internal static IEnumerable <T> PNN <T>(IEnumerable <T> source, int from, int to, int step)
{
to = -to;
step = -step;
int count;
var buffer = new DynamicRotativeBuffer <T>();
using (var enumerator = source.GetEnumerator())
{
var fromCount = from + 1;
var toCount = to - 1;
var minToBuffer = Math.Min(fromCount, toCount);
buffer.BufferUpToCount(enumerator, minToBuffer);
if (buffer.length == 0)
{
yield break;
}
var reachedEnd = buffer.length < minToBuffer;
if (!reachedEnd)
{
if (fromCount > toCount)
{
var toSkip = fromCount - toCount;
reachedEnd = toSkip != buffer.RotateUpToCount(enumerator, toSkip);
}
else if (toCount > fromCount)
{
count = toCount - fromCount;
while (count > 0 && enumerator.MoveNext())
{
--count;
}
reachedEnd = count > 0;
}
}
count = buffer.length;
if (!reachedEnd)
{
count -= enumerator.Skip(count);
}
}
if (count <= 0)
{
yield break;
}
var head = (buffer.head - 1 + buffer.length) % buffer.length;
while (count > 0)
{
count -= step;
yield return(buffer.items[head]);
head = (head - step + buffer.length) % buffer.length;
}
}
internal static IEnumerable <T> NPP <T>(IEnumerable <T> source, int from, int?to, int step)
{
from = -from;
var buffer = new DynamicRotativeBuffer <T>();
int count;
var offset = 0;
if (to.HasValue)
{
var toValue = to.Value;
if (from >= toValue)
{
using (var enumerator = source.GetEnumerator())
{
buffer.BufferUpToCount(enumerator, toValue);
if (buffer.length != toValue)
{
offset = 0;
count = buffer.length;
}
else
{
var toSkip = from - toValue;
if (toSkip != enumerator.Skip(toSkip))
{
offset = 0;
count = buffer.length;
}
else
{
offset = enumerator.Skip(toValue);
count = toValue - offset;
}
}
}
}
else
{
using (var enumerator = source.GetEnumerator())
{
buffer.BufferUpToCount(enumerator, from);
count = buffer.length;
if (count != from)
{
offset = 0;
}
else
{
var toSkip = toValue - from;
if (toSkip != buffer.RotateUpToCount(enumerator, toSkip))
{
offset = 0;
count = buffer.length;
}
else
{
offset = enumerator.Skip(from);
count = from - offset;
}
}
}
}
}
else
{
using (var enumerator = source.GetEnumerator())
{
buffer.BufferUpToCount(enumerator, from);
if (buffer.length == from)
{
buffer.RotateUntilEndWithCount(enumerator);
}
}
count = buffer.length;
}
if (count <= 0)
{
yield break;
}
count = (count + step - 1) / step;
buffer.head += offset;
while (count-- > 0)
{
yield return(buffer.items[buffer.head % buffer.length]);
buffer.head += step;
}
}
