;;; 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)

(cl-defstruct (land (:constructor land--create)
                      (:copier nil))
  "Contaner for the land layout"
  grid (width nil :read-only t) (height nil :read-only t))

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

(should (equal (solver-directions 6 (land--create :width 5 :height 5)) '(7 11 5 1)))
(should (equal (solver-directions 0 (land--create :width 5 :height 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-next-steps (pos land paths)
  (let ((elevation (aref (land-grid land) pos)))
    (unless (= 27 elevation) ;; reached destination
      (mapcan (lambda (option)
                (when (and (not (gethash option paths))
                           (>= (1+ elevation) (aref (land-grid land) option))) ;; allowed move
                  (puthash option (cons option (gethash pos paths)) paths)
                  (list option)))
              (solver-directions pos land)))))


(defun solver-search (queue land paths)
  (when-let ((next (mapcan (lambda (place) (solver-next-steps place land paths)) queue)))
      (solver-search next land paths)))

(with-temp-buffer
;;   (insert "Sabqponm
;; abcryxxl
;; accszExk
;; acctuvwj
;; abdefghi")
  (insert-file-contents "input")
  (let* ((data (split-string (buffer-string) "\n" t))
         (height (length data))
         (width (length (car data)))
         (land (land--create :grid (solver-land data) :width width :height height))
         (start (seq-position (land-grid land) 0 #'eq))
         (finish (seq-position (land-grid land) 27 #'eq))
         (paths (make-hash-table :test #'eq)))
    (puthash start (list start) paths)
    (solver-search (list start) land paths)
    (1- (length (gethash finish paths)))
    ))