Day 14

day14.peggy

@@ -0,0 +1,4 @@
+lines = (@line "\n")*
+line = "p=" @num "," @num _ "v=" @num "," @num
+num = n:$("-"?[0-9]+) { return parseInt(n, 10) }
+_ = [ \t]+

day14.ts

@@ -0,0 +1,78 @@
+import { Point } from './lib/rect.ts';
+import { div, type MainArgs, mod, parseFile } from './lib/utils.ts';
+import { Counter } from './lib/counter.ts';
+
+type Parsed = [px: number, py: number, vx: number, vy: number][];
+
+const STEPS = 100;
+const WIDTH = 101;
+const HEIGHT = 103;
+
+function part1(inp: Parsed): number {
+  const pos: Point[] = [];
+  for (const [px, py] of inp) {
+    pos.push(new Point(px, py));
+  }
+
+  for (let j = 0; j < pos.length; j++) {
+    const p = pos[j];
+    const [_px, _py, vx, vy] = inp[j];
+    pos[j] = new Point(
+      mod(p.x + (vx * STEPS), WIDTH),
+      mod(p.y + (vy * STEPS), HEIGHT),
+    );
+  }
+
+  const hx = div(WIDTH, 2);
+  const hy = div(HEIGHT, 2);
+  const tots = [0, 0, 0, 0];
+  for (const p of pos) {
+    if (p.x < hx) {
+      if (p.y < hy) {
+        tots[0]++;
+      } else if (p.y > hy) {
+        tots[1]++;
+      }
+    } else if (p.x > hx) {
+      if (p.y < hy) {
+        tots[2]++;
+      } else if (p.y > hy) {
+        tots[3]++;
+      }
+    }
+  }
+  return tots.reduce((t, v) => t * v, 1);
+}
+
+function part2(inp: Parsed): number {
+  const pos: Point[] = [];
+  for (const [px, py] of inp) {
+    pos.push(new Point(px, py));
+  }
+
+  // I found the setpoints by writing images to the terminal and fiddling with
+  // the constants.  Used Jimp and term-img.
+  for (let i = 0; i < Infinity; i++) {
+    const xc = new Counter<number>();
+    const yc = new Counter<number>();
+    for (let j = 0; j < pos.length; j++) {
+      const p = pos[j];
+      const [_px, _py, vx, vy] = inp[j];
+      const [nx, ny] = [mod(p.x + vx, WIDTH), mod(p.y + vy, HEIGHT)];
+      pos[j] = new Point(nx, ny);
+      xc.add(nx); // Count the number of pixels in each vertical line
+      yc.add(ny); // Each horizontal line
+    }
+
+    if (((xc.max()?.[1] ?? 0) > 30) && ((yc.max()?.[1] ?? 0) > 30)) {
+      // The box around the tree.
+      return i + 1;
+    }
+  }
+  return NaN;
+}
+
+export default async function main(args: MainArgs): Promise<[number, number]> {
+  const inp = await parseFile<Parsed>(args);
+  return [part1(inp), part2(inp)];
+}

lib/rect.ts

@@ -435,6 +435,12 @@ export class Rect<T = string> {
     return prev;
   }
 
+  fill(val: T): void {
+    for (let y = 0; y < this.height; y++) {
+      this.#vals[y].fill(val);
+    }
+  }
+
   filter(callbackFn: RectFilterCallback<T>): Point[] {
     const res: Point[] = [];
     this.forEach((v, x, y) => {

lib/utils.ts

@@ -166,6 +166,35 @@ export function adjacentFile(
   return join(p, '..', ...dir, `day${args.day}.${ext}`);
 }
 
+/**
+ * Create an array of the given length from a callback.
+ *
+ * @param length
+ * @param cb
+ * @returns
+ */
+export function toArray<T>(length: number, cb: (k: number) => T): T[] {
+  return Array.from({ length }, (_v, k) => cb(k));
+}
+
+export function div<T extends number | bigint>(x: T, y: T): T {
+  let q = (x / y) as unknown as T;
+  if (typeof x === 'bigint') {
+    const r = mod(x, y);
+    // Not only does Math.floor not work for BigInt, it's not needed because
+    // `/` does the right thing in the first place.
+
+    // except for numbers of opposite sign
+    if ((q < 0n) && (r > 0n)) {
+      // There was a remainder.  JS rounded toward zero, but python
+      // rounds down.
+      q--;
+    }
+    return q;
+  }
+  return Math.floor(q as number) as T;
+}
+
 /**
  * Modulo, minus the JS bug with negative numbers.
  * `-5 % 4` should be `3`, not `-1`.

test/day14.js

@@ -0,0 +1 @@
+export default [216772608, 6888];