path: root/day04.lisp

(defun input-lines (f)
  (with-open-file (s f)
    (loop for line = (read-line s nil 'eof)
          until (eq line 'eof)
          collect line)))

(defun split-by-char (char string)
  (loop for i = 0 then (1+ j)
        as j = (position char string :start i)
        collect (subseq string i j)
        while j))

(defun parse-integers (string)
  (loop for i = 0 then (cadr o)
        as o = (multiple-value-list (parse-integer string :start i :junk-allowed t))
        as n = (car o)
        while n
        collect n))

(defun card-winning (winning numbers)
  (loop for number in numbers
        count (member number winning)))

(defun line-winning (line)
  (let ((card-info (split-by-char #\| (cadr (split-by-char #\: line)))))
    (let ((winning (parse-integers (car card-info)))
          (numbers (parse-integers (cadr card-info))))
      (card-winning winning numbers))))

(defun line-points (line)
  (ash 1 (1- (line-winning line))))

(defun solve1 (f)
  (let ((lines (input-lines f)))
    (reduce #'+ (mapcar 'line-points lines))))

(defun solve2 (f)
  (let ((lines (input-lines f)))
    (let ((arr (make-array (length lines) :initial-element 1)))
      (loop for i = 0 then (1+ i)
            for line in lines
            as count = (line-winning line)
            as v = (aref arr i)
            do (loop for j from 1 to count
                     as e = (aref arr (+ i j))
                     do (setf (aref arr (+ i j)) (+ v e)))
               sum v))))

(print (solve1 "data/04/example.txt")) ;; 13
(print (solve1 "data/04/input.txt"))   ;; 24160
(print (solve2 "data/04/example.txt")) ;; 30
(print (solve2 "data/04/input.txt"))   ;; 5659035