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#lang racket/base
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(require (for-syntax racket/base))
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(require racket/list racket/set)
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(require "define.rkt" "len.rkt" "coerce.rkt")
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(define+provide/contract (trimf xs test-proc)
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(list? procedure? . -> . list?)
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(dropf-right (dropf xs test-proc) test-proc))
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(define+provide/contract (slicef-at xs pred [force? #f])
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((list? procedure?) (boolean?) . ->* . (listof list?))
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(cond
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[(null? xs) null]
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[force? (slicef-at (dropf xs (compose1 not pred)) pred)]
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[else
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(define-values (car-match others) (splitf-at xs pred))
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(define-values (head tail) (splitf-at others (compose1 not pred)))
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(cons (append (or car-match null) head) (slicef-at tail pred force?))]))
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(define+provide/contract (slice-at xs len [force? #f])
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((list? (and/c integer? positive?)) (boolean?) . ->* . (listof list?))
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(cond
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[(equal? xs null) null]
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[(len . > . (length xs)) (if force? null (list xs))]
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[else (cons (take xs len) (slice-at (drop xs len) len force?))]))
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(define+provide/contract (filter-split xs split-test)
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(list? predicate/c . -> . (listof list?))
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(let loop ([xs (trimf xs split-test)] [acc '()])
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(if (empty? xs)
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(reverse acc) ; because accumulation is happening backward
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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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(loop rest (cons item acc))))))
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(define+provide/contract (frequency-hash x)
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(list? . -> . hash?)
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(define counter (make-hash))
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(for ([item (in-list (flatten x))])
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(hash-set! counter item (add1 (hash-ref counter item 0))))
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counter)
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(define+provide/contract (members-unique? x)
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((or/c list? vector? string?) . -> . boolean?)
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(cond
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[(list? x) (= (len (remove-duplicates x)) (len x))]
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[(vector? x) (->list x)]
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[(string? x) (string->list x)]
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[else (error (format "members-unique? cannot be determined for ~a" x))]))
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(define+provide/contract (members-unique?/error x)
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(any/c . -> . boolean?)
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(define result (members-unique? x))
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(if (not result)
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(let* ([duplicate-keys (filter-not empty? (hash-map (frequency-hash x)
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(λ(k v) (if (> v 1) k '()))))])
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(error (string-append "members-unique? failed because " (if (= (len duplicate-keys) 1)
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"item isn’t"
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"items aren’t") " unique:") duplicate-keys))
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result))
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;; for use inside quasiquote
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;; instead of ,(when ...) use ,@(when/splice ...)
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;; to avoid voids
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(provide when/splice)
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(define-syntax (when/splice stx)
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(syntax-case stx ()
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[(_ test body)
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#'(if test (list body) '())]))
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(provide values->list)
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(define-syntax (values->list stx)
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(syntax-case stx ()
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[(_ values-expr) #'(call-with-values (λ () values-expr) list)]))
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(define+provide/contract (sublist xs i j)
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(list? (and/c integer? (not/c negative?)) (and/c integer? (not/c negative?)) . -> . list?)
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(cond
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[(> j (length xs)) (error 'sublist (format "ending index ~a exceeds length of list" j))]
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[(>= j i) (take (drop xs i) (- j i))]
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[else (error 'sublist (format "starting index ~a is larger than ending index ~a" i j))]))
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(define increasing-nonnegative? (λ(xs) (apply < -1 xs)))
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(define increasing-nonnegative-list? (and/c list? increasing-nonnegative?))
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(define+provide/contract (break-at xs bps)
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(list? (and/c coerce/list? (or/c empty? increasing-nonnegative-list?)) . -> . (listof list?))
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(when (ormap (λ(bp) (>= bp (length xs))) bps)
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(error 'break-at (format "breakpoint in ~v is greater than or equal to input list length = ~a" bps (length xs))))
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;; easier to do back to front, because then the list index for each item won't change during the recursion
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;; cons a zero onto bps (which may already start with zero) and then use that as the terminating condition
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;; because breaking at zero means we've reached the start of the list
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(reverse (let loop ([xs xs][bps (reverse (cons 0 bps))])
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(if (= (car bps) 0)
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(cons xs null) ; return whatever's left, because no more splits are possible
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(let-values ([(head tail) (split-at xs (car bps))])
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(cons tail (loop head (cdr bps)))))))) |