChanges()
+	if changes[len(changes)-1].Action != ActionRemoveNumber {
+		t.Fatalf("last action = %v, want ActionRemoveNumber", changes[len(changes)-1].Action)
+	}
+}
+
+func TestStringRendersDigitsAndNotes(t *testing.T) {
+	f := mustField(t, "900000000"+strings.Repeat("0", 72))
+	cell, _ := f.GetCell(0, 1)
+	if err := cell.GetNotes().Add(f, cell, 1, "test", nil); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasPrefix(f.String(), "9 · ·") {
+		t.Fatalf("String() = %q", f.String())
+	}
+	if got := f.StringNotesForNumber(1); got != "0/1" {
+		t.Fatalf("StringNotesForNumber() = %q, want 0/1", got)
+	}
+}
+
+func TestGettersReturnDefensiveCopies(t *testing.T) {
+	f := mustField(t, strings.Repeat("0", 81))
+	props, err := f.GetProperties()
+	if err != nil {
+		t.Fatal(err)
+	}
+	props.Rows = 4
+	stored, _ := f.GetProperties()
+	if stored.Rows != 9 {
+		t.Fatal("GetProperties exposed internal properties")
+	}
+
+	cell, _ := f.GetCell(0, 0)
+	if err := cell.GetNotes().Add(f, cell, 1, "test", nil); err != nil {
+		t.Fatal(err)
+	}
+	notes := cell.GetNotes().Get()
+	notes[0] = 9
+	if !cell.GetNotes().Has(1) || cell.GetNotes().Has(9) {
+		t.Fatal("Notes.Get exposed its backing slice")
+	}
+
+	changes := f.GetChanges()
+	changes[0].Value = 9
+	if f.GetChanges()[0].Value != 1 {
+		t.Fatal("GetChanges exposed its backing slice")
+	}
+}

field/parts.go

@@ -7,83 +7,72 @@ import (
 
 type Part interface {
 	IsSolved() bool
-	ForEachCell(fn func(cell *Cell))
+	ForEachCell(func(*Cell))
 	GetMissingNumbers() []int
-	RemoveNote(field *Field, note int, trigger string, marks []Mark)
+	RemoveNote(*Field, int, string, []Mark) error
 	String() string
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// LINE STRUCTURE                                                                 //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Line struct {
 	cells []*Cell
 }
 
 func (l *Line) IsSolved() bool {
-	//TODO
+	return partIsSolved(l)
-	return false
 }
 
-func (l *Line) ForEachCell(fn func(cell *Cell)) {
+func (l *Line) ForEachCell(fn func(*Cell)) {
+	if l == nil || fn == nil {
+		return
+	}
 	for _, cell := range l.cells {
 		fn(cell)
 	}
 }
 
 func (l *Line) GetMissingNumbers() []int {
-	return getMissingNumbersHelper(len(l.cells), l.ForEachCell)
+	if l == nil {
+		return nil
+	}
+	return getMissingNumbers(len(l.cells), l.ForEachCell)
 }
 
-func (l *Line) RemoveNote(field *Field, note int, trigger string, marks []Mark) {
+func (l *Line) RemoveNote(f *Field, note int, trigger string, marks []Mark) error {
-	l.ForEachCell(func(cell *Cell) {
+	return removeNoteFromPart(l, f, note, trigger, marks)
-		cell.Notes.Remove(field, cell, note, trigger, marks)
-	})
 }
 
 func (l *Line) String() string {
-	return lineString(l, "Line")
+	return partString(l, "Line")
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// ROW STRUCTURE                                                                  //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Row struct {
 	Line
 }
 
 func (r *Row) String() string {
-	return lineString(r, "Row")
+	return partString(r, "Row")
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// COLUMN STRUCTURE                                                               //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Column struct {
 	Line
 }
 
 func (c *Column) String() string {
-	return lineString(c, "Column")
+	return partString(c, "Column")
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
-// BLOCK STRUCTURE                                                                //
-// ────────────────────────────────────────────────────────────────────────────── //
-
 type Block struct {
 	cells [][]*Cell
 }
 
 func (b *Block) IsSolved() bool {
-	//TODO
+	return partIsSolved(b)
-	return false
 }
 
-func (b *Block) ForEachCell(fn func(cell *Cell)) {
+func (b *Block) ForEachCell(fn func(*Cell)) {
+	if b == nil || fn == nil {
+		return
+	}
 	for _, row := range b.cells {
 		for _, cell := range row {
 			fn(cell)
@@ -92,56 +81,79 @@ func (b *Block) ForEachCell(fn func(cell *Cell)) {
 }
 
 func (b *Block) GetMissingNumbers() []int {
-	max := len(b.cells) * len(b.cells[0])
+	if b == nil || len(b.cells) == 0 {
-	return getMissingNumbersHelper(max, b.ForEachCell)
+		return nil
+	}
+	size := 0
+	for _, row := range b.cells {
+		size += len(row)
+	}
+	return getMissingNumbers(size, b.ForEachCell)
 }
 
-func (b *Block) RemoveNote(field *Field, note int, trigger string, marks []Mark) {
+func (b *Block) RemoveNote(f *Field, note int, trigger string, marks []Mark) error {
-	b.ForEachCell(func(cell *Cell) {
+	return removeNoteFromPart(b, f, note, trigger, marks)
-		cell.Notes.Remove(field, cell, note, trigger, marks)
-	})
 }
 
 func (b *Block) String() string {
-	var str strings.Builder
+	return partString(b, "Block")
-	str.WriteString("Block:\t")
-	lastLine := 0
-	b.ForEachCell(func(cell *Cell) {
-		fmt.Fprintf(&str, "%d ", cell.number)
-		if lastLine == cell.Pos.inBlockRow {
-
-		}
-	})
-
-	return str.String()
 }
 
