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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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13 changes: 13 additions & 0 deletions src/library/scala/collection/immutable/LazyList.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -1230,6 +1230,19 @@ object LazyList extends SeqFactory[LazyList] {
at(0)
}

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

override def range[A](start: A, end: A, step: A)(implicit ev: Integral[A]): LazyList[A] =
newLL {
if (ev.equiv(step, ev.zero)) throw new IllegalArgumentException("step cannot be 0.")
else 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)))
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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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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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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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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 {
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14 changes: 13 additions & 1 deletion test/junit/scala/collection/immutable/LazyListTest.scala
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Expand Up @@ -444,8 +444,20 @@ class LazyListTest {
LazyList(1).lazyAppendedAll({ count += 1; Seq(2)}).toList
assertEquals(1, count)
}

@Test
def lazyRangeAllowsMoreThanIntMaxValue(): Unit = {
val totalElements: Long = Int.MaxValue.toLong + 2L
val count: Long =
LazyList
.range(start = 0L, end = totalElements)
.foldLeft(0L) { case (acc, _) =>
acc + 1L
}
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even with inlining turned on, it's a lot to ask of a unit test. Maybe it's sufficient to test construction and head.

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Yeah I was wondering the same.
However, not sure if just asking for the head ensures there are more than Int.MaxValue elements.

So not sure if there is a smart way to test this without it being a CPU trap.

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Using a reverse range we don't need to compute a lot of elements.
Let me know what you think of the new test.
C.C. @NthPortal

assertEquals(totalElements, count)
}
}

object LazyListTest {
var serializationForceCount = 0
}
}