(import (chicken string)
        (chicken format))
(define input "
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  ")

(define-record part-number number line start-col end-col)
(define-record part-symbol sym line start-col end-col)
(define-record buffer-char char line col)

(define (index-input input #!optional (indexed-input '()) (line-index 0) (col-index 0))
  (if (= 0 (length input))
      (reverse indexed-input)
      (let ((next-char (car input))
            (new-rest-input (cdr input)))
        (if (eqv? #\newline next-char)
            (index-input new-rest-input
                         indexed-input
                         (+ line-index 1)
                         0)
            (index-input new-rest-input
                         (cons (make-buffer-char next-char line-index col-index)
                               indexed-input)
                         line-index
                         (+ col-index 1))))))

(define (number-char? c)
  (case (buffer-char-char c)
    ((#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9) #t)
    (else #f)))

(define (finalize-token buffer)
  (let* ((rev-buffer (reverse buffer))
         (line (buffer-char-line (car buffer)))
         (start-col (buffer-char-col (car buffer)))
         (end-col (buffer-char-col (car rev-buffer))))
    (if (number-char? (car buffer))
        (make-part-number (string->number
                           (apply string (map buffer-char-char buffer)))
                          line start-col end-col)
        (make-part-symbol (string->symbol
                           (apply string (map buffer-char-char buffer)))
                          line start-col end-col))))

(define (compatible-with-buffer? buffer char)
  (and
   (not (eqv? #\. (buffer-char-char char)))
   (or (= 0 (length buffer))
       (and (number-char? (car buffer))
            (number-char? char))
       (and (not (number-char? (car buffer)))
            (not (number-char? char))))))

(define (tokenize-indexed-input indexed-input #!optional (token-buffer '()) (part-nums '()) (part-syms '()))
  (if (= 0 (length indexed-input))
      (values (reverse part-nums)
              (reverse part-syms))
      (let ((next-char (car indexed-input)))
        (cond
           ((compatible-with-buffer? token-buffer (car indexed-input))
            (tokenize-indexed-input (cdr indexed-input)
                                    (cons (car indexed-input) token-buffer)
                                    part-nums part-syms))
           ((= 0 (length token-buffer))
            (tokenize-indexed-input
              (cdr indexed-input)
              (if (eqv? #\. (buffer-char-char (car indexed-input)))
                token-buffer (list (car indexed-input)))
              part-nums part-syms))
           (else
            (let ((token (finalize-token (reverse token-buffer))))
              (tokenize-indexed-input
               (cdr indexed-input)
               (if (eqv? #\. (buffer-char-char next-char)) '() (list (car indexed-input)))
               (if (part-number? token) (cons token part-nums) part-nums)
               (if (part-symbol? token) (cons token part-syms) part-syms))))))))

(define (neighbours? part-num part-sym)
  (let ((part-num-line (part-number-line part-num))
        (col-min (- (part-number-start-col part-num) 1))
        (col-max (+ (part-number-end-col part-num) 1)))
    (and (>= (part-symbol-line part-sym) (- part-num-line 1))
         (<= (part-symbol-line part-sym) (+ part-num-line 1))
         (>= (part-symbol-start-col part-sym) col-min)
         (<= (part-symbol-start-col part-sym) col-max)
         (>= (part-symbol-end-col part-sym) col-min)
         (<= (part-symbol-end-col part-sym) col-max))))

(define (is-real-part-number? part-num part-syms)
  (and (< 0 (length part-syms))
       (or (neighbours? part-num (car part-syms))
           (is-real-part-number? part-num (cdr part-syms)))))

(define (fold-part-numbers part-nums part-syms)
  (foldl (lambda (output input)
           (if (is-real-part-number? input part-syms)
               (+ output (part-number-number input))
               output))
         0 part-nums))

(define (calc-part-1)
  (let-values (((part-nums part-syms) (tokenize-indexed-input (index-input (string->list input)))))
    (fold-part-numbers part-nums part-syms)))

(define (gather-neighbours part-sym part-nums)
  (foldl (lambda (output input)
           (if (neighbours? input part-sym)
               (cons input output)
               output))
         (list) part-nums))

(define (filter-gear-candidates part-syms)
  (foldl (lambda (output input)
           (if (eqv? '* (part-symbol-sym input))
               (cons input output) output))
         (list) part-syms))

(define (calc-gear-ratio gears)
  (foldl (lambda (output input)
           (if (= 2 (length input))
               (+ output (apply * (map part-number-number input)))
               output))
         0 gears))

(define (calc-part-2)
  (let-values (((part-nums part-syms) (tokenize-indexed-input (index-input (string->list input)))))
    (let ((gear-candidates (map (lambda (x) (gather-neighbours x part-nums)) (filter-gear-candidates part-syms))))
      (calc-gear-ratio gears))))