path: root/day11.lisp

(defun read-lines (filespec)
  (with-open-file (stream filespec)
    (loop for line = (read-line stream nil)
          while line
          collect line)))

(defun empty-rows (lines)
  (loop for line in lines
        collect (every (lambda (c) (equal c #\.)) line)))

(defun empty-cols (lines)
  (loop for i below (length (first lines))
        collect (every (lambda (line) (equal (aref line i) #\.)) lines)))

(defun galaxies-coords (lines erows ecols eincrement)
  (loop for i = 0 then (+ i (if erow eincrement 1))
        for erow in erows
        for line in lines
        append (loop for j = 0 then (+ j (if ecol eincrement 1))
                     for ecol in ecols
                     for c across line
                     if (equal c #\#)
                       collect (cons i j))))

(defun solve (filespec eincrement)
  (let* ((lines (read-lines filespec))
         (erows (empty-rows lines))
         (ecols (empty-cols lines))
         (coords (galaxies-coords lines erows ecols eincrement)))
    (loop for coord in coords
          for rem on (cdr coords)
          while rem
          sum (loop for other in rem
                    sum (+ (abs (- (car coord) (car other)))
                           (abs (- (cdr coord) (cdr other))))))))

(defun solve1 (filespec)
  (solve filespec 2))

(defun solve2 (filespec)
  (solve filespec 1000000))

(print (solve1 "data/11/example.txt"))  ; 374
(print (solve1 "data/11/input.txt"))    ; 9563821
(print (solve2 "data/11/example.txt"))  ; 82000210
(print (solve2 "data/11/input.txt"))    ; 827009909817