/// <summary>
/// Given a partially solved grid, try each remaining possibility in turn and backtrack if necessary
/// </summary>
/// <param name="grid"></param>
public void Solve(Grid grid)
{
var workingGrid = grid.Clone();
var unsolvedSquares = GetUnsolvedSquarePossibilities(grid);
for (var remainingSquareIdx = 0; remainingSquareIdx < unsolvedSquares.Count; ++remainingSquareIdx)
{
var foundSquareSolution = false;
var currentUnsolvedSquare = unsolvedSquares[remainingSquareIdx];
foreach (var possibleDigit in currentUnsolvedSquare.PossibleDigits)
{
if (possibleDigit >
workingGrid.Squares[currentUnsolvedSquare.RowIdx, currentUnsolvedSquare.ColIdx].Digit)
{
var isValidToSet = workingGrid.EnsureValidToSetTheDigit(currentUnsolvedSquare.RowIdx,
currentUnsolvedSquare.ColIdx, possibleDigit, false);
if (isValidToSet)
{
workingGrid.Squares[currentUnsolvedSquare.RowIdx, currentUnsolvedSquare.ColIdx].SetDigit(possibleDigit);
foundSquareSolution = true;
}
}
if (foundSquareSolution) break;
}
// If have a working solution for the current unsolved square, move to the next
if (foundSquareSolution) continue;
// Otherwise roll back
workingGrid.Squares[currentUnsolvedSquare.RowIdx, currentUnsolvedSquare.ColIdx].ClearDigit();
remainingSquareIdx = remainingSquareIdx - 2; // Move to the next possibility for the previous unsolved square (for loop moves forward one again)
if (remainingSquareIdx < -1) break;
}
if (!workingGrid.IsSolved) return;
// Copy the newly solved squares over
for (var rowIdx = 0; rowIdx < 9; ++rowIdx)
{
for (var colIdx = 0; colIdx < 9; ++colIdx)
{
if (!grid.Squares[rowIdx, colIdx].IsSolved)
{
grid.SetDigit(rowIdx, colIdx, workingGrid.Squares[rowIdx, colIdx].Digit);
}
}
}
}
