advent-2024/p10/p10.org

Solution to p10

Load the file into a list of lines

(require 'dash)
  (with-temp-buffer
    (insert-file-contents "input")
    (goto-char (point-min))
    (replace-regexp "^" "\"")
       (goto-char (point-min))
       (replace-regexp "$" "\"")
    (goto-char (point-min))
    (insert "(setq data '(")
    (goto-char (point-max))
    (insert "))")
    (eval-buffer))

and split into a list of list of heights, calculating height and width

  (setq data-heights (-map (lambda (str) (--map (- (string-to-char it) ?0) (split-string str "\\|.+" t)))
			 data)
	  height (length data-heights)
	  width (length (car data-heights)))

41

here we define some helper functions

  (defun acceptable-p (p)
    (let ((x (car p))
	  (y (cadr p)))
      (and (>= x 0) (>= y 0) (< x width) (< y height)) ))

  (defun neighbours (p)
    (let ((x (car p))
	  (y (cadr p)))
      (-filter #'acceptable-p
	       (-map (lambda (q) (list (+ x (car q)) (+ y (cadr q)))) '((+1 0) (-1 0) (0 1) (0 -1))))))

  (defun height (p)
    (nth (car p) (nth (cadr p) data-heights)))

height

This yields the coordinates of the trailheads

  (setq trail-heads (-map #'cadr
	  (--filter (eq (car it) 0)
		    (-mapcat #'identity
			     (-map-indexed (lambda (y l) (-map-indexed (lambda (x el) (list el (list x y))) l)) data-heights)))))

2	0
14	0
17	0
22	0
33	0
35	0
12	1
18	1
32	1
1	2
3	2
35	2
6	3
14	3
19	3
24	3
26	3
37	3
7	4
14	4
20	4
21	4
23	4
28	4
30	4
36	4
38	4
0	5
24	5
29	5
33	5
7	6
9	6
14	6
19	6
25	6
26	6
28	6
38	6
39	6
2	7
6	7
4	8
17	8
27	8
35	8
40	8
8	9
12	9
20	9
39	9
8	10
9	10
27	10
31	10
33	10
0	11
12	11
24	11
39	11
0	12
1	12
4	12
8	12
10	12
13	12
25	12
14	13
22	13
27	13
37	13
10	14
13	14
15	14
33	14
36	14
5	15
17	15
21	15
32	15
16	16
20	16
31	16
38	16
8	17
9	17
19	17
0	18
7	18
17	18
19	18
28	18
34	18
10	19
17	19
23	19
30	19
32	19
33	19
2	20
7	20
13	20
39	20
5	21
6	21
19	21
30	21
0	22
7	22
16	22
22	22
24	22
29	22
30	22
35	22
37	22
12	23
22	23
29	23
32	23
22	24
23	24
38	24
3	25
10	25
21	25
26	25
31	25
36	25
39	25
40	26
6	27
11	27
16	27
23	27
28	27
31	27
33	27
39	27
8	28
18	28
0	29
1	29
3	29
11	29
37	29
2	30
19	30
21	30
31	30
8	31
15	31
26	31
27	31
4	32
5	32
7	32
11	32
12	32
29	32
38	32
13	33
24	33
40	33
5	34
7	34
20	34
32	34
13	35
15	35
21	35
29	35
34	35
37	35
39	35
40	35
26	36
29	36
33	36
39	36
10	37
13	37
15	37
17	37
38	37
1	38
25	38
2	39
39	39
1	40
2	40
4	40
14	40
22	40
23	40
24	40

these follow the trails to the top, calculating the two properties of each trailhead

     (defun tails (p h)
       (when (eq (height p) h)
	 (if (eq h 9) (list p)
	   (--mapcat (tails it (+ 1 h)) (neighbours p)))))
     (defun score (p)
       (length (-distinct (tails p 0))))
     (defun rating (p)
       (length (tails p 0)))

  (-reduce #'+ (-map #'score trail-heads))
  (-reduce #'+ (-map #'rating trail-heads))

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