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Fix lengthCompare for indexed types #6930

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Jul 18, 2018
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Fix lengthCompare for indexed types #6930

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Fix lengthCompare for indexed types
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@NthPortal
NthPortal committed Jul 13, 2018
commit 604f8d41c972656529969376a1c35bb9025c2053
2 changes: 1 addition & 1 deletion src/library/scala/collection/ArrayOps.scala
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Expand Up @@ -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.

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.


/** Selects an interval of elements. The returned array is made up
* of all elements `x` which satisfy the invariant:
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2 changes: 1 addition & 1 deletion src/library/scala/collection/IndexedSeq.scala
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Expand Up @@ -52,7 +52,7 @@ trait IndexedSeqOps[+A, +CC[_], +C] extends Any with SeqOps[A, CC, C] { self =>

override def last: A = apply(length - 1)

override def lengthCompare(len: Int): Int = length - len
override def lengthCompare(len: Int): Int = Integer.compare(length, len)

final override def knownSize: Int = length

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2 changes: 2 additions & 0 deletions src/library/scala/collection/StringOps.scala
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Expand Up @@ -155,6 +155,8 @@ final class StringOps(private val s: String) extends AnyVal {
/** Get the char at the specified index. */
@`inline` def apply(i: Int): Char = s.charAt(i)

def lengthCompare(len: Int): Int = Integer.compare(s.length, len)

/** Builds a new collection by applying a function to all chars of this String.
*
* @param f the function to apply to each char.
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2 changes: 0 additions & 2 deletions src/library/scala/collection/immutable/Vector.scala
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Expand Up @@ -80,8 +80,6 @@ final class Vector[+A] private[immutable] (private[collection] val startIndex: I

def length: Int = endIndex - startIndex

override def lengthCompare(len: Int): Int = length - len

private[collection] def initIterator[B >: A](s: VectorIterator[B]): Unit = {
s.initFrom(this)
if (dirty) s.stabilize(focus)
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18 changes: 18 additions & 0 deletions test/junit/scala/collection/IndexedSeqTest.scala
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Expand Up @@ -39,6 +39,12 @@ abstract class IndexedTest[T, E] {
}
}

/** check that lengthCompare compares values correctly */
@Test def checkLengthCompare(): Unit = {
val test = underTest(size)
assert(lengthCompare(test, Int.MinValue) > 0)
}

/**
* check simple equallity of the initial data.
* More a test of the infra that we use in this est than a full test of equallity
Expand Down Expand Up @@ -231,6 +237,8 @@ abstract class IndexedTest[T, E] {
//the length of underTest
def length(underTest: T): Int

def lengthCompare(underTest: T, len: Int): Int

//the value at index i of underTest
def get(underTest: T, i: Int): E

Expand Down Expand Up @@ -316,6 +324,8 @@ package IndexedTestImpl {
val TYPE: Class[_]) extends IndexedTest[Array[E], E]{
override final def length(underTest: Array[E]) = underTest.length

override final def lengthCompare(underTest: Array[E], len: Int): Int = underTest.lengthCompare(len)

override def get(underTest: Array[E], i: Int) = underTest(i)

override def slice(underTest: Array[E], from: Int, to: Int) = underTest.slice(from, to)
Expand Down Expand Up @@ -347,6 +357,8 @@ package IndexedTestImpl {
import mutable.ArraySeq
override final def length(underTest: ArraySeq[E]) = underTest.length

override final def lengthCompare(underTest: ArraySeq[E], len: Int): Int = underTest.lengthCompare(len)

override def get(underTest: ArraySeq[E], i: Int) = underTest(i)

override def slice(underTest: ArraySeq[E], from: Int, to: Int) = underTest.slice(from, to)
Expand Down Expand Up @@ -377,6 +389,8 @@ package IndexedTestImpl {
abstract class MutableIndexedSeqTest[T <: mutable.Seq[E], E] extends IndexedTest[T, E] with DataProvider[E]{
override final def length(underTest: T) = underTest.length

override final def lengthCompare(underTest: T, len: Int): Int = underTest.lengthCompare(len)

override def get(underTest: T, i: Int) = underTest(i)

override def slice(underTest: T, from: Int, to: Int) = underTest.slice(from, to).asInstanceOf[T]
Expand Down Expand Up @@ -407,6 +421,8 @@ package IndexedTestImpl {
abstract class ImmutableIndexedSeqTest[T <: SeqOps[E, Seq, T], E] extends IndexedTest[T, E] with DataProvider[E] {
override final def length(underTest: T) = underTest.length

override final def lengthCompare(underTest: T, len: Int): Int = underTest.lengthCompare(len)

override def get(underTest: T, i: Int) = underTest(i)

override def slice(underTest: T, from: Int, to: Int) = underTest.slice(from, to)
Expand All @@ -428,6 +444,8 @@ package IndexedTestImpl {
abstract class StringOpsBaseTest extends IndexedTest[StringOps, Char] with DataProvider[Char] {
override final def length(underTest: StringOps) = underTest.size

override final def lengthCompare(underTest: StringOps, len: Int): Int = underTest.lengthCompare(len)

override def get(underTest: StringOps, i: Int) = underTest(i)

override def slice(underTest: StringOps, from: Int, to: Int) = underTest.slice(from, to)
Expand Down