Day 16: Solve part 1 and 2

Author: nihas101

Diff for: src/advent_of_code_2023/day16.clj

@@ -1,9 +1,97 @@
 (ns advent-of-code-2023.day16
   (:require
-   [advent-of-code-2023.utils.string :as u]))
+   [advent-of-code-2023.utils.graph :as g]))
 
-(defn parse-input [input])
+;; For debug purposes
+(defn- assoc-beams [{:keys [positions] :as c} [x y dir]]
+  (if (#{\| \- \\ \/} (positions [x y]))
+    c
+    (assoc-in c [:positions [x y]]
+              (condp = dir
+                :north \^
+                :east \>
+                :south \v
+                :west \<
+                :?))))
 
-(defn day16-1 [parsed-input])
+;; For debug purposes
+(defn- assoc-energized [c [x y]]
+  (assoc-in c [:positions [x y]] \#))
 
-(defn day16-2 [parsed-input])
+(defn parse-input [input]
+  (g/parse-positional-map input identity))
+
+(defn- move-beam [[_ _ direction :as position]]
+  (condp = direction
+    :north (update position 1 dec)
+    :east (update position 0 inc)
+    :south (update position 1 inc)
+    :west (update position 0 dec)))
+
+(defn- split-beam [dir val [x y]]
+  (condp = [dir val]
+    [:east \|] [[x y :north] [x y :south]]
+    [:west \|] [[x y :north] [x y :south]]
+    [:north \-] [[x y :east] [x y :west]]
+    [:south \-] [[x y :east] [x y :west]]
+    nil))
+
+(defn- trace-beam
+  ([contraption start-pos]
+   (trace-beam contraption start-pos #{}))
+  ([{:keys [positions] :as contraption} beam-position beam-positions]
+   (let [[x y dir :as np] (move-beam beam-position)
+         val (positions (butlast np))]
+     ;; It is not enough to currently be in the same position and same direction
+     ;; for a cycle. We also need to have come from the same position
+     (if (and (beam-positions beam-position) (beam-positions np))
+       beam-positions
+       (condp = val
+         ;; We are outside of the grid -> stop
+         nil (conj beam-positions beam-position)
+         \. (recur contraption np (conj beam-positions beam-position))
+         \| (if-let [[dir1 dir2] (split-beam dir val np)]
+              (let [up-beam-positions
+                    (trace-beam contraption dir1 (conj beam-positions beam-position))]
+                (recur contraption dir2 up-beam-positions))
+              ;; We simply pass through the mirror
+              (recur contraption np (conj beam-positions beam-position)))
+         \- (if-let [[dir1 dir2] (split-beam dir val np)]
+              (let [left-beam-positions
+                    (trace-beam contraption dir1 (conj beam-positions beam-position))]
+                (trace-beam contraption dir2 left-beam-positions))
+              ;; We simply pass through the mirror
+              (recur contraption np (conj beam-positions beam-position)))
+         \\ (recur contraption [x y (condp = dir
+                                      :north :west
+                                      :south :east
+                                      :east :south
+                                      :west :north)]
+                   (conj beam-positions beam-position))
+         \/ (recur contraption [x y (condp = dir
+                                      :north :east
+                                      :south :west
+                                      :east :north
+                                      :west :south)]
+                   (conj beam-positions beam-position)))))))
+
+(defn- energy [beam-positions]
+  ;; We have to subtract one, because our result also contains the
+  ;; start point otuside of the grid
+  (dec (count (set (mapv butlast beam-positions)))))
+
+(defn- day16 [contraption beam-start]
+  (energy (trace-beam contraption beam-start)))
+
+(defn day16-1 [contraption]
+  (day16 contraption [-1 0 :east]))
+
+(defn day16-2 [{:keys [height width] :as contraption}]
+  (let [beam-starts (concat
+                     (for [x (range width)] [x -1 :south])
+                     (for [x (range width)] [x height :north])
+                     (for [y (range height)] [-1 y :east])
+                     (for [y (range height)] [width y :west]))]
+    (->> beam-starts
+         (pmap (partial day16 contraption))
+         (reduce max))))

Diff for: test/advent_of_code_2023/day16_test.clj

@@ -4,26 +4,31 @@
    [advent-of-code-2023.day16 :refer :all]
    [advent-of-code-2023.test-utils :as tu]))
 
-(defonce ^:private example-input (parse-input ""))
+(defonce ^:private example-input (parse-input ".|...\\....
+|.-.\\.....
+.....|-...
+........|.
+..........
+.........\\
+..../.\\\\..
+.-.-/..|..
+.|....-|.\\
+..//.|...."))
 
 (def ^:private input (parse-input (tu/slurp-input "resources/day16.txt")))
 
 (deftest day16-1-example-test
   (testing "day16-1 example"
-    (is (= nil
-           (day16-1 example-input)))))
+    (is (= 46 (day16-1 example-input)))))
 
 (deftest day16-1-test
   (testing "day16-1"
-    (is (= nil
-           (day16-1 input)))))
+    (is (= 6855 (day16-1 input)))))
 
 (deftest day16-2-example-test
   (testing "day16-2 example"
-    (is (= nil 
-           (day16-2 example-input)))))
+    (is (= 51 (day16-2 example-input)))))
 
 (deftest day16-2-test
   (testing "day16-2"
-    (is (= nil
-           (day16-2 input)))))
+    (is (= 7513 (day16-2 input)))))