/// <summary>
/// Modifies the given array in-place such that true-valued and false-valued items (determined by <paramref name="predicate" />) are contiguous rather than intermixed
/// </summary>
public static void Partition <TValue>(
this TValue[] values,
Func <TValue, bool> predicate,
out ArrayView <TValue> trueValues,
out ArrayView <TValue> falseValues)
{
Contract.RequiresNotNull(values);
Contract.RequiresNotNull(predicate);
if (values.Length == 0)
{
trueValues = ArrayView <TValue> .Empty;
falseValues = ArrayView <TValue> .Empty;
return;
}
int leftIndex = 0;
int rightIndex = values.Length - 1;
// left and right indices will eventually meet (immediate for a single element array).
while (rightIndex != leftIndex)
{
// Advance the left cursor to the next out-of-place false-value.
// (but don't go past right; consider an all-true array meaning that the right cursor initially also points to a true value).
while (leftIndex < rightIndex && predicate(values[leftIndex]))
{
leftIndex++;
}
// Similarly for the right cursor - find the next out-of-place true-value
while (leftIndex < rightIndex && !predicate(values[rightIndex]))
{
rightIndex--;
}
// Swap the out-of-place pair (each is no longer out of place).
if (leftIndex != rightIndex)
{
TValue temp = values[leftIndex];
values[leftIndex] = values[rightIndex];
values[rightIndex] = temp;
// We've established both loop conditions above:
// leftIndex < rightIndex && predicate(values[leftIndex]) && !predicate(values[rightIndex])
// (due to the swap).
// So, we can advance one or both cursors to avoid re-evaluating their predicates on the next iteration.
leftIndex++;
if (leftIndex < rightIndex)
{
rightIndex--;
}
Contract.Assert(leftIndex <= rightIndex);
}
}
Contract.Assert(
leftIndex == rightIndex,
"Loop terminates when left and right cursors meet; we then have a pivot which could go in either the true or false view.");
int pivotIndex = leftIndex;
// The true values are [0, pivot] or [0, pivot - 1].
// Shift so that the true values are instead [0, pivot - 1]
if (predicate(values[pivotIndex]))
{
pivotIndex++;
}
trueValues = new ArrayView <TValue>(values, 0, pivotIndex);
falseValues = new ArrayView <TValue>(values, pivotIndex, values.Length - pivotIndex);
}