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[DEAD] Simplify lag/lead implementation #6

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[DEAD] Simplify lag/lead implementation #6

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76b08af
Simplify lag implementation
Orace Oct 31, 2019
0d8de14
Simplifi lead implementation
Orace Oct 31, 2019
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27 changes: 4 additions & 23 deletions MoreLinq/Lag.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,31 +66,12 @@ public static IEnumerable<TResult> Lag<TSource, TResult>(this IEnumerable<TSourc

return _(); IEnumerable<TResult> _()
{
using var iter = source.GetEnumerator();

var i = offset;
var lagQueue = new Queue<TSource>(offset);
// until we progress far enough, the lagged value is defaultLagValue
var hasMore = true;
// NOTE: The if statement below takes advantage of short-circuit evaluation
// to ensure we don't advance the iterator when we reach the lag offset.
// Do not reorder the terms in the condition!
while (i-- > 0 && (hasMore = iter.MoveNext()))
{
lagQueue.Enqueue(iter.Current);
// until we reach the lag offset, the lagged value is the defaultLagValue
yield return resultSelector(iter.Current, defaultLagValue);
}

if (hasMore) // check that we didn't consume the sequence yet
foreach (var value in source)
{
// now the lagged value is derived from the sequence
while (iter.MoveNext())
{
var lagValue = lagQueue.Dequeue();
yield return resultSelector(iter.Current, lagValue);
lagQueue.Enqueue(iter.Current);
}
var lagValue = lagQueue.Count == offset ? lagQueue.Dequeue() : defaultLagValue;
yield return resultSelector(value, lagValue);
lagQueue.Enqueue(value);
}
}
}
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19 changes: 5 additions & 14 deletions MoreLinq/Lead.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,22 +65,13 @@ public static IEnumerable<TResult> Lead<TSource, TResult>(this IEnumerable<TSour
return _(); IEnumerable<TResult> _()
{
var leadQueue = new Queue<TSource>(offset);
var iter = source.GetEnumerator();

bool hasMore;
// first, prefetch and populate the lead queue with the next step of
// items to be streamed out to the consumer of the sequence
while ((hasMore = iter.MoveNext()) && leadQueue.Count < offset)
leadQueue.Enqueue(iter.Current);
// next, while the source sequence has items, yield the result of
// the projection function applied to the top of queue and current item
while (hasMore)
foreach (var value in source)
{
yield return resultSelector(leadQueue.Dequeue(), iter.Current);
leadQueue.Enqueue(iter.Current);
hasMore = iter.MoveNext();
if (leadQueue.Count == offset)
yield return resultSelector(leadQueue.Dequeue(), value);
leadQueue.Enqueue(value);
}
// yield the remaining values in the lead queue with the default lead value

while (leadQueue.Count > 0)
yield return resultSelector(leadQueue.Dequeue(), defaultLeadValue);
}
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