Refactor Sudoku field and solver implementation

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
 }