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racket/contract: improve fast-append

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test whether the 2nd arg is null once, rather than N times
This commit is contained in:
Ben Greenman 2019-05-01 23:35:27 -04:00
parent 721ef2050f
commit a13070f302
1 changed files with 6 additions and 5 deletions
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11
racket/collects/racket/contract/private/collapsible-common.rkt
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@ -217,11 +217,12 @@
;; A specialized version of append that will immediately return if either ;; A specialized version of append that will immediately return if either
;; argument is empty ;; argument is empty
(define (fast-append l1 l2) (define (fast-append l1 l2)
(cond (if (null? l2)
[(null? l2) l1] l1
[(null? l1) l2] (let fast-append ((l1 l1))
[else (if (null? l1)
(cons (car l1) (fast-append (cdr l1) l2))])) l2
(cons (car l1) (fast-append (cdr l1)))))))
;; Assuming that merging is symmetric, ie old-can-merge? iff new-can-merge? ;; Assuming that merging is symmetric, ie old-can-merge? iff new-can-merge?
;; This is true of the current c-c implementation, but if it ever changes ;; This is true of the current c-c implementation, but if it ever changes