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Merge pull request #540 from felixr/better-quicksort

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Improve quicksort to avoid worst case performance on sorted input
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Calvin Rose 2020-12-26 15:23:01 -06:00 committed by GitHub
commit 7242ee0186
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1 changed files with 25 additions and 22 deletions
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45
src/boot/boot.janet
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@ -790,27 +790,30 @@
### ###
### ###
(defn- sort-part (defn- median-of-three [a b c]
[a lo hi by] (if (not= (> a b) (> a c))
(def pivot (in a hi)) a
(var i lo) (if (not= (> b a) (> b c)) b c)))
(forv j lo hi
(def aj (in a j))
(when (by aj pivot)
(def ai (in a i))
(set (a i) aj)
(set (a j) ai)
(++ i)))
(set (a hi) (in a i))
(set (a i) pivot)
i)
(defn- sort-help (defn sort-help [a lo hi by]
[a lo hi by] (when (< lo hi)
(when (> hi lo) (def pivot
(def piv (sort-part a lo hi by)) (median-of-three (in a hi) (in a lo)
(sort-help a lo (- piv 1) by) (in a (math/floor (/ (+ lo hi) 2)))))
(sort-help a (+ piv 1) hi by)) (var left lo)
(var right hi)
(while true
(while (by (in a left) pivot) (++ left))
(while (by pivot (in a right)) (-- right))
(when (<= left right)
(def tmp (in a left))
(set (a left) (in a right))
(set (a right) tmp)
(++ left)
(-- right))
(if (>= left right) (break)))
(sort-help a lo right by)
(sort-help a left hi by))
a) a)
(defn sort (defn sort
@ -818,7 +821,7 @@
[a &opt by] [a &opt by]
(sort-help a 0 (- (length a) 1) (or by <))) (sort-help a 0 (- (length a) 1) (or by <)))
(undef sort-part) (undef median-of-three)
(undef sort-help) (undef sort-help)
(defn sort-by (defn sort-by