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+;;; solver.el --- Day 09 -*- lexical-binding: t; -*-
+;;
+;; Copyright (C) 2022 Óscar Nájera
+;;
+;; Author: Óscar Nájera <hi@oscarnajera.com>
+;; Maintainer: Óscar Nájera <hi@oscarnajera.com>
+;; Created: December 09, 2022
+;; Modified: December 09, 2022
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; Commentary:
+;;
+;;  Day 09
+;;
+;;; Code:
+
+(defsubst solver-diff (head tail)
+  (cons (- (car head) (car tail)) (- (cdr head) (cdr tail))))
+
+(defsubst solver-distance-1 (vec)
+  "Norm 1 distance."
+  (+ (abs (car vec)) (abs (cdr vec))))
+
+(defsubst solver-diagonal (vec)
+  (and (= 1 (abs (car vec))) (= 1 (abs (cdr vec)))))
+
+
+(defun solver-move (direction)
+  (cl-ecase direction
+    ('R (lambda (x) (cl-incf (car x))))
+    ('L (lambda (x) (cl-decf (car x))))
+    ('U (lambda (x) (cl-incf (cdr x))))
+    ('D (lambda (x) (cl-decf (cdr x))))))
+;; WARN This need to be bytecompiled otherwise the lambda is not capturing diff-v in the closure
+(defun solver-puller (diff-v)
+  (let ((distance (solver-distance-1 diff-v)))
+    (cond
+     ((or (<= distance 1)
+          (solver-diagonal diff-v)) #'identity)
+     ((= distance 2)
+      (pcase diff-v
+        (`(0 . ,d) (solver-move (if (= d 2) 'U 'D)))
+        (`(,d . 0) (solver-move (if (= d 2) 'R 'L)))))
+     ((= distance 3)
+      (lambda (x)
+        (funcall (solver-move (if (< 0 (car diff-v)) 'R 'L)) x)
+        (funcall (solver-move (if (< 0 (cdr diff-v)) 'U 'D)) x)))
+     (t (error "Head moved too far")))))
+
+(with-temp-buffer
+  (insert-file-contents "input")
+;;   (insert "R 4
+;; U 4
+;; L 3
+;; D 1
+;; R 4
+;; D 1
+;; L 5
+;; R 2")
+  (let (path
+        (head (cons 0 0))
+        (tail (cons 0 0))
+        (moves
+         (thread-last
+           (split-string (buffer-string) "\n" t)
+           (mapcar (lambda (inst) (let ((move (split-string inst)))
+                                    (cons (intern (car move))
+                                          (string-to-number (cadr move))))))
+           )))
+    (dolist (move moves)
+      (dotimes (_ (cdr move))
+        (funcall (solver-move (car move)) head)
+        (funcall (solver-puller (solver-diff head tail)) tail)
+        (push (cons (car tail) (cdr tail)) path)))
+    (= 6384 (length (cl-remove-duplicates path :test #'equal))))
+
+