52 lines
1.2 KiB
Scheme
52 lines
1.2 KiB
Scheme
(library
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(list-utils)
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(export tails
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intersperse
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iterate
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comparing
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list-group-by
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accumulate)
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(import (rnrs)
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(srfi s8 receive))
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(define (tails xs)
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(if (null? xs)
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'()
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(cons xs (tails (cdr xs)))))
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(define (intersperse sep xs)
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(cond
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((null? xs) '())
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((null? (cdr xs)) xs)
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(else (cons (car xs)
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(cons sep
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(intersperse sep (cdr xs)))))))
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(define (iterate fn count)
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(let loop ((count count))
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(if (zero? count)
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'()
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(cons (fn) (loop (- count 1))))))
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(define (comparing cmp key)
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(lambda (x y)
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(cmp (key x) (key y))))
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;; Assumes the list is already sorted
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(define (list-group-by pred xs)
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(if (null? xs)
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'()
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(let ((fst (car xs)))
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(receive (g1 gs) (partition (lambda (x) (pred fst x)) (cdr xs))
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(cons (cons fst g1)
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(list-group-by pred gs))))))
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;; calculates a running total for a list. Returns a list.
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(define (accumulate xs)
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(let loop ((xs xs) (total 0))
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(if (null? xs)
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'()
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(cons total (loop (cdr xs) (+ total (car xs))))))))
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