Add day 20 of year 2024 🧑‍🎄

2024/days/day-20/README.md

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+# Benchmark
+
+```
+Platform: darwin arm64
+CPU: Apple M3 Pro 11 Cores
+Memory: 18.00 GB
+```
+
+## Sample 1
+
+| part | time (~) | μs                 |
+| ---- | -------- | ------------------ |
+| 1    | 4.48ms   | 4.4783750000000015 |
+
+## Answer
+
+| part | time (~) | μs           |
+| ---- | -------- | ------------ |
+| 1    | 11.930s  | 11929.655666 |

2024/days/day-20/in.sample.txt

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+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############

2024/days/day-20/main.js

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+function parse(source) {
+  return source
+    .trim()
+    .split('\n')
+    .map((e) => e.split(''));
+}
+
+function findTile(grid, char) {
+  const row = grid.findIndex((row) => row.includes(char));
+  return [row, grid[row].indexOf(char)];
+}
+
+const symbols = { start: 'S', end: 'E', wall: '#', track: '.' };
+const dirs = {
+  orthogonal: [
+    [0, -1],
+    [-1, 0],
+    [0, 1],
+    [1, 0],
+  ],
+};
+
+function inRange(data, row, col) {
+  return row >= 0 && row < data.length && col >= 0 && col < data[0].length;
+}
+
+const hash = (row, col) => `${row},${col}`;
+
+function bfs(grid, start, end, visited, trackPath, memo = new Map()) {
+  const startKey = `${hash(...start)}->${hash(...end)}`;
+  if (memo.has(startKey)) return memo.get(startKey);
+
+  const q = [[0, start]],
+    source = new Map();
+
+  while (q.length) {
+    const [steps, [row, col]] = q.shift();
+    const key = hash(row, col);
+    if (visited.has(key)) continue;
+    visited.add(key);
+
+    if (row === end[0] && col === end[1]) {
+      if (!trackPath) {
+        memo.set(startKey, steps);
+        return steps;
+      }
+      const path = [[row, col]];
+      let head = source.get(key);
+      while (head) {
+        path.push(head.split(',').map(Number));
+        head = source.get(head);
+      }
+      const result = [steps, path.reverse()];
+      memo.set(startKey, result);
+      return result;
+    }
+
+    dirs.orthogonal.forEach(([drow, dcol]) => {
+      const [nextRow, nextCol] = [row + drow, col + dcol];
+      if (
+        inRange(grid, nextRow, nextCol) &&
+        grid[nextRow][nextCol] !== symbols.wall &&
+        !visited.has(hash(nextRow, nextCol))
+      ) {
+        q.push([steps + 1, [nextRow, nextCol]]);
+        source.set(hash(nextRow, nextCol), key);
+      }
+    });
+  }
+  memo.set(startKey, -1);
+  return -1;
+}
+
+function part1(data) {
+  const end = findTile(data, symbols.end),
+    start = findTile(data, symbols.start);
+  const memo = new Map();
+  const [minSteps, path] = bfs(data, start, end, new Set(), true, memo);
+
+  const visited = new Set(),
+    saves = new Map();
+  path.forEach(([row, col], steps) => {
+    visited.add(hash(row, col));
+    dirs.orthogonal.forEach(([drow, dcol]) => {
+      const [nextRow, nextCol] = [row + drow, col + dcol];
+      const nextKey = hash(nextRow, nextCol);
+      if (
+        inRange(data, nextRow, nextCol) &&
+        data[nextRow][nextCol] === symbols.wall &&
+        !visited.has(nextKey)
+      ) {
+        const bfsres = bfs(
+          data,
+          [nextRow, nextCol],
+          end,
+          new Set(visited),
+          false,
+          memo
+        );
+        if (bfsres !== -1) {
+          const candidate = steps + bfsres + 1;
+          if (candidate < minSteps)
+            saves.set(candidate, (saves.get(candidate) ?? 0) + 1);
+        }
+      }
+    });
+  });
+
+  return [
+    minSteps,
+    [...saves.entries()]
+      .filter(([k]) => minSteps - k >= 100)
+      .reduce((sum, [_, v]) => sum + v, 0),
+  ];
+}
+
+function part2() {
+  return null;
+}
+
+module.exports = { parse, part1, part2 };