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perf: optimize scanl O(n²) --> O(n) #37

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perf: optimize scanl O(n²) --> O(n) #37

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e931fca
perf: optimize scanl
walking-octopus Apr 17, 2025
eafd15b
feat: optional seed for scanl
walking-octopus Apr 17, 2025
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22 changes: 15 additions & 7 deletions main.rkt
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Original file line number Diff line number Diff line change
Expand Up @@ -114,13 +114,21 @@
[else (tail-call lst)]))


(define (scanl proc lst)
(foldl
(λ (val acc)
(append acc (list (proc val (last acc)))))
(list (first lst))
(rest lst)))

(define scanl
(case-lambda
[(proc lst)
(scanl proc (car lst) (cdr lst))]
[(proc seed lst)
(let loop ([acc (list seed)]
[val seed]
[rest lst])
(if (empty? rest)
(reverse acc)
(let ([next (proc val (car rest))])
(loop (cons next acc)
next
(cdr rest)))))]))


(define (scanr proc lst)
(foldl
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