Created
December 27, 2011 01:23
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memoize @ SICP
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| (define (memoize f) | |
| (let ((table (make-table))) | |
| (lambda (x) | |
| (let ((previously-computed-result (lookup x table))) | |
| (or previously-computed-result | |
| (let ((result (f x))) | |
| (insert! x result table) | |
| result)))))) | |
| (define (lookup key table) | |
| (let ((record (assoc key (cdr table)))) | |
| (if record | |
| (cdr record) | |
| #f))) | |
| (define (assoc key records) | |
| (cond ((null? records) #f) | |
| ((equal? key (caar records)) (car records)) | |
| (else (assoc key (cdr records))))) | |
| (define (insert! key value table) | |
| (let ((record (assoc key (cdr table)))) | |
| (if record | |
| (set-cdr! record value) | |
| (set-cdr! table | |
| (cons (cons key value) (cdr table))))) | |
| 'ok) | |
| (define (make-table) | |
| (list '*table*)) | |
| (define (fib n) | |
| (cond ((= n 0) 0) | |
| ((= n 1) 1) | |
| (else (+ (fib (- n 1)) | |
| (fib (- n 2)))))) | |
| (define memo-fib | |
| (memoize (lambda (n) | |
| (cond ((= n 0) 0) | |
| ((= n 1) 1) | |
| (else (+ (memo-fib (- n 1)) | |
| (memo-fib (- n 2)))))))) | |
| (memo-fib 3) ;=> 2 | |
| (memo-fib 4) ;=> 3 | |
| (memo-fib 5) ;=> 5 | |
| (memo-fib 6) ;=> 8 | |
| (memo-fib 7) ;=> 13 |
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