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[stdlib][NFC] Make partition(by:)’s implementation easier to follow

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Also, fix a case in the description of the loop invariant
This commit is contained in:
Karoy Lorentey
2020-11-18 14:37:39 -08:00
parent 244442939d
commit cb8ceed2af
1 changed files with 17 additions and 22 deletions
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39
stdlib/public/core/CollectionAlgorithms.swift
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@@ -401,35 +401,30 @@ extension MutableCollection where Self: BidirectionalCollection {
) rethrows -> Index {
var lo = startIndex
var hi = endIndex
while true {
// Invariants at this point:
//
// * `lo <= hi`
// * all elements in `startIndex ..< lo` belong in the first partition
// * all elements in `hi ..< endIndex` belong in the second partition
// 'Loop' invariants (at start of Loop, all are true):
// * lo < hi
// * predicate(self[i]) == false, for i in startIndex ..< lo
// * predicate(self[i]) == true, for i in hi ..< endIndex
// Find next element from `lo` that may not be in the right place.
while true {
guard lo < hi else { return lo }
if try belongsInSecondPartition(self[lo]) { break }
formIndex(after: &lo)
}
Loop: while true {
FindLo: repeat {
while lo < hi {
if try belongsInSecondPartition(self[lo]) { break FindLo }
formIndex(after: &lo)
}
break Loop
} while false
FindHi: repeat {
// Find next element down from `hi` that we can swap `lo` with.
while true {
formIndex(before: &hi)
while lo < hi {
if try !belongsInSecondPartition(self[hi]) { break FindHi }
formIndex(before: &hi)
}
break Loop
} while false
guard lo < hi else { return lo }
if try !belongsInSecondPartition(self[hi]) { break }
}
swapAt(lo, hi)
formIndex(after: &lo)
}
return lo
}
}