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(library (lib)
(export solve-part1 solve-part2)
(import (chezscheme))
; Split list at first delimiter into `prefix' and `suffix'
; Return value is a pair like `((p r e f i x) s u f f i x)'
(define (list-split-left delimiter? xs)
(let loop ((prefix '())
(suffix xs))
(if (null? suffix)
(cons prefix suffix)
(let ((x (car suffix)))
(if (delimiter? x)
; Found first delimiter, so return immediately
(cons prefix (cdr suffix))
; `x' isn't a delimiter, so append it to `prefix'
(loop (append prefix (list x)) (cdr suffix)))))))
; Split list at given delimiter into list of sublists
(define (list-split delimiter? xs)
(let loop ((pieces '())
(rest xs))
(if (null? rest)
; Fix order and remove all empty sublists from output
; (which are caused by consecutive delimiters in `xs')
(reverse (remp null? pieces))
; Extract next piece from `rest' and prepend it to `pieces'
(let ((next (list-split-left delimiter? rest)))
(loop (cons (car next) pieces) (cdr next))))))
; Split string at whitespace into list of words
(define (string-split str)
(map list->string (list-split char-whitespace? (string->list str))))
; Sort chars in string according to `proc'
(define (string-sort proc str)
(list->string (list-sort proc (string->list str))))
; Partially applied equality check
(define (check-identical? word1)
(lambda (word2) (equal? word1 word2)))
; Partially applied anagram check
(define (check-anagram? word1)
(lambda (word2)
; Sort chars by Unicode value, then check string equality
(string=? (string-sort char<? word1)
(string-sort char<? word2))))
; Count for how many elements of `xs' `proc' returns true
(define (count proc xs)
(length (filter proc xs)))
; Count how many passphrases in `lines' are reported valid by `proc'
(define (solve-puzzle proc lines)
(count
(lambda (phrase)
; For each word in `phrase', does it have any copies/anagrams?
(let loop ((words phrase))
(if (null? words)
#t
; Count copies/anagrams, including `(car words)' itself
(if (> (count (proc (car words)) phrase) 1)
#f
(loop (cdr words))))))
(map string-split lines)))
(define (solve-part1 lines)
(solve-puzzle check-identical? lines))
(define (solve-part2 lines)
(solve-puzzle check-anagram? lines))
)
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