-// ────────────────────────────────────────────────────────────────────────────── //
+func partIsSolved(part Part) bool {
-// HELPER                                                                         //
+	if part == nil {
-// ────────────────────────────────────────────────────────────────────────────── //
+		return false
-
+	}
-func lineString(part Part, name string) string {
+	cellCount := 0
-	var str strings.Builder
+	seen := make(map[int]struct{})
-	fmt.Fprintf(&str, "%s:\t", name)
+	valid := true
 	part.ForEachCell(func(cell *Cell) {
-		fmt.Fprintf(&str, "%d ", cell.number)
+		cellCount++
+		if cell == nil || cell.number <= 0 {
+			valid = false
+			return
+		}
+		if _, exists := seen[cell.number]; exists {
+			valid = false
+		}
+		seen[cell.number] = struct{}{}
 	})
-	return str.String()
+	return valid && cellCount > 0 && len(seen) == cellCount
 }
 
-func getMissingNumbersHelper(max int, iterate func(fn func(cell *Cell))) []int {
+func removeNoteFromPart(part Part, f *Field, note int, trigger string, marks []Mark) error {
-	present := make(map[int]bool, max)
+	var firstErr error
+	part.ForEachCell(func(cell *Cell) {
+		if firstErr != nil || !cell.notes.Has(note) {
+			return
+		}
+		firstErr = cell.notes.Remove(f, cell, note, trigger, marks)
+	})
+	return firstErr
+}
 
+func partString(part Part, name string) string {
+	if part == nil {
+		return name + ":"
+	}
+	var result strings.Builder
+	fmt.Fprintf(&result, "%s:\t", name)
+	part.ForEachCell(func(cell *Cell) {
+		fmt.Fprintf(&result, "%d ", cell.GetNumber())
+	})
+	return strings.TrimSuffix(result.String(), " ")
+}
+
+func getMissingNumbers(max int, iterate func(func(*Cell))) []int {
+	present := make(map[int]struct{}, max)
 	iterate(func(cell *Cell) {
-		if cell.number != 0 {
+		if cell != nil && cell.number != 0 {
-			present[cell.number] = true
+			present[cell.number] = struct{}{}
 		}
 	})
-
+	missing := make([]int, 0, max-len(present))
-	var missing []int
+	for number := 1; number <= max; number++ {
-	for i := 1; i <= max; i++ {
+		if _, exists := present[number]; !exists {
-		if !present[i] {
+			missing = append(missing, number)
-			missing = append(missing, i)
 		}
 	}
 	return missing

field/position.go

@@ -22,37 +22,64 @@ func NewPosition(row, column, blockRow, blockColumn, inBlockRow, inBlockColumn i
 }
 
 func (p *Position) GetRow() int {
+	if p == nil {
+		return 0
+	}
 	return p.row
 }
 
 func (p *Position) GetColumn() int {
+	if p == nil {
+		return 0
+	}
 	return p.column
 }
 
 func (p *Position) GetBlockRow() int {
+	if p == nil {
+		return 0
+	}
 	return p.blockRow
 }
 
 func (p *Position) GetBlockColumn() int {
+	if p == nil {
+		return 0
+	}
 	return p.blockColumn
 }
 
 func (p *Position) GetInBlockRow() int {
+	if p == nil {
+		return 0
+	}
 	return p.inBlockRow
 }
 
 func (p *Position) GetInBlockColumn() int {
+	if p == nil {
+		return 0
+	}
 	return p.inBlockColumn
 }
 
 func (p *Position) GetCoords() (int, int) {
+	if p == nil {
+		return 0, 0
+	}
 	return p.row, p.column
 }
 
 func (p *Position) GetBlockCoords() (int, int) {
+	if p == nil {
+		return 0, 0
+	}
 	return p.blockRow, p.blockColumn
 }
 
 func (p *Position) GetInBlockCoords() (int, int) {
+	if p == nil {
+		return 0, 0
+	}
 	return p.inBlockRow, p.inBlockColumn
 }

logic/solver.go

@@ -1,65 +1,83 @@
 package logic
 
 import (
+	"errors"
+	"fmt"
+
 	"git.kleiax.de/homepage/field"
 	"git.kleiax.de/homepage/logic/strategies"
 )
 
-// ────────────────────────────────────────────────────────────────────────────── //
+var ErrNoProgress = errors.New("no strategy can solve the puzzle")
-// SOLVER STRUCTURE                                                               //
-// ────────────────────────────────────────────────────────────────────────────── //
 
 type Solver struct {
 	strategies []strategies.Strategy
-	returnTo   int
 	field      *field.Field
-	conf       struct {
-		all    bool
-		repeat bool
-	}
 }
 
 func (s *Solver) Add(strategy strategies.Strategy) {
+	if strategy != nil {
 		s.strategies = append(s.strategies, strategy)
+	}
 }
 
-func (s *Solver) InitStragies(field *field.Field) {
+func (s *Solver) InitStrategies(f *field.Field) error {
-	s.returnTo = 1
+	if f == nil || !f.IsValid() {
-	s.field = field
+		return field.ErrInvalidField
+	}
+	s.field = f
 	for _, strategy := range s.strategies {
-		strategy.Init(field)
+		strategy.Init(f)
 	}
+	return nil
 }
 
-func (s *Solver) Run(i int) bool {
+// InitStragies is kept for compatibility. New code should use InitStrategies.
-	for j := 0; j < len(s.strategies); j++ {
+func (s *Solver) InitStragies(f *field.Field) error {
-		if s.strategies[j].SearchProgressableCells() == 0 {
+	return s.InitStrategies(f)
-			continue
+}
-		}
+
-
+// Run applies strategies from the requested index until the field is solved or
-		for _, change := range s.strategies[j].ApplyAll() {
+// no strategy can make progress. It returns false without an error for a valid
-			if change.Action != field.ActionSetNumber {
+// but currently unsolved field.
-				continue
+func (s *Solver) Run(start int) (bool, error) {
-			}
+	if s.field == nil || !s.field.IsValid() {
-			s.field.ForEachPartAtPos(change.Cell.Pos, func(part field.Part) {
+		return false, field.ErrInvalidField
-				part.ForEachCell(func(cell *field.Cell) {
+	}
-					cell.Notes.Remove(s.field, cell, change.Value, "remove note after insert of a number", nil)
+	if start < 0 || start > len(s.strategies) {
-				})
+		return false, fmt.Errorf("strategy index %d out of range", start)
-			})
-		}
-
-		j = s.returnTo - 1
 	}
 
