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Nov 12, 2020
Merged

Add random-access methods to SlidingWindows #42

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42aaf05
Add random-access methods to SlidingWindows
timvermeulen Oct 28, 2020
0e4e40c
Add error message
timvermeulen Nov 10, 2020
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197 changes: 195 additions & 2 deletions Sources/Algorithms/SlidingWindows.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -84,9 +84,202 @@ extension SlidingWindows: Collection {
)
}

// TODO: Implement distance(from:to:), index(_:offsetBy:) and
// index(_:offsetBy:limitedBy:)
public func index(_ i: Index, offsetBy distance: Int) -> Index {
guard distance != 0 else { return i }

return distance > 0
? offsetForward(i, by: distance)
: offsetBackward(i, by: -distance)
}

public func index(
_ i: Index,
offsetBy distance: Int,
limitedBy limit: Index
) -> Index? {
guard distance != 0 else { return i }
guard limit != i else { return nil }

if distance > 0 {
return limit > i
? offsetForward(i, by: distance, limitedBy: limit)
: offsetForward(i, by: distance)
} else {
return limit < i
? offsetBackward(i, by: -distance, limitedBy: limit)
: offsetBackward(i, by: -distance)
}
}

private func offsetForward(_ i: Index, by distance: Int) -> Index {
guard let index = offsetForward(i, by: distance, limitedBy: endIndex)
else { fatalError("Index is out of bounds") }
return index
}

private func offsetBackward(_ i: Index, by distance: Int) -> Index {
guard let index = offsetBackward(i, by: distance, limitedBy: startIndex)
else { fatalError("Index is out of bounds") }
return index
}

private func offsetForward(
_ i: Index, by distance: Int, limitedBy limit: Index
) -> Index? {
assert(distance > 0)
assert(limit > i)

// `endIndex` and the index before it both have `base.endIndex` as their
// upper bound, so we first advance to the base index _before_ the upper
// bound of the output, in order to avoid advancing past the end of `base`
// when advancing to `endIndex`.
//
// Advancing by 4:
//
// input: [x|x x x x x|x x x x] [x x|x x x x x|x x x]
// |> > >|>| or |> > >|
// output: [x x x x x|x x x x x] [x x x x x x x x x x] (`endIndex`)
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👏 This is excellent and a massive help when reviewing this code


if distance >= size {
// Avoid traversing `self[i.lowerBound..<i.upperBound]` when the lower
// bound of the output is greater than or equal to the upper bound of the
// input.

// input: [x|x x x x|x x x x x x x]
// |> >|> > >|>|
// output: [x x x x x x x|x x x x|x]

guard limit.lowerBound >= i.upperBound,
let lowerBound = base.index(
i.upperBound,
offsetBy: distance - size,
limitedBy: limit.lowerBound),
let indexBeforeUpperBound = base.index(
lowerBound,
offsetBy: size - 1,
limitedBy: limit.upperBound)
else { return nil }

// If `indexBeforeUpperBound` equals `base.endIndex`, we're advancing to
// `endIndex`.
guard indexBeforeUpperBound != base.endIndex else { return endIndex }

return Index(
lowerBound: lowerBound,
upperBound: base.index(after: indexBeforeUpperBound))
} else {
// input: [x|x x x x x x|x x x x x]
// |> > > >| |> > >|>|
// output: [x x x x x|x x x x x x|x]

guard let indexBeforeUpperBound = base.index(
i.upperBound,
offsetBy: distance - 1,
limitedBy: limit.upperBound)
else { return nil }

// If `indexBeforeUpperBound` equals the limit, the upper bound itself
// exceeds it.
guard indexBeforeUpperBound != limit.upperBound || limit == endIndex
else { return nil }

