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31 lines
1.2 KiB
Racket
31 lines
1.2 KiB
Racket
11 years ago
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#lang racket/base
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(require racket/contract racket/list)
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(provide trim splitf-at*)
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;; trim from beginning & end of list
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(define (trim items test-proc)
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(list? procedure? . -> . list?)
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(dropf-right (dropf items test-proc) test-proc))
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;; split list into list of sublists using test-proc
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(define/contract (splitf-at* xs split-test)
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;; todo: better error message when split-test is not a predicate
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(list? predicate/c . -> . (listof list?))
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(define (&splitf-at* xs [acc '()]) ; use acc for tail recursion
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(if (empty? xs)
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;; reverse because accumulation is happening backward
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;; (because I'm using cons to push latest match onto front of list)
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(reverse acc)
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(let-values ([(item rest)
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;; drop matching elements from front
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;; then split on nonmatching
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;; = nonmatching item + other elements (which will start with matching)
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(splitf-at (dropf xs split-test) (compose1 not split-test))])
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;; recurse, and store new item in accumulator
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(&splitf-at* rest (cons item acc)))))
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;; trim off elements matching split-test
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(&splitf-at* (trim xs split-test)))
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