[Repo Assist] [Dart] Fix Array.compareWith comparing lengths before elements

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Dart] Fix `Array.compareWith` comparing lengths before elements, producing wrong results for arrays with common prefixes (fixes #2961)
 * [Dart] Enable explicit variable typing for more data types (by @ncave)
 * [Dart] Fix tests for `round`, `sign`, `truncate`, `log`, `log2`, `log10`, `pow`, `DivRem`, `Min`, `Max`, `Clamp`, `MinMagnitude`, `MaxMagnitude`, `cosh`, `sinh`, `tanh`, and float `Parse` (by @ncave)
 * [Python] Add `Math.DivRem` support for int, int64, and bigint (by @dbrattli)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Dart] Fix `Array.compareWith` comparing lengths before elements, producing wrong results for arrays with common prefixes (fixes #2961)
 * [Dart] Enable explicit variable typing for more data types (by @ncave)
 * [Dart] Fix tests for `round`, `sign`, `truncate`, `log`, `log2`, `log10`, `pow`, `DivRem`, `Min`, `Max`, `Clamp`, `MinMagnitude`, `MaxMagnitude`, `cosh`, `sinh`, `tanh`, and float `Parse` (by @ncave)
 * [Python] Add `Math.DivRem` support for int, int64, and bigint (by @dbrattli)

src/fable-library-dart/Array.fs

@@ -713,28 +713,33 @@ let splitAt (index: int) (array: 'T[]) : 'T[] * 'T[] =
 
     Native.sublist (array, 0, index), Native.sublist (array, index)
 
+// Note that, unlike the `compare` operator, Array.compareWith compares elements first,
+// then falls back to length comparison. See #2961.
 let compareWith (comparer: 'T -> 'T -> int) (array1: 'T[]) (array2: 'T[]) : int =
     // Null checks not necessary because Dart provides null safety
-    //    if isNull array1 then
-    //        if isNull array2 then 0 else -1
-    //    elif isNull array2 then
-    //        1
-    //    else
     let mutable i = 0
     let mutable result = 0
     let length1 = array1.Length
     let length2 = array2.Length
 
-    if length1 > length2 then
+    let minLength =
+        if length1 < length2 then
+            length1
+        else
+            length2
+
+    while i < minLength && result = 0 do
+        result <- comparer array1[i] array2[i]
+        i <- i + 1
+
+    if result <> 0 then
+        result
+    elif length1 > length2 then
         1
     elif length1 < length2 then
         -1
     else
-        while i < length1 && result = 0 do
-            result <- comparer array1[i] array2[i]
-            i <- i + 1
-
-        result
+        0
 
 let equalsWith (equals: 'T -> 'T -> bool) (array1: 'T[]) (array2: 'T[]) : bool =
     // Null checks not necessary because Dart provides null safety

tests/Dart/src/ArrayTests.fs

@@ -1102,17 +1102,17 @@ let tests () =
         throwsAnyError (fun () -> Array.removeManyAt -1 2 [|1|] |> ignore)
         throwsAnyError (fun () -> Array.removeManyAt 2 2 [|1|] |> ignore)
 
-    // testCase "Array.compareWith works" <| fun () -> // See #2961
-    //     let a = [|1;3|]
-    //     let b = [|1;2;3|]
-    //     // compares lengths first, then elements
-    //     let c1 = a < b
-    //     let c2 = compare a b
-    //     // should compare elements first, then lengths
-    //     let c3 = Array.compareWith compare a b
-    //     equal c1 true
-    //     equal c2 -1
-    //     equal c3 1
+    testCase "Array.compareWith works" <| fun () -> // See #2961
+        let a = [|1;3|]
+        let b = [|1;2;3|]
+        // compares lengths first, then elements
+        let c1 = a < b
+        let c2 = compare a b
+        // should compare elements first, then lengths
+        let c3 = Array.compareWith compare a b
+        equal c1 true
+        equal c2 -1
+        equal c3 1
 
     // testCase "System.Array.Resize works" <| fun () ->
     //     let mutable xs = [|1; 2; 3; 4; 5|]