// If `indexBeforeUpperBound` equals `base.endIndex`, we're advancing to
// `endIndex`.
guard indexBeforeUpperBound != base.endIndex else { return endIndex }

return Index(
lowerBound: base.index(i.lowerBound, offsetBy: distance),
upperBound: base.index(after: indexBeforeUpperBound))
}
}

private func offsetBackward(
_ i: Index, by distance: Int, limitedBy limit: Index
) -> Index? {
assert(distance > 0)
assert(limit < i)

if i == endIndex {
// Advance `base.endIndex` by `distance - 1`, because the index before
// `endIndex` also has `base.endIndex` as its upper bound.
//
// Advancing by 4:
//
// input: [x x x x x x x x x x] (`endIndex`)
// |< < < < <|< < <|
// output: [x x|x x x x x|x x x]

guard let upperBound = base.index(
base.endIndex,
offsetBy: -(distance - 1),
limitedBy: limit.upperBound)
else { return nil }

return Index(
lowerBound: base.index(upperBound, offsetBy: -size),
upperBound: upperBound)
} else if distance >= size {
// Avoid traversing `self[i.lowerBound..<i.upperBound]` when the upper
// bound of the output is less than or equal to the lower bound of the
// input.
//
// input: [x x x x x x x|x x x x|x]
// |< < < <|< <|
// output: [x|x x x x|x x x x x x x]

guard limit.upperBound <= i.lowerBound,
let upperBound = base.index(
i.lowerBound,
offsetBy: -(distance - size),
limitedBy: limit.upperBound)
else { return nil }

return Index(
lowerBound: base.index(upperBound, offsetBy: -size),
upperBound: upperBound)
} else {
// input: [x x x x x|x x x x x x|x]
// |< < < <| |< < < <|
// output: [x|x x x x x x|x x x x x]

guard let lowerBound = base.index(
i.lowerBound,
offsetBy: -distance,
limitedBy: limit.lowerBound)
else { return nil }

return Index(
lowerBound: lowerBound,
upperBound: base.index(i.lowerBound, offsetBy: -distance))
}
}

public func distance(from start: Index, to end: Index) -> Int {
guard start <= end else { return -distance(from: end, to: start) }
guard start != end else { return 0 }
guard end < endIndex else {
// We add 1 here because the index before `endIndex` also has
// `base.endIndex` as its upper bound.
return base[start.upperBound...].count + 1
}

if start.upperBound <= end.lowerBound {
// The distance between `start.lowerBound` and `start.upperBound` is
// already known.
//
// start: [x|x x x x|x x x x x x x]
// |- - - -|> >|
// end: [x x x x x x x|x x x x|x]

return size + base[start.upperBound..<end.lowerBound].count
} else {
// start: [x|x x x x x x|x x x x x]
// |> > > >|
// end: [x x x x x|x x x x x x|x]

return base[start.lowerBound..<end.lowerBound].count
}
}
}

extension SlidingWindows: BidirectionalCollection where Base: BidirectionalCollection {
Expand Down
20 changes: 19 additions & 1 deletion Tests/SwiftAlgorithmsTests/SlidingWindowsTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
//===----------------------------------------------------------------------===//

import XCTest
import Algorithms
@testable import Algorithms

final class SlidingWindowsTests: XCTestCase {

Expand Down Expand Up @@ -88,4 +88,22 @@ final class SlidingWindowsTests: XCTestCase {
XCTAssertEqualSequences(a[i], [1, 2])
}

func testWindowsIndexTraversals() {
validateIndexTraversals(
"".slidingWindows(ofCount: 1),
"a".slidingWindows(ofCount: 1),
"ab".slidingWindows(ofCount: 1),
"abc".slidingWindows(ofCount: 1),
"".slidingWindows(ofCount: 3),
"a".slidingWindows(ofCount: 3),
"abc".slidingWindows(ofCount: 3),
"abcdefgh".slidingWindows(ofCount: 3),
indices: { windows in
let endIndex = windows.base.endIndex
let indices = windows.base.indices + [endIndex]
return zip(indices, indices.dropFirst(windows.size))
.map { .init(lowerBound: $0, upperBound: $1) }
+ [.init(lowerBound: endIndex, upperBound: endIndex)]
})
}
}