+	for {
 		if s.field.IsSolved() {
-		return true
+			return true, nil
+		}
+
+		progress := false
+		for index := start; index < len(s.strategies); index++ {
+			strategy := s.strategies[index]
+			if strategy.SearchProgressableCells() == 0 {
+				continue
+			}
+			changes, err := strategy.ApplyAll()
+			if err != nil {
+				return false, fmt.Errorf("apply strategy %q: %w", strategy.Name(), err)
+			}
+			if len(changes) == 0 {
+				continue
+			}
+			progress = true
+			break
+		}
+
+		if !progress {
+			return false, nil
+		}
+		start = 0
 	}
-	return false
 }
 
-func (s *Solver) Search() {
+func (s *Solver) Search() {}
-
-}
 
 func (s *Solver) GetSolutionPath() []field.ExternalChange {
 	return nil

logic/solver_test.go

@@ -0,0 +1,71 @@
+package logic_test
+
+import (
+	"errors"
+	"testing"
+
+	"git.kleiax.de/homepage/field"
+	"git.kleiax.de/homepage/logic"
+	"git.kleiax.de/homepage/logic/strategies"
+	"git.kleiax.de/homepage/parser"
+)
+
+const solvedPuzzle = "123456789456789123789123456234567891567891234891234567345678912678912345912345678"
+
+func parsedField(t *testing.T, puzzle string) *field.Field {
+	t.Helper()
+	input := &parser.PuzzleString{}
+	if err := input.Parse([]byte(puzzle)); err != nil {
+		t.Fatal(err)
+	}
+	result, err := input.GetField(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return result
+}
+
+func TestSolverRunSolvesAndDetectsNoProgress(t *testing.T) {
+	t.Run("solved", func(t *testing.T) {
+		f := parsedField(t, "0"+solvedPuzzle[1:])
+		solver := &logic.Solver{}
+		solver.Add(&strategies.LastDigit{})
+		if err := solver.InitStrategies(f); err != nil {
+			t.Fatal(err)
+		}
+		solved, err := solver.Run(0)
+		if err != nil || !solved || !f.IsSolved() {
+			t.Fatalf("Run() = (%v, %v), field solved = %v", solved, err, f.IsSolved())
+		}
+	})
+
+	t.Run("no progress", func(t *testing.T) {
+		f := parsedField(t, "0"+solvedPuzzle[1:])
+		solver := &logic.Solver{}
+		if err := solver.InitStrategies(f); err != nil {
+			t.Fatal(err)
+		}
+		solved, err := solver.Run(0)
+		if err != nil || solved {
+			t.Fatalf("Run() = (%v, %v), want (false, nil)", solved, err)
+		}
+	})
+}
+
+func TestSolverRejectsInvalidStateAndStrategyIndex(t *testing.T) {
+	solver := &logic.Solver{}
+	if _, err := solver.Run(0); !errors.Is(err, field.ErrInvalidField) {
+		t.Fatalf("Run() before initialization error = %v", err)
+	}
+	if err := solver.InitStrategies(nil); !errors.Is(err, field.ErrInvalidField) {
+		t.Fatalf("InitStrategies(nil) error = %v", err)
+	}
+
+	f := parsedField(t, "0"+solvedPuzzle[1:])
+	if err := solver.InitStrategies(f); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := solver.Run(1); err == nil {
+		t.Fatal("Run() with invalid strategy index error = nil")
+	}
+}

logic/strategies/divers.go

@@ -17,13 +17,16 @@ func (ld *LastDigit) SearchProgressableCells() int {
 		missingNumbers := part.GetMissingNumbers()
 		if len(missingNumbers) == 1 {
 			var emptyCell *field.Cell
+			emptyCount := 0
 			part.ForEachCell(func(cell *field.Cell) {
 				if cell.GetNumber() == 0 {
 					emptyCell = cell
+					emptyCount++
 				}
 			})
-			// fmt.Println(part)
+			if emptyCount != 1 {
-			// fmt.Printf("Gefundene Zelle: %d/%d - %d, missungNumber: %v, Typ: %T\n", emptyCell.Pos.GetRow(), emptyCell.Pos.GetColumn(), emptyCell.GetNumber(), missingNumbers, part)
+				return
+			}
 			change := field.ExternalChange{
 				Cell:       emptyCell,
 				Action:     field.ActionSetNumber,
@@ -31,10 +34,9 @@ func (ld *LastDigit) SearchProgressableCells() int {
 				From:       emptyCell.GetNumber(),
 				TriggerdBy: ld.getName(),
 			}
-			ld.changes = append(ld.changes, change)
+			ld.queue(change)
 		}
 	})
-	// fmt.Printf("LastDigit Changes %d\n", len(ld.changes))
 	return len(ld.changes)
 }
 
@@ -42,6 +44,10 @@ func (ld *LastDigit) getName() string {
 	return "Last Digit"
 }
 
+func (ld *LastDigit) Name() string {
+	return ld.getName()
+}
+
 // ────────────────────────────────────────────────────────────────────────────── //
 // CRP STRUCTURE                                                                  //
 // ────────────────────────────────────────────────────────────────────────────── //

logic/strategies/hidden.go

@@ -15,8 +15,8 @@ func (hs *HiddenSingle) SearchProgressableCells() int {
 		m := make(map[int]int)
 
 		part.ForEachCell(func(cell *field.Cell) {
-			notes := cell.Notes.Get()
+			notes := cell.GetNotes().Get()
-			for note := range notes {
+			for _, note := range notes {
 				m[note]++
 			}
 		})
@@ -28,9 +28,9 @@ func (hs *HiddenSingle) SearchProgressableCells() int {
 			}
 		}
 
