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remove local length? utility

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svn: r9420
This commit is contained in:
Eli Barzilay 2008-04-23 11:48:06 +00:00
parent e2c4500e41
commit 5d4256921c
1 changed files with 31 additions and 40 deletions
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71
collects/scheme/list.ss
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@ -109,44 +109,35 @@
;; ls
;; (append l (car ls) (loop (cdr ls))))))]))
;; utility: returns the length for a proper list, #f otherwise; does not handle
;; circular lists
(define (length? x)
(let loop ([x x] [n 0])
(if (pair? x)
(loop (cdr x) (add1 n))
(and (null? x) n))))
(define (remove-duplicates l [=? equal?])
(let ([len (length? l)])
(unless len (raise-type-error 'remove-duplicates "list" l))
(let ([h (cond [(< len 40) #f]
[(eq? =? eq?) (make-hasheq)]
[(eq? =? equal?) (make-hash)]
[else #f])])
(if h
;; Using a hash table when the list is long enough and a using `equal?'
;; or `eq?'. The length threshold (40) was determined by trying it out
;; with lists of length n holding (random n) numbers.
(let loop ([l l])
(if (null? l)
l
(let ([x (car l)] [l (cdr l)])
(if (hash-ref h x #f)
(loop l)
(begin (hash-set! h x #t) (cons x (loop l)))))))
;; plain n^2 list traversal (optimized for common cases)
(let-syntax ([loop (syntax-rules ()
[(_ search)
(let loop ([l l] [seen null])
(if (null? l)
l
(let ([x (car l)] [l (cdr l)])
(if (search x seen)
(loop l seen)
(cons x (loop l (cons x seen)))))))])])
(cond [(eq? =? equal?) (loop member)]
[(eq? =? eq?) (loop memq)]
[(eq? =? eqv?) (loop memv)]
[else (loop (lambda (x seen)
(ormap (lambda (y) (=? x y)) seen)))]))))))
(unless (list? l) (raise-type-error 'remove-duplicates "list" l))
(let ([h (cond [(< (length l) 40) #f]
[(eq? =? eq?) (make-hasheq)]
[(eq? =? equal?) (make-hash)]
[else #f])])
(if h
;; Using a hash table when the list is long enough and a using `equal?'
;; or `eq?'. The length threshold (40) was determined by trying it out
;; with lists of length n holding (random n) numbers.
(let loop ([l l])
(if (null? l)
l
(let ([x (car l)] [l (cdr l)])
(if (hash-ref h x #f)
(loop l)
(begin (hash-set! h x #t) (cons x (loop l)))))))
;; plain n^2 list traversal (optimized for common cases)
(let-syntax ([loop (syntax-rules ()
[(_ search)
(let loop ([l l] [seen null])
(if (null? l)
l
(let ([x (car l)] [l (cdr l)])
(if (search x seen)
(loop l seen)
(cons x (loop l (cons x seen)))))))])])
(cond [(eq? =? equal?) (loop member)]
[(eq? =? eq?) (loop memq)]
[(eq? =? eqv?) (loop memv)]
[else (loop (lambda (x seen)
(ormap (lambda (y) (=? x y)) seen)))])))))