commit - c5b150512827fae8980848c3f48a685db29d3db6
commit + c7c064a8bf3e2b6aaaa8c6dc5e00d0553fc20728
blob - /dev/null
blob + 7ae77b7dc1c88a2cef7ac36aca2be4e67f3e133c (mode 644)
--- /dev/null
+++ input/6.txt
@@ -0,0 +1,130 @@
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blob - /dev/null
blob + cfbc66e5a32a3916228ab874d072a61e7e9b951d (mode 644)
--- /dev/null
+++ src/day-6.lisp
@@ -0,0 +1,71 @@
+(defpackage #:aoc/day-6
+  (:use #:cl #:aoc/utils)
+  (:export #:day-6))
+(in-package #:aoc/day-6)
+
+(defun find-guard (map)
+  (loop for y from 0 below (input-map-height map)
+        thereis (loop for x from 0 below (input-map-width map)
+                      for point = (cons x y)
+                      for cell = (map-cell map point)
+                      when (char= cell #\^)
+                        do (return point))))
+
+(defun next (point direction)
+  (point+
+   point
+   (ecase direction
+     (:up (cons 0 -1))
+     (:left (cons -1 0))
+     (:right (cons 1 0))
+     (:down (cons 0 1)))))
+
+(defun turn (direction)
+  (ecase direction
+    (:up :right)
+    (:right :down)
+    (:down :left)
+    (:left :up)))
+
+(defun walk-map (map pos)
+  (loop with dir = :up
+        with visited = (list pos)
+        with visited-dir = (make-hash-table :test #'equal)
+        with map-width = (input-map-width map)
+        with map-height = (input-map-height map)
+        with in-bounds-p = (lambda (point)
+                             (and (>= (point-x point) 0)
+                                  (>= (point-y point) 0)
+                                  (< (point-x point) map-width)
+                                  (< (point-y point) map-height)))
+        while (funcall in-bounds-p pos)
+        when (gethash (cons pos dir) visited-dir)
+          do (return :loop)
+        do (setf (gethash (cons pos dir) visited-dir) t
+                 pos (loop with next = (next pos dir)
+                           while (and (funcall in-bounds-p next)
+                                      (char= (map-cell map next) #\#))
+                           do (setf dir (turn dir)
+                                    next (next pos dir))
+                           finally (return next)))
+           (push pos visited)
+        finally (return visited)))
+
+(defun task-2 (map initial-pos)
+  (loop with task-2 = 0
+        for y from 0 below (input-map-height map)
+        do (loop for x from 0 below (input-map-width map)
+                 for point = (cons x y)
+                 for cell = (map-cell map point)
+                 when (char= cell #\.)
+                   do (setf (map-cell map point) #\#)
+                      (when (eq (walk-map map initial-pos) :loop)
+                        (incf task-2))
+                      (setf (map-cell map point) #\.))
+        finally (return task-2)))
+
+(defun day-6 (input)
+  (let* ((map (make-map input))
+         (pos (find-guard map)))
+    (values (1- (length (remove-duplicates (walk-map map pos) :test #'equal)))
+            (task-2 map pos))))
blob - /dev/null
blob + eda1e012b0af55eca87efabf3442092527cbad6c (mode 644)
--- /dev/null
+++ t/day-6.lisp
@@ -0,0 +1,20 @@
+(defpackage #:aoc-test/day-6
+  (:use #:cl #:lisp-unit2)
+  (:import-from #:aoc/day-6))
+(in-package #:aoc-test/day-6)
+
+(define-test test-day-6
+    ()
+  (multiple-value-bind (task-1 task-2)
+      (aoc:run-day 6 "....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...")
+    (assert= 41 task-1)
+    (assert= 6 task-2)))