From 757d0c2af8bfa7ac2350183872cf3f07fdc06743 Mon Sep 17 00:00:00 2001
From: Tim Kurvers <tim@moonsphere.net>
Date: Fri, 6 Dec 2024 10:16:12 +0100
Subject: [PATCH] feat(js): add solutions for 2024 day 6

---
 js/src/2024/04/index.js       | 18 ++-------
 js/src/2024/06/index.js       | 72 +++++++++++++++++++++++++++++++++++
 js/src/utils/navigation.js    | 16 ++++++++
 puzzles/2024/06/examples.yaml | 17 +++++++++
 4 files changed, 108 insertions(+), 15 deletions(-)
 create mode 100644 js/src/2024/06/index.js
 create mode 100644 puzzles/2024/06/examples.yaml

diff --git a/js/src/2024/04/index.js b/js/src/2024/04/index.js
index 8e5750c..a15242d 100644
--- a/js/src/2024/04/index.js
+++ b/js/src/2024/04/index.js
@@ -1,20 +1,8 @@
-import { Grid, solution } from '../../utils/index.js';
+import { ALL_DIRECTIONS, INTERCARDINAL_DIRECTIONS, Grid, solution } from '../../utils/index.js';
 
 const parse = (input) => Grid.from(input);
 
-const DIRECTIONS = [
-  [-1, -1],
-  [0, -1],
-  [1, -1],
-  [-1, 0],
-  [1, 0],
-  [-1, 1],
-  [0, 1],
-  [1, 1],
-];
-
-// Keep diagonals only
-const CROSS_PATTERN_DIRECTIONS = DIRECTIONS.filter(([dx, dy]) => dx !== 0 && dy !== 0);
+const CROSS_PATTERN_DIRECTIONS = INTERCARDINAL_DIRECTIONS;
 
 // Whether given letters are all encountered when walking the grid from starting point in the dx/dy direction
 const matches = (letters, { grid, point, dx, dy }) =>
@@ -27,7 +15,7 @@ const search = (grid, word) => {
   const letters = word.split('');
   let count = 0;
   for (const point of grid) {
-    for (const [dx, dy] of DIRECTIONS) {
+    for (const [dx, dy] of ALL_DIRECTIONS) {
       count += +matches(letters, { grid, point, dx, dy });
     }
   }
diff --git a/js/src/2024/06/index.js b/js/src/2024/06/index.js
new file mode 100644
index 0000000..005607d
--- /dev/null
+++ b/js/src/2024/06/index.js
@@ -0,0 +1,72 @@
+import { Grid, Orientation, Rotation, solution, dx, dy, normalizeOrientation } from '../../utils/index.js';
+
+const GUARD = '^';
+const OBSTACLE = '#';
+
+const parse = (input) => {
+  const grid = Grid.from(input);
+  const start = grid.find((point) => point.value === GUARD);
+  return { grid, start };
+};
+
+const explore = (grid, start) => {
+  let orientation = Orientation.UP;
+
+  // Holds points visited by the guard
+  const visited = new Set();
+
+  // Holds points the guard faced (in front)
+  const faced = new Set();
+
+  // Used to track whether the guard patrols in a loop
+  const cyclic = new Set();
+  const hash = (point, direction) => `${point.x}:${point.y}:${direction}`;
+
+  let current = start;
+  let isCyclic = false;
+  while (current) {
+    const entry = hash(current, orientation);
+    if (cyclic.has(entry)) {
+      isCyclic = true;
+      break;
+    }
+    cyclic.add(entry);
+    visited.add(current);
+
+    const next = grid.getPoint(current.x + dx(orientation), current.y + dy(orientation));
+    if (next) {
+      faced.add(next);
+    }
+    if (next?.value === OBSTACLE) {
+      orientation = normalizeOrientation(orientation + Rotation.TURN_RIGHT);
+    } else {
+      current = next;
+    }
+  }
+  return { visited: Array.from(visited), faced, isCyclic };
+};
+
+export const partOne = solution((input) => {
+  const { grid, start } = parse(input);
+  return explore(grid, start).visited.length;
+});
+
+export const partTwo = solution.inefficient((input) => {
+  const { grid, start } = parse(input);
+
+  const initial = explore(grid, start);
+
+  let count = 0;
+  for (const candidate of initial.faced) {
+    const original = candidate.value;
+    candidate.value = OBSTACLE;
+
+    const result = explore(grid, start);
+    if (result.isCyclic) {
+      count++;
+    }
+
+    candidate.value = original;
+  }
+  return count;
+});
diff --git a/js/src/utils/navigation.js b/js/src/utils/navigation.js
index bc0080c..80366b3 100644
--- a/js/src/utils/navigation.js
+++ b/js/src/utils/navigation.js
@@ -4,6 +4,22 @@ import { TAU } from './math.js';
 
 const EPSILON = 0.000000001;
 
+export const CARDINAL_DIRECTIONS = [
+  [0, -1],
+  [-1, 0],
+  [1, 0],
+  [0, 1],
+];
+
+export const INTERCARDINAL_DIRECTIONS = [
+  [-1, -1],
+  [1, -1],
+  [-1, 1],
+  [1, 1],
+];
+
+export const ALL_DIRECTIONS = CARDINAL_DIRECTIONS.concat(INTERCARDINAL_DIRECTIONS);
+
 export const Orientation = {
   // UP / DOWN are flipped to ensure proper vertical movements in grids
   UP: Math.PI * 1.5,
diff --git a/puzzles/2024/06/examples.yaml b/puzzles/2024/06/examples.yaml
new file mode 100644
index 0000000..a9e1caa
--- /dev/null
+++ b/puzzles/2024/06/examples.yaml
@@ -0,0 +1,17 @@
+part-one:
+  - input: &example1 |
+      ....#.....
+      .........#
+      ..........
+      ..#.......
+      .......#..
+      ..........
+      .#..^.....
+      ........#.
+      #.........
+      ......#...
+    answer: 41
+
+part-two:
+  - input: *example1
+    answer: 6