Array.adjacentPairs() is incorrect

[domkm]

Swift Algorithms version: 1.2.0 Swift version: swift-driver version: 1.87.1 Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1) Target: arm64-apple-macosx14.0

Checklist

• [x] If possible, I've reproduced the issue using the main branch of this package
• [x] I've searched for existing GitHub issues

Steps to Reproduce

let pairs = [0, 1, 2].adjacentPairs().map{ $0 }
XCTAssertEqualSequences(pairs, [(0, 1), (1, 2)], by: ==) //failed - element (0, 2) on first sequence does not match element (1, 2) on second sequence at position 1

Expected behavior

[0, 1, 2].adjacentPairs() should return [(0, 1), (1,2)]

Actual behavior

[0, 1, 2].adjacentPairs() actually returns [(0, 1), (0, 2), (1, 2)]

[LucianoPAlmeida]

Couldn't repro this, did this have another detail for the repro?

[domkm]

Odd. It's still happening for me. I copied AdjacentPairsTests.swift and TestUtilities.swift into my project and updated the testThreeElementSequence() test.

  func testThreeElementSequence() {
    let pairs = (0...).prefix(3).adjacentPairs()
    XCTAssertEqualSequences(pairs, [(0, 1), (1, 2)], by: ==)
+   let pairs2 = [0, 1, 2].adjacentPairs().map {x in x}
+   XCTAssertEqualSequences(pairs2, [(0, 1), (1, 2)], by: ==)
  }

The second assertion fails:

Perhaps related, [0, 1, 2].adjacentPairs() without the .map {x in x} fails to type check with Ambiguous use of 'adjacentPairs()'.

Any suggestions?

[LucianoPAlmeida]

Wait, do the "ambiguos" diagnostic notes point to the candidates it found, it looks like maybe you would have to versions of "adjacentPairs" and the one being pick for array is incorrect. You can also click in Xcode and see where it takes you.

[domkm]

You're right! I'm new to Swift but should have checked this first anyway, sorry. It looks like SwifterSwift defines a different version of adjacentPairs. I haven't yet narrowed down how the two implementations are causing incorrect results though.

[LucianoPAlmeida]

Coincidently I happen to be a maintainer of SwifterSwift so I can take a look on the implementation there and see if this is indeed a bug there. Thank you for the report :)

[jshier]

Seems to be the intended behavior:

/// SwifterSwift: Unique pair of elements in a collection.
///
///        let array = [1, 2, 3]
///        for (first, second) in array.adjacentPairs() {
///            print(first, second) // print: (1, 2) (1, 3) (2, 3)
///        }
///
///

Seems to be what swift-algorithms calls permutations.

[LucianoPAlmeida]

Seems to be what swift-algorithms calls permutations.

Yeah, this is actually something I've missed on review