Day 6

day6.peggy

@@ -0,0 +1 @@
+lines = (@[^\n]+ "\n")*

day6.ts

@@ -0,0 +1,78 @@
+import { Dir, Point, Rect } from './lib/rect.ts';
+import { type MainArgs, parseFile } from './lib/utils.ts';
+
+type Parsed = string[][];
+
+function path(r: Rect): [Set<number>, Point] {
+  const [start] = r.filter(val => val === '^');
+  let pos = start;
+  let dir = Dir.N;
+  const visited = new Set<number>();
+  while (true) {
+    visited.add(pos.toNumber());
+    const ahead = pos.inDir(dir);
+    if (!r.check(ahead)) {
+      break;
+    }
+    const char = r.get(ahead);
+    if (char === '.' || char === '^') {
+      pos = ahead;
+    } else {
+      dir = (dir + 1) % 4
+    }
+  }
+  return [visited, start];
+}
+
+function path2(r: Rect): boolean {
+  const [start] = r.filter(val => val === '^');
+  let pos = start;
+  let dir = Dir.N;
+  const visitedWithDir = new Set<string>();
+  while (true) {
+    const pwd = `${pos},${dir}`;
+    if (visitedWithDir.has(pwd)) {
+      return true;
+    }
+    visitedWithDir.add(pwd);
+    const ahead = pos.inDir(dir);
+    if (!r.check(ahead)) {
+      return false;
+    }
+    const char = r.get(ahead);
+    if (char === '.' || char === '^') {
+      pos = ahead;
+    } else {
+      dir = (dir + 1) % 4
+    }
+  }
+}
+
+function part1(inp: Parsed): number {
+  const r = new Rect(inp);
+  const [visited] = path(r)
+  return visited.size;
+}
+
+function part2(inp: Parsed): number {
+  const r = new Rect(inp);
+  const [visited, start] = path(r)
+  const points =  [...visited].map(v => Point.fromNumber(v))
+  let tot = 0;
+  for (const p of points) {
+    if (p.equals(start)) {
+      continue;
+    }
+    r.set(p, '#');
+    if (path2(r)) {
+      tot++;
+    }
+    r.set(p, '.');
+  }
+  return tot;
+}
+
+export default async function main(args: MainArgs): Promise<[number, number]> {
+  const inp = await parseFile<Parsed>(args);
+  return [part1(inp), part2(inp)];
+}

lib/rect.ts

@@ -145,6 +145,22 @@ export class Point implements PointLike {
     return `${this.x},${this.y}`;
   }
 
+  toNumber(size = 16): number {
+    return (this.x << size) | this.y;
+  }
+
+  static fromNumber(num: number, size = 16): Point {
+    return new Point(num >> size, num & ((1 << size) - 1));
+  }
+
+  static fromString(str: string): Point {
+    const m = str.match(/(\d+),(\d+)/);
+    if (!m) {
+      throw new Error(`Invalid format: "${str}"`);
+    }
+    return new Point(Number(m[1]), Number(m[2]));
+  }
+
   [Symbol.for('Deno.customInspect')](): string {
     return this.toString();
   }

test/day6.js

@@ -0,0 +1 @@
+export default [4374, 1705];