;;; solver.el --- Day 12 -*- lexical-binding: t; -*-
;;
;; Copyright (C) 2022 Óscar Nájera
;;
;; Author: Óscar Nájera <hi@oscarnajera.com>
;; Maintainer: Óscar Nájera <hi@oscarnajera.com>
;; Created: December 12, 2022
;; Modified: December 12, 2022
;;
;; This file is not part of GNU Emacs.
;;
;;; Commentary:
;;
;;  Day 12
;;
;;; Code:

(require 'ert)

(defun solver-directions (pos width height)
  ;; point on grid is p=x+y*width
  (let ((x (mod pos width))
        (y (/ pos width)))
    (delq nil
          (list
           (when (< -1 x  (1- width)) (1+ pos))      ;; right move
           (when (<  0 x width) (1- pos))            ;; left move
           (when (< -1 y (1- height)) (+ pos width)) ;; down move
           (when (<  0 y height) (- pos width)))))) ;; up move

(should (equal (solver-directions 6 5 5) '(7 5 11 1)))
(should (equal (solver-directions 0 5 5) '(1 5)))

(defun solver-land (data)
  (vconcat (mapconcat (lambda (row)
                        (cl-map 'string (lambda (chr)
                                          (cl-case chr
                                            (?S 0)
                                            (?E 27)
                                            (t (- chr 96))))
                                row))
                      data "")))

(defun solver-search (path land)
  (let* ((pos (car path))
         (elevation (aref land pos))
         (next-cells (solver-directions pos width height)))
    (if (= 27 elevation) ;; reached destination
        (list (reverse path))
      (mapcan (lambda (option)
                (when (and (not (memq option path))
                           (>= (1+ elevation) (aref land option))) ;; allowed move
                  (solver-search (cons option path) land)))
              next-cells))))

(with-temp-buffer
  (insert "Sabqponm
abcryxxl
accszExk
acctuvwj
abdefghi")
  (let* ((data (split-string (buffer-string) "\n"))
         (height (length data))
         (width (length (car data)))
         (land (solver-land data))
         (start (seq-position land 0 #'eq)))
    (thread-first
      (cl-sort
       (solver-search (list start) land)
       #'<
       :key #'length)
      (car)
      (length)
      (1-))
    ))