private static IList <int> CompleteConsecutiveness(
Board board,
IEnumerable <Consecutiveness> incompleteMoves)
{
var cells = board.Cells;
var boardDimensions = board.Dimensions;
var moves = new List <int>();
void collect(CellCoordinates coordinates)
{
if (CellCoordinates.IsInside(coordinates, boardDimensions))
{
var cellAt = CellCoordinates.ToCellAt(coordinates, boardDimensions);
if (cells[cellAt] == CellOwner.None)
{
moves.Add(cellAt);
}
}
}
var prioritized = incompleteMoves
.OrderByDescending(consecutiveness => consecutiveness.CellsAt.Count);
foreach (var consecutiveness in prioritized)
{
// single moves cannot be completed in one turn
if (consecutiveness.CellsAt.Count < 2)
{
continue;
}
var first = CellCoordinates.FromCellAt(consecutiveness.CellsAt.First(), boardDimensions);
var second = CellCoordinates.FromCellAt(consecutiveness.CellsAt.Skip(1).First(), boardDimensions);
var last = CellCoordinates.FromCellAt(consecutiveness.CellsAt.Last(), boardDimensions);
var stepSpan = second - first;
collect(first - stepSpan);
collect(last + stepSpan);
}
return(moves);
}
private static IList <int> FindWinningMoves(
Board board,
IEnumerable <Consecutiveness> incompleteMoves,
Predicate <CellOwner> cellOwnerPredicate)
{
var dimensions = board.Dimensions;
var cells = board.Cells;
var filteredIncompleteMoves = incompleteMoves.Where(
(consecutiveness) => cellOwnerPredicate(cells[consecutiveness.CellsAt.First()]));
// When there are moves, which could complete a winning consecutiveness on the board,
// we should preferably take it.
var completingMoves = CompleteConsecutiveness(board, filteredIncompleteMoves);
if (completingMoves.Count > 0)
{
return(Cells.TakeAnyCell(completingMoves));
}
// Gaps between taken cells could also complete a winning consecutiveness
var gaps = new List <Gap>();
var smallestSpread = int.MaxValue;
for (int i = 0; i < cells.Count; i++)
{
// Skip cells which are not set (we find the gaps otherwise)
// and skip cells which are not of interest for now
var leftCellOwner = cells[i];
if (leftCellOwner == CellOwner.None || !cellOwnerPredicate(leftCellOwner))
{
continue;
}
// Bubble upwards and find a complementing cell of the same cellOwner
var leftCoordinates = CellCoordinates.FromCellAt(i, dimensions);
for (int j = i + 1; j < cells.Count; j++)
{
var rightCellOwner = cells[j];
if (rightCellOwner != leftCellOwner)
{
continue;
}
// When the half distance between them is an integer (or distance from the middle to both
// sides is equal), we have found a potential gap.
var distance = j - i;
var halfDistance = distance / 2;
if (halfDistance + halfDistance != distance)
{
continue;
}
// The middle cell as gap must be claimable (only concerns boards bigger than 3x3)
var middleCellAt = i + halfDistance;
var middleCellOwner = cells[middleCellAt];
if (middleCellOwner != CellOwner.None)
{
continue;
}
// Now we need to determine the spread between left cell and middle cell in terms of
// board coordinates. This is actually only a priorization help for boards bigger than 3x3.
// On 3x3 we will always have a spread of 1.
var middleCoordinates = CellCoordinates.FromCellAt(middleCellAt, dimensions);
var spread = (middleCoordinates - leftCoordinates).Spread();
smallestSpread = Math.Min(smallestSpread, spread);
gaps.Add(new Gap(middleCellAt, spread));
}
}
// Gaps with smaller spread will be filled first
var gapsWithSmallestSpread = gaps.Where((gap) => gap.Spread == smallestSpread)
.Select((gap) => gap.CellAt);
return(Cells.TakeAnyCell(gapsWithSmallestSpread.ToList()));
}