Fix lengthCompare for indexed types #6930
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| @@ -221,7 +221,7 @@ final class ArrayOps[A](val xs: Array[A]) extends AnyVal { | |||
| * x > 0 if this.length > len | |||
| * }}} | |||
| */ | |||
| def lengthCompare(len: Int): Int = xs.length - len | |||
| def lengthCompare(len: Int): Int = Integer.compare(xs.length, len) | |||
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I would prefer if this reverted to the original behavior (i.e. it returns the difference in lengths) in case anyone is depending on it.
Since we can count on length being non-negative, it could be
val ans = xs.length - len
if (len < 0 && ans < 0) Int.MaxValue else ans
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the default implementation never returned the difference in lengths, though. and if they happen to know it's an IndexedSeq (which is pretty much the only way to even maybe rely on that behaviour), they can just call .length
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I think I'm with @NthPortal on this, there is no reason or need to stick to a contract that we never documented. what we documented was to only rely on the sign
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It's fair enough to say that we didn't agree to obey this contract so we're abandoning it, but I don't think it's hard to preserve it for IndexedSeq, where it worked pretty reliably (Array, ArrayBuffer, String, Vector, etc.). I guess I don't see the downside of supporting it.
| /** check that lengthCompare compares values correctly */ | ||
| @Test def checkLengthCompare(): Unit = { | ||
| val test = underTest(size) | ||
| assert(lengthCompare(test, Integer.MIN_VALUE) > 0) |
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Was this test previously failing?
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Yes.
size - Int.MinValue (also, I accidentally used Java's, oops; I'll fix that) is the same as size + Int.MaxValue + 1, which overflows to size + Int.MinValue or Int.MinValue + size. Since size <= Int.MaxValue, Int.MinValue + size <= Int.MinValue + Int.MaxValue == -1. However, size > Int.MinValue, so you ought to get a positive value.
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(I actually wrote and ran the test before fixing ArrayOps, just to be sure)
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| @@ -221,7 +221,7 @@ final class ArrayOps[A](val xs: Array[A]) extends AnyVal { | |||
| * x > 0 if this.length > len | |||
| * }}} | |||
| */ | |||
| def lengthCompare(len: Int): Int = xs.length - len | |||
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what's the scenario where this fails? The test below has Int.MinValue, which seems impossible for a length...
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it passes in Int.MinValue as the value of len, which is perfectly possible. See this comment.
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Well OK, but why would length be negative?
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because someone passed a negative number in ¯\_(ツ)_/¯
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@isaacl - As a concrete example, suppose you have a collection xs that you will use to pair with any overflow of ys which is normally going to be paired with zs. You might want to know if xs is big enough. So you ask xs.lengthCompare(ys.length - zs.length).
Beyond that, even if we can't think of a sensible use case for a negative value being passed in, it still should return the logically correct result, because we can't always anticipate what someone might try to use it for.
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OK. Well, this function doesn't feel super useful to me, but I guess if there's no preconditions it should return correctly.
Requested change is not essential for correctness nor is it universally desired.
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It looks like this is ready for merge |
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thanks Nth! |
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no problem! |
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