/// <summary>
/// Applies a folding function to the list of intervals.
/// </summary>
/// <typeparam name="T">The type of value being folded.</typeparam>
/// <param name="xs">The first list of intervals.</param>
/// <param name="ys">The second list of intervals.</param>
/// <param name="initial">The initial value.</param>
/// <param name="fold">The folding function.</param>
/// <param name="final">The final fold function.</param>
/// <returns>The resulting value.</returns>
private static T Fold <T>(IReadOnlyList <int> xs, IReadOnlyList <int> ys, T initial, IntervalFold <T> fold, IntervalFinalFold <T> final)
{
#region Contract
Debug.Assert(xs != null);
Debug.Assert(ys != null);
Debug.Assert(fold != null);
Debug.Assert(final != null);
#endregion
T current = initial;
// Neither lists has any elements, so we'll return.
if (xs.Count == 0 && ys.Count == 0)
{
return(current);
}
// Indices point to the elements in the lists that are greater than or equal to `cursor`.
int xi = 0;
int yi = 0;
// The cursor is the current position.
int comparison = CompareIntervals(xs, ys, xi, yi, out int cursor);
if (comparison <= 0)
{
xi += 1;
}
if (comparison >= 0)
{
yi += 1;
}
while (xi < xs.Count || yi < ys.Count)
{
comparison = CompareIntervals(xs, ys, xi, yi, out int nextCursor);
int start = cursor;
int end = nextCursor;
bool hasX = (xi % 2) != 0;
bool hasY = (yi % 2) != 0;
current = fold(start, end, hasX, hasY, current);
if (comparison <= 0)
{
xi += 1;
}
if (comparison >= 0)
{
yi += 1;
}
cursor = nextCursor;
}
current = final(cursor, current);
return(current);
}
/// <summary> /// Applies a folding function to the list of intervals. /// </summary> /// <typeparam name="T">The type of value being folded.</typeparam> /// <param name="xs">The first list of intervals.</param> /// <param name="ys">The second list of intervals.</param> /// <param name="initial">The initial value.</param> /// <param name="fold">The folding function.</param> /// <returns>The resulting value.</returns> private static T Fold <T>(IReadOnlyList <int> xs, IReadOnlyList <int> ys, T initial, IntervalFold <T> fold) => Fold(xs, ys, initial, fold, (_, v) => v);
