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

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Aug 17, 2021
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Ensure chunked(by:) always compares consecutive elements #162

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22 changes: 14 additions & 8 deletions Sources/Algorithms/Chunked.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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 {
Expand Down
19 changes: 19 additions & 0 deletions Tests/SwiftAlgorithmsTests/ChunkedTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -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 }))
Expand Down