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(define-module (day03)
#:use-module (srfi srfi-41)
#:use-module (ice-9 match)
#:use-module (ice-9 rdelim)
#:use-module (ice-9 regex)
#:export (part1 part2))
(define mul-regexp (make-regexp "mul\\(([0-9]+),([0-9]+)\\)|do\\(\\)|(don't\\(\\))"))
(define (parse-instruction match)
(cond ((match:start match 3) 'dont)
((match:start match 1) (list 'mul
(string->number (match:substring match 1))
(string->number (match:substring match 2))))
(else 'do)))
(define* (input-instructions #:optional (port (current-input-port)))
(stream-let loop ((line (read-line port)) (start 0))
(if (eof-object? line)
stream-null
(let ((mch (regexp-exec mul-regexp line start)))
(if mch
(stream-cons (parse-instruction mch)
(loop line (match:end mch)))
(loop (read-line port) 0))))))
(define (mul-op? x)
(and (pair? x) (eq? 'mul (car x))))
(define* (part1 #:optional (port (current-input-port)))
(stream-fold + 0
(stream-map (lambda (op) (apply * (cdr op)))
(stream-filter mul-op? (input-instructions port)))))
(define* (part2 #:optional (port (current-input-port)))
(let ((sum 0) (on? #t))
(stream-for-each
(match-lambda
('do (set! on? #t))
('dont (set! on? #f))
(('mul x y) (when on? (set! sum (+ sum (* x y))))))
(input-instructions port))
sum))
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