public bool Solve(Crossword crossword)
{
var allLetters = GetAllLetters().ToArray();
var unresolvedCells = crossword.GetCells().ToDictionary(cell => cell, cell => allLetters.ToList());
while (true)
{
foreach (var unresolvedCell in unresolvedCells)
{
var cell = unresolvedCell.Key;
var questions = crossword.GetQuestionsForCell(cell).ToArray();
var availableCharacters = unresolvedCell.Value;
foreach (var character in availableCharacters.ToArray())
{
crossword.ApplyCharacter(cell, character);
if (!CheckQuestions(crossword, questions))
{
availableCharacters.Remove(character);
if (availableCharacters.Count == 0)
{
throw new ArgumentException("Invalid crossword data", "crossword");
}
}
}
crossword.ApplyCharacter(cell, '\0');
}
var resolvedCellInfos = unresolvedCells.Where(info => info.Value.Count == 1).ToArray();
if (resolvedCellInfos.Length == 0)
{
break;
}
foreach (var cellInfo in resolvedCellInfos)
{
crossword.ApplyCharacter(cellInfo.Key, cellInfo.Value[0]);
unresolvedCells.Remove(cellInfo.Key);
}
}
if (unresolvedCells.Count == 0)
{
return true;
}
while (unresolvedCells.Count > 0)
{
var unrosolvedCell = unresolvedCells.First();
if (!Attempt(crossword, unrosolvedCell.Key, unresolvedCells))
return false;
unresolvedCells.Remove(unrosolvedCell.Key);
}
return true;
}
private bool Attempt(Crossword crossword, CrosswordCell cell, IDictionary<CrosswordCell, List<char>> unresolvedCells)
{
var characters = unresolvedCells[cell];
var questions = crossword.GetQuestionsForCell(cell).ToArray();
foreach (var character in characters)
{
crossword.ApplyCharacter(cell, character);
if (!CheckQuestions(crossword, questions))
{
continue;
}
if (crossword.GetCells().All(c => crossword.GetCellCharacter(c) != 0))
{
return true;
}
foreach (var affectedCell in crossword.GetQuestionsForCell(cell).SelectMany(crossword.GetCellsForQuestion).Where(c => crossword.GetCellCharacter(c) == 0))
{
if (Attempt(crossword, affectedCell, unresolvedCells))
{
return true;
}
}
}
crossword.ApplyCharacter(cell, '\0');
return false;
}