-		for num := range hiddenSingles {
+		for _, num := range hiddenSingles {
 			part.ForEachCell(func(cell *field.Cell) {
-				if cell.Notes.Has(num) {
+				if cell.GetNumber() == 0 && cell.GetNotes().Has(num) {
 					change := field.ExternalChange{
 						Cell:       cell,
 						Action:     field.ActionSetNumber,
@@ -38,7 +38,7 @@ func (hs *HiddenSingle) SearchProgressableCells() int {
 						From:       cell.GetNumber(),
 						TriggerdBy: hs.getName(),
 					}
-					hs.changes = append(hs.changes, change)
+					hs.queue(change)
 				}
 			})
 		}
@@ -50,6 +50,10 @@ func (hs *HiddenSingle) getName() string {
 	return "Hidden Single"
 }
 
+func (hs *HiddenSingle) Name() string {
+	return hs.getName()
+}
+
 // ────────────────────────────────────────────────────────────────────────────── //
 // HIDDEN_PAIR STRUCTURE                                                          //
 // ────────────────────────────────────────────────────────────────────────────── //

logic/strategies/naked.go

@@ -13,7 +13,7 @@ type NakedSingle struct {
 func (ns *NakedSingle) SearchProgressableCells() int {
 	ns.field.ForEachPart(func(part field.Part) {
 		part.ForEachCell(func(cell *field.Cell) {
-			candidates := cell.Notes.Get()
+			candidates := cell.GetNotes().Get()
 			if len(candidates) == 1 && cell.GetNumber() == 0 {
 				change := field.ExternalChange{
 					Cell:       cell,
@@ -22,7 +22,7 @@ func (ns *NakedSingle) SearchProgressableCells() int {
 					From:       cell.GetNumber(),
 					TriggerdBy: ns.getName(),
 				}
-				ns.changes = append(ns.changes, change)
+				ns.queue(change)
 			}
 		})
 	})
@@ -33,6 +33,10 @@ func (ns *NakedSingle) getName() string {
 	return "Naked Single"
 }
 
+func (ns *NakedSingle) Name() string {
+	return ns.getName()
+}
+
 // ────────────────────────────────────────────────────────────────────────────── //
 // NAKED_DOUBLE STRUCTURE                                                         //
 // ────────────────────────────────────────────────────────────────────────────── //

logic/strategies/notes.go

@@ -1,6 +1,8 @@
 package strategies
 
 import (
+	"sort"
+
 	"git.kleiax.de/homepage/field"
 )
 
@@ -15,11 +17,12 @@ type Notes struct {
 func (n *Notes) SearchProgressableCells() int {
 	n.field.ForEachRow(func(row *field.Row) {
 		row.ForEachCell(func(cell *field.Cell) {
-			column, _ := n.field.GetColumn(cell.Pos.GetColumn())
+			pos := cell.GetPosition()
-			block, _ := n.field.GetBlock(cell.Pos.GetBlockRow(), cell.Pos.GetBlockColumn())
+			column, _ := n.field.GetColumn(pos.GetColumn())
+			block, _ := n.field.GetBlock(pos.GetBlockRow(), pos.GetBlockColumn())
 			candidates := intersection3(row.GetMissingNumbers(), column.GetMissingNumbers(), block.GetMissingNumbers())
 			for _, note := range candidates {
-				if cell.Notes.Has(note) || cell.GetNumber() != 0 {
+				if cell.GetNotes().Has(note) || cell.GetNumber() != 0 {
 					continue
 				}
 				ch := field.ExternalChange{
@@ -30,7 +33,7 @@ func (n *Notes) SearchProgressableCells() int {
 					Marks:      nil,
 					From:       0,
 				}
-				n.changes = append(n.changes, ch)
+				n.queue(ch)
 			}
 		})
 	})
@@ -41,6 +44,10 @@ func (n *Notes) getName() string {
 	return "Make Notes"
 }
 
+func (n *Notes) Name() string {
+	return n.getName()
+}
+
 func intersection3(a, b, c []int) []int {
 	set := make(map[int]bool)
 
@@ -76,6 +83,7 @@ func intersection3(a, b, c []int) []int {
 	for v := range set {
 		result = append(result, v)
 	}
+	sort.Ints(result)
 
