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  1. ; string-replace
  2. ; replace all occurrences of pat in str with rep
  3.  
  4. (define (string-find pat str . s)
  5.   (let* ((plen (string-length pat))
  6.          (slen (string-length str))
  7.          (skip (make-vector plen 0)))
  8.     (let loop ((i 1) (j 0))
  9.       (cond ((= i plen))
  10.             ((char=? (string-ref pat i) (string-ref pat j))
  11.               (vector-set! skip i (+ j 1))
  12.               (loop (+ i 1) (+ j 1)))
  13.             ((< 0 j) (loop i (vector-ref skip (- j 1))))
  14.             (else (vector-set! skip i 0)
  15.                   (loop (+ i 1) j))))
  16.     (let loop ((p 0) (s (if (null? s) 0 (car s))))
  17.       (cond ((= s slen) #f)
  18.             ((char=? (string-ref pat p) (string-ref str s))
  19.               (if (= p (- plen 1))
  20.                   (- s plen -1)
  21.                   (loop (+ p 1) (+ s 1))))
  22.             ((< 0 p) (loop (vector-ref skip (- p 1)) s))
  23.             (else (loop p (+ s 1)))))))
  24.  
  25. (define (string-replace str pat rep) ; quadratic
  26.   (let ((x (string-find pat str)))
  27.     (if (not x) str
  28.       (string-append (substring str 0 x) rep
  29.         (string-replace (substring str (+ x (string-length pat))
  30.           (string-length str)) pat rep)))))
  31.  
  32. (time (begin (string-replace (make-string 1000 #\a) "a" "b") 'done))
  33.  
  34. (define (string-replace str pat rep) ; linear
  35.   (let ((str-len (string-length str))
  36.         (pat-len (string-length pat)))
  37.     (let loop ((pos 0) (xs (list)))
  38.       (if (<= str-len pos)
  39.           (apply string-append (reverse xs))
  40.           (let ((x (string-find pat str pos)))
  41.             (if x
  42.                 (loop (+ pos pat-len)
  43.                       (cons rep (cons (substring str pos x) xs)))
  44.                 (loop str-len (cons (substring str pos str-len) xs))))))))
  45.  
  46. (time (begin (string-replace (make-string 1000 #\a) "a" "b") 'done))
Success #stdin #stdout #stderr 0.06s 7364KB
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0.03s CPU time, 0.009s GC time (major), 13022 mutations, 6/141 GCs (major/minor)
0.023s CPU time, 0.005s GC time (major), 12017 mutations, 3/133 GCs (major/minor)


    

	
			

					
			
			
			
			
			
			
			
			

			
			
			
		

	
		
		

			

			                        







			
			

				

	
					
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