From 78f92480fd5eb975dd0a9e7bafc21ae2f721fdea Mon Sep 17 00:00:00 2001
From: Nikita Hasert <nikitahasert@gmail.com>
Date: Fri, 22 Dec 2023 21:42:11 +0100
Subject: [PATCH] Day 21: Solve part 1 and look up a solutions for  part 2

---
 src/advent_of_code_2023/day21.clj       | 58 +++++++++++++++++++++++--
 test/advent_of_code_2023/day21_test.clj | 26 ++++++-----
 2 files changed, 68 insertions(+), 16 deletions(-)

diff --git a/src/advent_of_code_2023/day21.clj b/src/advent_of_code_2023/day21.clj
index 04278bb..282702b 100644
--- a/src/advent_of_code_2023/day21.clj
+++ b/src/advent_of_code_2023/day21.clj
@@ -1,9 +1,59 @@
 (ns advent-of-code-2023.day21
   (:require
-   [advent-of-code-2023.utils.string :as u]))
+   [advent-of-code-2023.utils.graph :as g]))
 
-(defn parse-input [input])
+(defn parse-input [input]
+  (let [{:keys [positions] :as garden} (g/parse-positional-map input identity)]
+    (assoc garden :start
+           (first (mapv first (filterv (fn [[p v]] (= v \S)) positions))))))
 
-(defn day21-1 [parsed-input])
+(defn- neighbours [p]
+  [(update p 0 inc)
+   (update p 0 dec)
+   (update p 1 inc)
+   (update p 1 dec)])
 
-(defn day21-2 [parsed-input])
\ No newline at end of file
+(defn- garden-positions-step-fn [positions adjusted-pos]
+  (fn [garden-positions]
+    (persistent!
+     (transduce
+      (comp
+       (mapcat neighbours)
+       (filter (comp #{\. \S} positions adjusted-pos))
+       (distinct))
+      conj! (transient []) garden-positions))))
+
+(defn- day21 [{:keys [positions]} starts ^long steps adjusted-pos]
+  (let [garden-positions-step (garden-positions-step-fn positions adjusted-pos)]
+    (loop [steps ^long steps
+           current-positions starts]
+      (if (pos? steps)
+        (let [new-positions (garden-positions-step current-positions)]
+          (recur (dec steps)
+                 new-positions))
+        current-positions))))
+
+(defn day21-1 [{:keys [start] :as garden} steps]
+  (count (day21 garden [start] steps identity)))
+
+(defn- adjust-infinity-fn [^long height ^long width]
+  (fn [[^long x ^long y]]
+    [(mod (if (<= 0 x) x (+ x width)) width)
+     (mod (if (<= 0 y) y (+ y width)) height)]))
+
+;; Monkey see, monkey do:
+;; https://github.com/goggle/AdventOfCode2023.jl/blob/main/src/day21.jl#L44
+(defn day21-2 [{:keys [^long height ^long width start] :as garden} ^long steps]
+  (let [adjust-infinity (adjust-infinity-fn height width)
+        w width
+        w2 (quot width 2)
+        pos1 (day21 garden [start] w2 adjust-infinity)
+        pos2 (day21 garden pos1 w adjust-infinity)
+        r1 (count pos1)
+        r2 (count pos2)
+        r3 (count (day21 garden pos2 w adjust-infinity))
+        a ^long (quot (- (+ r3 r1) (* 2 r2)) 2)
+        b ^long (quot (- (- (* 4 r2) (* 3 r1)) r3) 2)
+        c r1
+        x ^long (quot (- steps w2) w)]
+    (+ (* a x x) (* b x) c)))
\ No newline at end of file
diff --git a/test/advent_of_code_2023/day21_test.clj b/test/advent_of_code_2023/day21_test.clj
index 0493c84..8bbff58 100644
--- a/test/advent_of_code_2023/day21_test.clj
+++ b/test/advent_of_code_2023/day21_test.clj
@@ -4,26 +4,28 @@
    [advent-of-code-2023.day21 :refer :all]
    [advent-of-code-2023.test-utils :as tu]))
 
-(defonce ^:private example-input (parse-input ""))
+(defonce ^:private example-input (parse-input "...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+..........."))
 
 (def ^:private input (parse-input (tu/slurp-input "resources/day21.txt")))
 
 (deftest day21-1-example-test
   (testing "day21-1 example"
-    (is (= nil
-           (day21-1 example-input)))))
+    (is (= 16 (day21-1 example-input 6)))))
 
 (deftest day21-1-test
   (testing "day21-1"
-    (is (= nil
-           (day21-1 input)))))
-
-(deftest day21-2-example-test
-  (testing "day21-2 example"
-    (is (= nil
-           (day21-2 example-input)))))
+    (is (= 3733 (day21-1 input 64)))))
 
 (deftest day21-2-test
   (testing "day21-2"
-    (is (= nil
-           (day21-2 input)))))
+    (is (= 617729401414635 (day21-2 input 26501365)))))