AdventOfCode-2023-functional/day01-easy/main.scm

#!/usr/bin/guile -s
!#

(use-modules (ice-9 rdelim))

(define (read-lines)
  (let loop ((line (read-line)))
    (if
      (eof-object? line)
      '()
      (cons line (loop (read-line))))))

(define (reduce-right reducer initial list)
  (if
    (null? list)
    initial
    (reducer (car list) (reduce-right reducer initial (cdr list)))))

(define (map mapper list)
  (reduce-right
    (lambda (current accumulator) (cons (mapper current) accumulator))
    '()
    list))

(define (combine combiner a b)
  (if (null? a) b (if (null? b) a (combiner a b))))

(define (sum numbers) (reduce-right + 0 numbers))

(define
  (repeat value number)
  (if
    (= number 0)
    '()
    (cons value (repeat value (- number 1)))))

(define (is-numeric-char char) (if (char-numeric? char) #t #f))

(define (first-last predicate list)
  (reduce-right
    (lambda (current accumulator)
      (combine
        (lambda (left right) (cons (car left) (cdr right)))
        current
        accumulator))
    '()
    (map
      (lambda (entry)
        (if (predicate entry) (repeat entry 2) '()))
      list)))

(define (solve-line line)
  ((lambda (first-last-result)
    (string->number (list->string first-last-result)))
    (first-last is-numeric-char (string->list line))))

(define (solve-all lines)
  (sum (map solve-line lines)))

(display (solve-all (read-lines)))