 	return result
 }

logic/strategies/strategies_test.go

@@ -0,0 +1,82 @@
+package strategies
+
+import (
+	"strings"
+	"testing"
+
+	"git.kleiax.de/homepage/field"
+)
+
+const solvedGrid = "123456789456789123789123456234567891567891234891234567345678912678912345912345678"
+
+func strategyField(t *testing.T, digits string) *field.Field {
+	t.Helper()
+	props := field.Properties{Rows: 9, Columns: 9, BlockRows: 3, BlockColumns: 3, BlockSizeRow: 3, BlockSizeColumn: 3}
+	cells := make([][]field.Cell, 9)
+	for row := range cells {
+		cells[row] = make([]field.Cell, 9)
+		for column := range cells[row] {
+			pos := field.NewPosition(row, column, row/3, column/3, row%3, column%3)
+			cells[row][column] = *field.NewCell(int(digits[row*9+column]-'0'), pos)
+		}
+	}
+	result, err := field.New(props, cells)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return result
+}
+
+func TestLastDigitDeduplicatesOverlappingParts(t *testing.T) {
+	f := strategyField(t, "0"+solvedGrid[1:])
+	strategy := &LastDigit{}
+	strategy.Init(f)
+	if got := strategy.SearchProgressableCells(); got != 1 {
+		t.Fatalf("SearchProgressableCells() = %d, want 1", got)
+	}
+	changes, err := strategy.ApplyAll()
+	if err != nil {
+		t.Fatalf("ApplyAll() error = %v", err)
+	}
+	if len(changes) != 1 || !f.IsSolved() {
+		t.Fatalf("changes = %d, solved = %v", len(changes), f.IsSolved())
+	}
+}
+
+func TestHiddenSingleUsesCandidateValuesNotSliceIndexes(t *testing.T) {
+	f := strategyField(t, strings.Repeat("0", 81))
+	first, _ := f.GetCell(0, 0)
+	second, _ := f.GetCell(0, 1)
+	third, _ := f.GetCell(0, 2)
+	for _, setup := range []struct {
+		cell *field.Cell
+		note int
+	}{{first, 9}, {second, 8}, {third, 8}} {
+		if err := setup.cell.GetNotes().Add(f, setup.cell, setup.note, "test", nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	strategy := &HiddenSingle{}
+	strategy.Init(f)
+	strategy.SearchProgressableCells()
+	found := false
+	for _, change := range strategy.changes {
+		if change.Cell == first && change.Value == 9 {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatal("hidden single for candidate value 9 was not found")
+	}
+}
+
+func TestIntersection3IsSorted(t *testing.T) {
+	got := intersection3([]int{9, 1, 5}, []int{5, 9, 1}, []int{9, 5, 1})
+	want := []int{1, 5, 9}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("intersection3() = %v, want %v", got, want)
+		}
+	}
+}

logic/strategies/strategy.go

@@ -12,8 +12,8 @@ import (
 
 type Strategy interface {
 	Init(f *field.Field)
-	ApplyAll() []field.ExternalChange
+	ApplyAll() ([]field.ExternalChange, error)
-	ApplyNext() field.ExternalChange
+	ApplyNext() (field.ExternalChange, error)
 	Name() string
 	SearchProgressableCells() int
 }
@@ -40,35 +40,40 @@ func (b *Base) Init(f *field.Field) {
 	b.field = f
 }
 
-func (b *Base) ApplyAll() []field.ExternalChange {
+func (b *Base) ApplyAll() ([]field.ExternalChange, error) {
 	changesCopy := make([]field.ExternalChange, len(b.changes))
 	copy(changesCopy, b.changes)
-
-	for _, change := range b.changes {
-		b.field.AddChange(&change)
-	}
-
 	b.changes = b.changes[:0]
-
+	if err := b.field.AddChanges(changesCopy); err != nil {
-	return changesCopy
+		return nil, err
+	}
+	return changesCopy, nil
 }
 
-func (b *Base) ApplyNext() field.ExternalChange {
+func (b *Base) ApplyNext() (field.ExternalChange, error) {
 	if len(b.changes) < 1 {
-		return field.ExternalChange{}
+		return field.ExternalChange{}, nil
 	}
-
 	changeCopy := b.changes[0]
-
-	b.field.AddChange(&b.changes[0])
-
 	b.changes = b.changes[1:]
+	if err := b.field.AddChange(&changeCopy); err != nil {
+		return field.ExternalChange{}, err
+	}
+	return changeCopy, nil
+}
 
-	return changeCopy
+func (b *Base) queue(change field.ExternalChange) {
+	for _, existing := range b.changes {
+		if existing.Cell == change.Cell && existing.Action == change.Action &&
+			existing.Value == change.Value && existing.From == change.From {
+			return
+		}
+	}
+	b.changes = append(b.changes, change)
 }
 
 func (b *Base) getName() string {
-	return "Unkown"
+	return "Unknown"
 }
 
 func (b *Base) Name() string {

main.go

@@ -17,7 +17,7 @@ func main() {
 	solver.Add(&strategies.LastDigit{})
 	solver.Add(&strategies.NakedSingle{})
 	solver.Add(&strategies.HiddenSingle{})
-	game, err := sudoku.New(&parser.PuzzleBank{}, solver, openFile())
+	game, err := sudoku.New(&parser.PuzzleBank{}, solver, openFile(), 2)
 	if err != nil {
 		fmt.Println(err.Error())
 		os.Exit(1)

parser/parser.go

@@ -1,30 +1,106 @@
 package parser
 
-import "git.kleiax.de/homepage/field"
+import (
+	"errors"
+	"fmt"
+	"slices"
+	"strings"
 
-//TODO: const für klassik sudoku board.Props
+	"git.kleiax.de/homepage/field"
-//TODO: funktion für [][]Cell generierung
+)
+
+var (
+	ErrFieldIndex   = errors.New("field index out of range")
+	ErrInvalidInput = errors.New("invalid puzzle input")
+	ErrNoPuzzles    = errors.New("input contains no puzzles")
+)
+
+func ClassicProperties() field.Properties {
+	return field.Properties{
+		Rows:            9,
+		Columns:         9,
+		BlockRows:       3,
+		BlockColumns:    3,
+		BlockSizeRow:    3,
+		BlockSizeColumn: 3,
+	}
+}
 
 type Parser interface {
-	Parse(data []byte) error
+	Parse([]byte) error
-	GetField(i int) *field.Field
+	GetField(int) (*field.Field, error)
-	GetAllFields() []field.Field
+	GetAllFields() []*field.Field
 }
 
 type ParserHelper struct {
-	fields []field.Field
+	fields []*field.Field
 }
 
-func (ph *ParserHelper) GetField(i int) *field.Field {
+func (ph *ParserHelper) GetField(index int) (*field.Field, error) {
-	if i < 0 || i >= len(ph.fields) {
+	if ph == nil || index < 0 || index >= len(ph.fields) {
-		return &field.Field{}
+		return nil, fmt.Errorf("%w: %d", ErrFieldIndex, index)
 	}
-	return &ph.fields[i]
+	return ph.fields[index], nil
 }
 
