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Fix LazyList.range [ci: last-only] #10767

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Fix LazyList.range [ci: last-only] #10767

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6de2bf8
Override LazyList.range
BalmungSan Apr 30, 2024
91088b5
Test LazyList.range
BalmungSan Apr 30, 2024
bd3db0d
Better LazyList.range impl
BalmungSan Apr 30, 2024
f7745a4
Move LazyList.range implementation closer to other overrides
BalmungSan Apr 30, 2024
2028dae
Ensure lazyness of `Integral[A].one` for overrided `range` implementa…
BalmungSan Apr 30, 2024
fbc2623
Ensure step can't be zero
BalmungSan Apr 30, 2024
1ac4f64
Allow reverse LazyList.range
BalmungSan Apr 30, 2024
b4eb90a
Make the lazyRangeAllowsMoreThanIntMaxValue test run fast
BalmungSan Apr 30, 2024
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Move LazyList.range implementation closer to other overrides
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@BalmungSan
BalmungSan committed Apr 30, 2024
commit f7745a48f0f71c71ed9558e9ed32164aec30fc53
21 changes: 10 additions & 11 deletions src/library/scala/collection/immutable/LazyList.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -968,17 +968,6 @@ final class LazyList[+A] private(private[this] var lazyState: () => LazyList.Sta
*/
@SerialVersionUID(3L)
object LazyList extends SeqFactory[LazyList] {
// Override SeqFactory methods.
override def range[A: Integral](start: A, end: A): LazyList[A] =
range(start, end, step = Integral[A].one)

override def range[A](start: A, end: A, step: A)(implicit ev: Integral[A]): LazyList[A] =
newLL(rangeImpl(start, end, step))

private[this] def rangeImpl[A](start: A, end: A, step: A)(implicit ev: Integral[A]): State[A] =
if (ev.lt(start, end)) sCons(start, newLL(rangeImpl(ev.plus(start, step), end, step)))
else State.Empty

// Eagerly evaluate cached empty instance
private[this] val _empty = newLL(State.Empty).force

Expand Down Expand Up @@ -1241,6 +1230,16 @@ object LazyList extends SeqFactory[LazyList] {
at(0)
}

override def range[A: Integral](start: A, end: A): LazyList[A] =
range(start, end, step = Integral[A].one)
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override def range[A](start: A, end: A, step: A)(implicit ev: Integral[A]): LazyList[A] =
newLL(rangeImpl(start, end, step))
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private[this] def rangeImpl[A](start: A, end: A, step: A)(implicit ev: Integral[A]): State[A] =
if (ev.lt(start, end)) sCons(start, newLL(rangeImpl(ev.plus(start, step), end, step)))
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@BalmungSan BalmungSan Apr 30, 2024

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Uhm, wait, this does not allow reversed ranges, does it? Like LazyList.range(start = 100, end = 0, step = -2)
I guess rather than check for lt we have to check for !ev.equiv(start, end)?

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@BalmungSan BalmungSan Apr 30, 2024

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Also, this made me think. This could be a good way to test this without a CPU trap. We just request the reversed range and then we can indeed ask for the head and check it is bigger than Int.MaxValue

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@NthPortal NthPortal Apr 30, 2024

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good point, the check is incorrect, hmm. probably need to check the sign or something as well. I'll have another look shortly

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@NthPortal NthPortal Apr 30, 2024

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my thought is to check the sign of step in the 3 parameter overload of range, and then pass it as a Boolean parameter (probably named positiveStep) to rangeImpl, thus checking the condition only once rather than for each element

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@BalmungSan BalmungSan Apr 30, 2024 •
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I just pushed an implementation that allows for reverse ranges.
Let me know what you think about it.

else State.Empty

// significantly simpler than the iterator returned by Iterator.unfold
override def unfold[A, S](init: S)(f: S => Option[(A, S)]): LazyList[A] =
newLL {
Expand Down