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diff --git a/AoC2022/03/solver.el b/AoC2022/03/solver.el
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+;;; solver.el --- Day 03 -*- lexical-binding: t; -*-
+;;
+;; Copyright (C) 2022 Óscar Nájera
+;;
+;; Author: Óscar Nájera <hi@oscarnajera.com>
+;; Maintainer: Óscar Nájera <hi@oscarnajera.com>
+;; Created: December 04, 2022
+;; Modified: December 04, 2022
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; Commentary:
+;;
+;;  Day 03
+;;
+;;; Code:
+
+(defsubst solver-recover-value (cross)
+  (let ((result 0))
+    (while (= 0 (logand cross 1))
+      (setq cross (ash cross -1))
+      (cl-incf result))
+    result))
+
+(ert-deftest test-recover ()
+  (should (eq ?t (+ 64 (solver-recover-value (ash 1 (- ?t 64))))))
+  (should (eq ?s (+ 64 (solver-recover-value (ash 1 (- ?s 64))))))
+  (should (eq ?C (+ 64 (solver-recover-value (ash 1 (- ?C 64))))))) ;; small cap starts at a=97, but already took 64
+
+(defsubst solver-to-priority (value)
+  (if (< value 31) ;; It is a capital letter in range [1;26]
+      (+ value 26)
+    (- value 32)))
+
+(defun solver-line-priority (str)
+  (cl-assert (length> str 0))
+  (let ((mid (/ (length str) 2))
+        (left-pack 0) (right-pack 0))
+    (cl-loop for l across (substring str 0 mid)
+             for r across (substring str mid)
+             do (progn
+                  (setf left-pack (logior left-pack (ash 1 (- l 64)))) ;; ASCII letters start at 65=A
+                  (setf right-pack  (logior right-pack(ash 1 (- r 64)))))
+             for cross = (logand left-pack right-pack)
+             when (< 0 cross)
+             return (solver-to-priority (solver-recover-value cross)))))
+
+(ert-deftest test-line-priority ()
+  (should (= 16 (solver-line-priority  "vJrwpWtwJgWrhcsFMMfFFhFp")))
+  (should (= 19 (solver-line-priority  "hMHLcmGLMLhHmsRMsSvsQSqrsrlJTTdV" )))
+  (should (= 20 (solver-line-priority  "ttgJtRGJQctTZtZT"))))
+
+(ert-deftest test-problems ()
+  (should (= 8072
+             (with-temp-buffer
+               ;; (insert "vJrwpWtwJgWrhcsFMMfFFhFp")
+               (insert-file-contents "input")
+               (goto-char (point-min))
+               (let ((result 0))
+                 (while (not (eobp))
+                   (cl-incf result (solver-line-priority
+                                    (buffer-substring-no-properties
+                                     (line-beginning-position) (line-end-position))))
+                   (forward-line))
+                 result)))))
+