-func (ph *ParserHelper) GetAllFields() []field.Field {
+func (ph *ParserHelper) GetAllFields() []*field.Field {
-	if len(ph.fields) == 0 {
+	if ph == nil {
 		return nil
 	}
-	return ph.fields
+	return slices.Clone(ph.fields)
+}
+
+func classicField(digits string, rating float64) (*field.Field, error) {
+	props := ClassicProperties()
+	if len(digits) != props.Rows*props.Columns {
+		return nil, fmt.Errorf("%w: puzzle has %d characters, expected 81", ErrInvalidInput, len(digits))
+	}
+
+	props.Rating = rating
+	cells := make([][]field.Cell, props.Rows)
+	for row := 0; row < props.Rows; row++ {
+		cells[row] = make([]field.Cell, props.Columns)
+		for column := 0; column < props.Columns; column++ {
+			char := digits[row*props.Columns+column]
+			if char < '0' || char > '9' {
+				return nil, fmt.Errorf("%w: puzzle contains invalid character %q at position %d", ErrInvalidInput, char, row*props.Columns+column)
+			}
+			position := field.NewPosition(
+				row,
+				column,
+				row/props.BlockSizeRow,
+				column/props.BlockSizeColumn,
+				row%props.BlockSizeRow,
+				column%props.BlockSizeColumn,
+			)
+			cells[row][column] = *field.NewCell(int(char-'0'), position)
+		}
+	}
+	result, err := field.New(props, cells)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidInput, err)
+	}
+	return result, nil
+}
+
+// PuzzleString parses one classic Sudoku represented by exactly 81 digits.
+// Whitespace around the complete string is ignored; 0 denotes an empty cell.
+type PuzzleString struct {
+	ParserHelper
+}
+
+func (p *PuzzleString) Parse(data []byte) error {
+	if p == nil {
+		return errors.New("nil puzzle string parser")
+	}
+	digits := strings.TrimSpace(string(data))
+	if digits == "" {
+		return ErrNoPuzzles
+	}
+	parsed, err := classicField(digits, 0)
+	if err != nil {
+		return fmt.Errorf("parse puzzle string: %w", err)
+	}
+	p.fields = []*field.Field{parsed}
+	return nil
 }

parser/parser_test.go

@@ -0,0 +1,110 @@
+package parser
+
+import (
+	"crypto/sha1"
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+const validSolvedPuzzle = "123456789456789123789123456234567891567891234891234567345678912678912345912345678"
+
+func bankRecord(digits string, rating float64) string {
+	hash := fmt.Sprintf("%x", sha1.Sum([]byte(digits)))[:12]
+	return fmt.Sprintf("%s %s   %.1f", hash, digits, rating)
+}
+
+func TestPuzzleBankParsesLFCRLFAndFinalLine(t *testing.T) {
+	first := "0" + validSolvedPuzzle[1:]
+	data := bankRecord(first, 1.2) + "\r\n\n" + bankRecord(validSolvedPuzzle, 2.5)
+	parser := &PuzzleBank{}
+	if err := parser.Parse([]byte(data)); err != nil {
+		t.Fatalf("Parse() error = %v", err)
+	}
+	if got := len(parser.GetAllFields()); got != 2 {
+		t.Fatalf("field count = %d, want 2", got)
+	}
+	second, err := parser.GetField(1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if second.GetRating() != 2.5 || !second.IsSolved() {
+		t.Fatalf("second field: rating=%v solved=%v", second.GetRating(), second.IsSolved())
+	}
+}
+
+func TestPuzzleBankRejectsMalformedInput(t *testing.T) {
+	valid := bankRecord(validSolvedPuzzle, 1.2)
+	tests := map[string]string{
+		"empty":         "\n\r\n",
+		"columns":       "broken",
+		"hash length":   "abc " + validSolvedPuzzle + " 1.2",
+		"hash mismatch": "ffffffffffff" + valid[12:],
+		"bad puzzle":    bankRecord("x"+validSolvedPuzzle[1:], 1.2),
+		"bad rating":    strings.TrimSuffix(valid, "1.2") + "nope",
+	}
+	for name, input := range tests {
+		t.Run(name, func(t *testing.T) {
+			if err := (&PuzzleBank{}).Parse([]byte(input)); err == nil || (name != "empty" && !errors.Is(err, ErrInvalidInput)) {
+				t.Fatalf("Parse() error = %v", err)
+			}
+		})
+	}
+}
+
+func TestPuzzleBankParseIsAtomicAndDoesNotAppend(t *testing.T) {
+	parser := &PuzzleBank{}
+	valid := bankRecord(validSolvedPuzzle, 1.2)
+	if err := parser.Parse([]byte(valid)); err != nil {
+		t.Fatal(err)
+	}
+	original, _ := parser.GetField(0)
+	if err := parser.Parse([]byte(valid + "\ninvalid")); err == nil {
+		t.Fatal("invalid Parse() error = nil")
+	}
+	afterFailure, _ := parser.GetField(0)
+	if afterFailure != original || len(parser.GetAllFields()) != 1 {
+		t.Fatal("failed parse changed previously parsed fields")
+	}
+	if err := parser.Parse([]byte(valid)); err != nil {
+		t.Fatal(err)
+	}
+	if len(parser.GetAllFields()) != 1 {
+		t.Fatal("repeated Parse() appended fields")
+	}
+}
+
+func TestParserHelperBoundsAndDefensiveSlice(t *testing.T) {
+	parser := &PuzzleString{}
+	if err := parser.Parse([]byte(validSolvedPuzzle)); err != nil {
+		t.Fatal(err)
+	}
+	for _, index := range []int{-1, 1} {
+		if _, err := parser.GetField(index); !errors.Is(err, ErrFieldIndex) {
+			t.Fatalf("GetField(%d) error = %v", index, err)
+		}
+	}
+	fields := parser.GetAllFields()
+	fields[0] = nil
+	if stored, err := parser.GetField(0); err != nil || stored == nil {
+		t.Fatal("GetAllFields exposed its backing slice")
+	}
+}
+
+func TestPuzzleStringValidatesInput(t *testing.T) {
+	parser := &PuzzleString{}
+	if err := parser.Parse([]byte(" \n" + validSolvedPuzzle + "\n")); err != nil {
+		t.Fatalf("Parse() error = %v", err)
+	}
+	if fields := parser.GetAllFields(); len(fields) != 1 || !fields[0].IsSolved() {
+		t.Fatal("valid puzzle string was not parsed")
+	}
+
+	invalid := []string{"", validSolvedPuzzle[:80], "x" + validSolvedPuzzle[1:], "11" + validSolvedPuzzle[2:]}
+	for _, input := range invalid {
+		if err := (&PuzzleString{}).Parse([]byte(input)); err == nil {
+			t.Fatalf("Parse(%q) error = nil", input)
+		}
+	}
+}

