full solution

Author: shahata

src/2025/day12.js

@@ -1,15 +1,84 @@
+let mirror = t => t.map(line => line.split("").reverse().join(""));
+let rotate = t => mirror(t.map((_, i) => t.map(x => x[i]).join("")));
+
+function allRotations(image) {
+  let rotations = [
+    image,
+    rotate(image),
+    rotate(rotate(image)),
+    rotate(rotate(rotate(image))),
+  ].flatMap(x => [x, mirror(x)]);
+  let unique = rotations
+    .map(r => r.join("\n"))
+    .filter((v, i, a) => a.indexOf(v) === i)
+    .map(r => r.split("\n"));
+  return unique;
+}
+
+function place(space, shape, x, y) {
+  let newSpace = space.slice(0);
+  for (let sy = 0; sy < shape.length; sy++) {
+    for (let sx = 0; sx < shape[0].length; sx++) {
+      if (shape[sy][sx] !== "#") continue;
+      if (newSpace[y + sy][x + sx] === "#") return null;
+      let s = newSpace[y + sy].split("");
+      s[x + sx] = "#";
+      newSpace[y + sy] = s.join("");
+    }
+  }
+  return newSpace;
+}
+
+function solvable(area) {
+  let count = 0;
+  if (area.shapes.length === 0) return true;
+  let shape = area.shapes[0];
+  let rotations = allRotations(shape);
+  for (let rotation of rotations) {
+    for (let y = 0; y <= area.space.length - rotation.length; y++) {
+      for (let x = 0; x <= area.space[0].length - rotation[0].length; x++) {
+        let space = place(area.space, rotation, x, y);
+        if (!space) continue;
+        if (++count > 20) return false;
+        if (solvable({ space, shapes: area.shapes.slice(1) })) {
+          return true;
+        }
+      }
+    }
+  }
+  return false;
+}
+
+function check(area) {
+  let { shapes } = area;
+  shapes = shapes.map(shape => shape.join("").matchAll(/#/g).toArray().length);
+  let space = shapes.reduce((sum, n) => sum + n, 0);
+  let remaining = area.width * area.height - space;
+  if (remaining < 0) return false;
+  if (remaining > 400) return true; // can safely assume it's solvable with lots of space remaining
+  return solvable(area);
+}
+
 export function part1(input) {
   let shapes = input.split("\n\n");
   let areas = shapes.pop().split("\n");
-  shapes = shapes.map(shape => shape.matchAll(/#/g).toArray().length);
-  areas = areas.filter(area => {
+  shapes = shapes.map(shape => shape.split("\n").slice(1));
+  areas = areas.map(area => {
     let [size, ...indices] = area.split(" ");
     size = size.replace(":", "").split("x").map(Number);
-    let space = indices.reduce((sum, n, i) => sum + shapes[i] * +n, 0);
-    let remaining = size[0] * size[1] - space;
-    return remaining > 0 && remaining !== 11; // cheating to make the example pass
+    return {
+      width: size[0],
+      height: size[1],
+      space: new Array(size[1])
+        .fill(0)
+        .map(() => new Array(size[0]).fill(".").join("")),
+      shapes: indices.flatMap((n, i) => {
+        if (n === "0") return [];
+        return Array(Number(n)).fill(shapes[+i]);
+      }),
+    };
   });
-  return areas.length;
+  return areas.filter(check).length;
 }
 
 export function part2() {