#lang racket

; You try to ask why they can't just use a weather machine ("not powerful enough")
; and where they're even sending you ("the sky") and why your map looks mostly
; blank ("you sure ask a lot of questions") and hang on did you just say the sky
; ("of course, where do you think snow comes from") when you realize that the
; Elves are already loading you into a trebuchet ("please hold still, we need
; to strap you in").

; As they're making the final adjustments, they discover that their calibration
; document (your puzzle input) has been amended by a very young Elf who was
; apparently just excited to show off her art skills. Consequently, the Elves
; are having trouble reading the values on the document.

; The newly-improved calibration document consists of lines of text; each line
; originally contained a specific calibration value that the Elves now need to
; recover. On each line, the calibration value can be found by combining the
; first digit and the last digit (in that order) to form a single two-digit
; number.

; For example:

; 1abc2
; pqr3stu8vwx
; a1b2c3d4e5f
; treb7uchet

; In this example, the calibration values of these four lines are 12, 38, 15,
; and 77. Adding these together produces 142.

; Consider your entire calibration document. What is the sum of all of the
; calibration values?

(define input '("1abc2"
                "pqr3stu8vwx"
                "a1b2c3d4e5f"
                "treb7uchet"))

; From a char list returns the first digit it finds as a char, or nil
(define (get-first-digit list)
  (if (= (length list) 0)
      null
      (let ([current-char (first list)])
        (if (char-numeric? current-char)
            current-char
            (get-first-digit (rest list))))))

(define (get-last-digit list carry)
  (if (= (length list) 0)
      carry
      (let* ([current-char (first list)]
             [next-char
              (if (char-numeric? current-char)
                  current-char
                  carry)])
        (get-last-digit (rest list) next-char))))

; Creates a new number from the first & last number found inside a string
(define (list->digits list)
    (let ([first-char (get-first-digit list)]
          [last-char (get-last-digit list -1)])
      (string->number (string first-char last-char))))


(define char-list (map string->list input))
(define result (foldl + 0 (map list->digits char-list)))

(print result)