parser/puzzleBank.go

@@ -2,73 +2,66 @@ package parser
 
 import (
 	"bytes"
+	"crypto/sha1"
+	"encoding/hex"
 	"errors"
 	"fmt"
+	"math"
 	"strconv"
+	"strings"
 
 	"git.kleiax.de/homepage/field"
 )
 
-// https://github.com/grantm/sudoku-exchange-puzzle-bank/tree/master
 type PuzzleBank struct {
 	ParserHelper
 }
 
 func (pb *PuzzleBank) Parse(data []byte) error {
+	if pb == nil {
+		return errors.New("nil puzzle bank parser")
+	}
 
-	lines := bytes.Split(data, []byte("\n"))
+	var parsed []*field.Field
-	for _, line := range lines {
+	for index, rawLine := range bytes.Split(data, []byte("\n")) {
-		if len(line) == 0 { // Überspringe leere Zeile
+		lineNumber := index + 1
+		line := strings.TrimSpace(string(rawLine))
+		if line == "" {
 			continue
 		}
 
-		if len(line) != 99 {
+		columns := strings.Fields(line)
-			return fmt.Errorf("Zeile hat die falsche länge. soll: 100, ist: %d", len(line))
+		if len(columns) != 3 {
+			return fmt.Errorf("line %d: %w: expected hash, puzzle and rating", lineNumber, ErrInvalidInput)
+		}
+		if len(columns[0]) != 12 {
+			return fmt.Errorf("line %d: %w: hash has %d characters, expected 12", lineNumber, ErrInvalidInput, len(columns[0]))
+		}
+		if _, err := hex.DecodeString(columns[0]); err != nil {
+			return fmt.Errorf("line %d: %w: invalid hash: %v", lineNumber, ErrInvalidInput, err)
+		}
+		digest := sha1.Sum([]byte(columns[1]))
+		if columns[0] != hex.EncodeToString(digest[:])[:12] {
+			return fmt.Errorf("line %d: %w: hash does not match puzzle", lineNumber, ErrInvalidInput)
 		}
 
-		//siehe Readme in github repo
+		rating, err := strconv.ParseFloat(columns[2], 64)
-		//sha1Hash := bytes.TrimSpace(line[0:12]) // Wird aktuell nicht gebraucht, später zum sudoku vergleichen
-		sudokuStr := string(line[13:94])
-		ratingStr := string(line[96:99])
-		//fmt.Println(sudokuStr)
-
-		var rating float64
-		_, err := fmt.Sscanf(ratingStr, "%f", &rating)
 		if err != nil {
-			return errors.New("can nor parse raiting")
+			return fmt.Errorf("line %d: %w: invalid rating %q: %v", lineNumber, ErrInvalidInput, columns[2], err)
 		}
-
+		if rating < 0 || math.IsNaN(rating) || math.IsInf(rating, 0) {
-		props := field.Properties{
+			return fmt.Errorf("line %d: %w: rating must be a finite non-negative number", lineNumber, ErrInvalidInput)
-			Rows:            9,
-			Columns:         9,
-			BlockRows:       3,
-			BlockColumns:    3,
-			BlockSizeRow:    3,
-			BlockSizeColumn: 3,
-			Rating:          rating,
 		}
-
+		parsedField, err := classicField(columns[1], rating)
-		cells := make([][]field.Cell, props.Rows)
-		for i := range cells {
-			cells[i] = make([]field.Cell, props.Columns)
-			for j := range cells[i] {
-				number, err := strconv.Atoi(string(sudokuStr[i*9+j]))
 		if err != nil {
-					return fmt.Errorf("Kann Zeichen nicht in Zahl konvertieren")
+			return fmt.Errorf("line %d: %w", lineNumber, err)
 		}
-				pos := field.NewPosition(
+		parsed = append(parsed, parsedField)
-					i,
-					j,
-					i/props.BlockRows,
-					j/props.BlockColumns,
-					i%props.BlockRows,
-					j%props.BlockColumns)
-
-				cells[i][j] = *field.NewCell(number, pos)
-			}
-		}
-		pb.fields = append(pb.fields, *field.New(props, cells))
 	}
 
