private static (ulong MagicMultiplier, ulong[] MovesMasks) FindMagicMultiplier(Dictionary <ulong, ulong> occupancyToMovesMask, int shift, ulong?knownMagicMultiplier)
{
var indexBits = 64 - shift;
var indexLength = (int)Math.Pow(2d, indexBits);
var movesMasks = new ulong[indexLength];
var occupancies = new List <ulong>(occupancyToMovesMask.Keys);
NextMagicMultiplier:
var magicMultiplier = knownMagicMultiplier ?? SafeRandom.NextULong();
// Clear moves masks.
for (var maskIndex = 0; maskIndex < movesMasks.Length; maskIndex++)
{
movesMasks[maskIndex] = 0;
}
for (var occupancyIndex = 0; occupancyIndex < occupancies.Count; occupancyIndex++)
{
var occupancy = occupancies[occupancyIndex];
var magicIndex = GetMagicIndex(occupancy, magicMultiplier, shift);
var movesMask = movesMasks[magicIndex];
if (movesMask == 0)
{
movesMasks[magicIndex] = occupancyToMovesMask[occupancy]; // Moves mask not yet added to unique moves array.
}
else if (movesMask != occupancyToMovesMask[occupancy])
{
goto NextMagicMultiplier; // Moves mask already added to unique moves array but mask is incorrect.
}
}
// Found magic multiplier that maps to correct moves index for all occupancies.
return(magicMultiplier, movesMasks);
}
