/// <summary>Performs hidden subset elimination on one dimension of possible numbers.</summary>
/// <param name="possibleNumbers">The row/column/box to analyze.</param>
/// <returns>The number of changes that were made to the possible numbers.</returns>
private int EliminateHiddenSubsets(FastBitArray [] possibleNumbers,
bool exitEarlyWhenSoleFound, out bool exitedEarly)
{
int changesMade = 0;
exitedEarly = false;
int numLocations;
int [] foundLocations = _foundLocations; // optimization, rather than allocating on each call
// We'll look starting with each cell in the row/column/box
for (int i = 0; i < possibleNumbers.Length; i++)
{
// Only look at the cell if it has at least subsetSize values set,
// otherwise it can't be part of a hidden subset
int numPossible = possibleNumbers[i].CountSet;
if (numPossible >= _subsetSize)
{
// For each combination
foreach (int [] combination in CreateCombinations(_subsetSize, possibleNumbers[i].GetSetBits()))
{
// Find other cells that contain that same combination,
// but only up to the subset size
numLocations = 0;
foundLocations[numLocations++] = i;
for (int j = i + 1; j < possibleNumbers.Length && numLocations < foundLocations.Length; j++)
{
if (AllAreSet(combination, possibleNumbers[j]))
{
foundLocations[numLocations++] = j;
}
}
if (numLocations == foundLocations.Length)
{
bool isValidHidden = true;
// Make sure that none of the numbers appear in any other cell
for (int j = 0; j < possibleNumbers.Length && isValidHidden; j++)
{
bool isFoundLocation = Array.BinarySearch(foundLocations, j) >= 0;
if (!isFoundLocation && AnyAreSet(combination, possibleNumbers[j]))
{
isValidHidden = false;
break;
}
}
// If this is a valid hidden subset, eliminate all other numbers
// from each cell in the subset
if (isValidHidden)
{
foreach (int foundLoc in foundLocations)
{
FastBitArray possibleNumbersForLoc = possibleNumbers[foundLoc];
foreach (int n in possibleNumbersForLoc.GetSetBits())
{
if (Array.BinarySearch(combination, n) < 0)
{
possibleNumbersForLoc[n] = false;
changesMade++;
}
}
if (exitEarlyWhenSoleFound &&
possibleNumbersForLoc.CountSet == 1)
{
exitedEarly = true;
return(changesMade);
}
}
break;
}
}
}
}
}
return(changesMade);
}