+	if len(parsed) == 0 {
+		return ErrNoPuzzles
+	}
+	pb.fields = parsed
 	return nil
 }

sudoku/sudoku.go

@@ -14,36 +14,43 @@ type Game struct {
 	solver logic.Solver
 }
 
-func New(parser parser.Parser, solver logic.Solver, data []byte) (*Game, error) {
+func New(input parser.Parser, solver logic.Solver, data []byte, fieldIndex int) (*Game, error) {
-	err := parser.Parse(data)
+	if input == nil {
-	if err != nil {
+		return nil, errors.New("cannot create game: nil parser")
-		return &Game{}, fmt.Errorf("can not create game: %w", err)
 	}
-	return &Game{solver: solver, field: parser.GetField(2)}, nil
+	if err := input.Parse(data); err != nil {
+		return nil, fmt.Errorf("cannot create game: %w", err)
+	}
+	parsedField, err := input.GetField(fieldIndex)
+	if err != nil {
+		return nil, fmt.Errorf("cannot create game: %w", err)
+	}
+	if parsedField == nil || !parsedField.IsValid() {
+		return nil, fmt.Errorf("cannot create game: %w", field.ErrInvalidField)
+	}
+	return &Game{solver: solver, field: parsedField}, nil
 }
 
 func (g *Game) Solve() error {
-	g.solver.InitStragies(g.field)
+	if g == nil || g.field == nil {
-	for !g.isFinished() {
+		return field.ErrInvalidField
-		if !g.nextSolveStep() {
-			return errors.New("no strategy can solve the puzzle")
 	}
+	if err := g.solver.InitStrategies(g.field); err != nil {
+		return err
+	}
+	solved, err := g.solver.Run(0)
+	if err != nil {
+		return err
+	}
+	if !solved {
+		return logic.ErrNoProgress
 	}
 	return nil
 }
 
-func (g *Game) nextSolveStep() bool {
+func (g *Game) GetField() *field.Field {
-	return g.solver.Run(0)
+	if g == nil {
-}
+		return nil
-
+	}
-func (g *Game) prevSolveStep() {
+	return g.field
-	// far far in the future
-}
-
-func (g *Game) isFinished() bool {
-	return g.field.IsSolved()
-}
-
-func (g *Game) GetField() field.Field {
-	return *g.field
 }

sudoku/sudoku_test.go

@@ -0,0 +1,65 @@
+package sudoku
+
+import (
+	"crypto/sha1"
+	"errors"
+	"fmt"
+	"testing"
+
+	"git.kleiax.de/homepage/logic"
+	"git.kleiax.de/homepage/logic/strategies"
+	"git.kleiax.de/homepage/parser"
+)
+
+const solvedSudoku = "123456789456789123789123456234567891567891234891234567345678912678912345912345678"
+
+func puzzleBankRecord(digits string, rating float64) string {
+	hash := fmt.Sprintf("%x", sha1.Sum([]byte(digits)))[:12]
+	return fmt.Sprintf("%s %s   %.1f", hash, digits, rating)
+}
+
+func TestNewSelectsExplicitPuzzleIndex(t *testing.T) {
+	data := puzzleBankRecord("0"+solvedSudoku[1:], 1.0) + "\n" + puzzleBankRecord(solvedSudoku, 2.0)
+	game, err := New(&parser.PuzzleBank{}, logic.Solver{}, []byte(data), 1)
+	if err != nil {
+		t.Fatalf("New() error = %v", err)
+	}
+	if game.GetField().GetRating() != 2 || !game.GetField().IsSolved() {
+		t.Fatal("New() did not select puzzle index 1")
+	}
+}
+
+func TestNewRejectsInvalidIndexAndInput(t *testing.T) {
+	valid := puzzleBankRecord(solvedSudoku, 1.0)
+	if _, err := New(&parser.PuzzleBank{}, logic.Solver{}, []byte(valid), 1); !errors.Is(err, parser.ErrFieldIndex) {
+		t.Fatalf("index error = %v, want ErrFieldIndex", err)
+	}
+	if _, err := New(&parser.PuzzleString{}, logic.Solver{}, []byte("invalid"), 0); err == nil {
+		t.Fatal("invalid input error = nil")
+	}
+}
+
+func TestSolveCompletesSimplePuzzle(t *testing.T) {
+	solver := logic.Solver{}
+	solver.Add(&strategies.LastDigit{})
+	game, err := New(&parser.PuzzleString{}, solver, []byte("0"+solvedSudoku[1:]), 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := game.Solve(); err != nil {
+		t.Fatalf("Solve() error = %v", err)
+	}
+	if !game.GetField().IsSolved() {
+		t.Fatal("field is not solved")
+	}
+}
+
+func TestSolveReportsNoProgress(t *testing.T) {
+	game, err := New(&parser.PuzzleString{}, logic.Solver{}, []byte("0"+solvedSudoku[1:]), 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := game.Solve(); !errors.Is(err, logic.ErrNoProgress) {
+		t.Fatalf("Solve() error = %v, want ErrNoProgress", err)
+	}
+}

todo.md

@@ -1,11 +1,10 @@
-- Feld immer wieder auf Validität checken (keine doppelten Zahlen in einem Part) damit keine Fehler bei den Strategien auftauchen können
+# Offene Punkte
-    - Validator in Logic oder Field?
+
-    - kommt in field
+Der priorisierte und aktuelle Stand befindet sich in [ROADMAP.md](ROADMAP.md).
-    - field sollte flag haben ob vor einem change geprüft wird, ob das Feld danach noch valid ist
+
-- Für die solver ein Interface implementieren, damit die folgenden Solver benutzt werden können:
+- gemeinsames Interface für menschlichen Solver, Backtracking und DLX
-    - backtracking Solver
+  implementieren
-    - dlx solver
+- strukturiertes, optionales Logging an den Anwendungsgrenzen ergänzen
-    - notes (menschlicher solver)
+- optionalen Diagnosemodus für zusätzliche Invariantenprüfungen entwickeln
-- Was passiert wenn zum Beispiel bei hiddenSingle ein Treffer zwei Changes auslöst? Muss ein Überprüfung her, ob die betreffende Zelle schon in einem vorheringen Change bearbeitet wird? Ich denka ja
+- Lösungsweg, Gruppierung von Änderungen und Undo/Redo implementieren
-- Debugging verbesser, Loglevel etc, überall sinnvolle Logs einbauen
+- weitere menschliche Lösungsstrategien in der Reihenfolge der Roadmap ergänzen
-- Flag oder so für sich selbst überprüfende Sachen, nur im debug modus nach jedem bisschen das Feld auf validität prüfen