Ensure chunked(by:) always compares consecutive elements (apple#162)

Author: Tim Vermeulen

Sources/Algorithms/Chunked.swift

@@ -77,9 +77,13 @@ extension ChunkedBy: LazyCollectionProtocol {
   /// at the given index.
   @inlinable
   internal func endOfChunk(startingAt start: Base.Index) -> Base.Index {
-    let subject = projection(base[start])
-    return base[base.index(after: start)...]
-      .endOfPrefix(while: { belongInSameGroup(subject, projection($0)) })
+    var subject = projection(base[start])
+    
+    return base[base.index(after: start)...].endOfPrefix(while: { element in
+      let nextSubject = projection(element)
+      defer { subject = nextSubject }
+      return belongInSameGroup(subject, nextSubject)
+    })
   }
 
   @inlinable
@@ -118,11 +122,13 @@ extension ChunkedBy: BidirectionalCollection
   @inlinable
   internal func startOfChunk(endingAt end: Base.Index) -> Base.Index {
     let indexBeforeEnd = base.index(before: end)
+    var subject = projection(base[indexBeforeEnd])
 
-    // Get the projected value of the last element in the range ending at `end`.
-    let subject = projection(base[indexBeforeEnd])
-    return base[..<indexBeforeEnd]
-      .startOfSuffix(while: { belongInSameGroup(projection($0), subject) })
+    return base[..<indexBeforeEnd].startOfSuffix(while: { element in
+      let nextSubject = projection(element)
+      defer { subject = nextSubject }
+      return belongInSameGroup(nextSubject, subject)
+    })
   }
 
   @inlinable
@@ -246,8 +252,8 @@ extension Collection {
       if try !belongInSameGroup(current, element) {
         result.append(self[start..<index])
         start = index
-        current = element
       }
+      current = element
     }
 
     if start != endIndex {

Tests/SwiftAlgorithmsTests/ChunkedTests.swift

@@ -78,6 +78,25 @@ final class ChunkedTests: XCTestCase {
     validateIndexTraversals(lazyChunks)
   }
 
+  func testChunkedByComparesConsecutiveElements() {
+    XCTAssertEqualSequences(
+      [1, 2, 3, 4, 6, 7, 8, 9].chunked(by: { $1 - $0 == 1 }),
+      [[1, 2, 3, 4], [6, 7, 8, 9]])
+    
+    XCTAssertEqualSequences(
+      [1, 2, 3, 4, 6, 7, 8, 9].lazy.chunked(by: { $1 - $0 == 1 }),
+      [[1, 2, 3, 4], [6, 7, 8, 9]])
+    
+    print(Array([1, 2, 3].lazy.chunked(by: { $1 - $0 == 1 })))
+    print(Array([1, 2, 3].lazy.chunked(by: { $1 - $0 == 1 }).reversed()))
+    
+    XCTAssertEqualSequences(
+      [1, 2, 3, 4, 6, 7, 8, 9].lazy.chunked(by: { $1 - $0 == 1 }).reversed(),
+      [[6, 7, 8, 9], [1, 2, 3, 4]])
+    
+    validateIndexTraversals([1, 2, 3].lazy.chunked(by: { $1 - $0 == 1 }))
+  }
+  
   func testChunkedLazy() {
     XCTAssertLazySequence(fruits.lazy.chunked(by: { $0.first == $1.first }))
     XCTAssertLazySequence(fruits.lazy.chunked(on: { $0.first }))