/// <summary>
/// Removes the current board hash from history.
/// </summary>
private void RemoveHistoryHash()
{
int hash = currentBoard.GetHashCode();
int freq = historyHashes[hash];
// if the frequency is more than 1 decrement it, otherwise remove the hash
if (freq > 1)
{
historyHashes[hash] = freq - 1;
}
else
{
historyHashes.Remove(hash);
}
}