Solve 2024-06 (using bruteforce)

2024/06/index.ts

@@ -0,0 +1,127 @@
+import { sumBy } from '../../utils/array.ts';
+
+export function replace(arr: string[], x: number, y: number, char: string) {
+  arr[y] = arr[y].substring(0, x) + char + arr[y].substring(x + 1);
+}
+
+const directions = [
+  { d: '^', x: 0, y: -1 },
+  { d: '>', x: 1, y: 0 },
+  { d: 'v', x: 0, y: 1 },
+  { d: '<', x: -1, y: 0 }
+] as const;
+
+export function solveFirst(input: string): number {
+  const rows = input.split('\n');
+
+  const columnCount = rows[0].length;
+  const rowCount = rows.length;
+
+  let y = rows.findIndex((r) => r.indexOf('^') >= 0);
+  let x = rows[y].indexOf('^');
+  let direction = 0;
+
+  while (x >= 0 && x < columnCount && y >= 0 && y < rowCount) {
+    const nextX = x + directions[direction].x;
+    const nextY = y + directions[direction].y;
+    if (rows[nextY]?.[nextX] === '#') {
+      direction = (direction + 1) % 4;
+    }
+
+    replace(rows, x, y, 'X');
+    x += directions[direction].x;
+    y += directions[direction].y;
+  }
+  return sumBy(rows, (r) => [...r.matchAll(/X/g)].length);
+}
+
+export function getNextObstacle(
+  rows: string[],
+  startX: number,
+  startY: number,
+  d: number
+): false | { x: number; y: number } {
+  let x = startX;
+  let y = startY;
+
+  while (true) {
+    const nextX = x + directions[d].x;
+    const nextY = y + directions[d].y;
+    const next = rows[nextY]?.[nextX];
+    if (!next) {
+      return false;
+    }
+    if (next === '#') {
+      return { x, y };
+    }
+    x = nextX;
+    y = nextY;
+  }
+}
+
+export function finishes(
+  rows: string[],
+  startX: number,
+  startY: number,
+  d: number
+): boolean {
+  let x = startX;
+  let y = startY;
+
+  let i = 0;
+  do {
+    const next = getNextObstacle(rows, x, y, (d + i) % 4);
+    if (!next) {
+      return true;
+    }
+    x = next.x;
+    y = next.y;
+    i++;
+
+    // TODO: be smarter
+  } while (i < 1_000_000);
+
+  return false;
+}
+
+export function solveRow(
+  rows: string[],
+  row: number,
+  startX: number,
+  startY: number,
+  startDirection: number
+) {
+  let found = 0;
+
+  for (let column = 0; column < rows[0].length; column++) {
+    if ((row === startX && column === startY) || rows[row][column] === '#') {
+      continue;
+    }
+
+    replace(rows, column, row, '#');
+    if (!finishes(rows, startX, startY, startDirection)) {
+      found++;
+    }
+
+    replace(rows, column, row, '.');
+  }
+  return found;
+}
+
+export function solveSecond(input: string): number {
+  const rows = input.split('\n');
+  const rowCount = rows.length;
+
+  const y = rows.findIndex((r) => r.indexOf('^') >= 0);
+  const x = rows[y].indexOf('^');
+  const direction = 0;
+
+  let found = 0;
+
+  for (let row = 0; row < rowCount; row++) {
+    found += solveRow(rows, row, x, y, direction);
+    console.log('row:', row, 'found', found);
+  }
+
+  return found;
+}