day 13

13a/index.mjs

@@ -0,0 +1,67 @@
+import { readFileSync } from "node:fs";
+
+const input = readFileSync("input0.txt", "utf-8");
+const patterns = input.split("\n\n");
+
+let result = 0;
+for (const pattern of patterns) {
+  const lines = pattern.split("\n").map((line) => line.split(""));
+
+  const height = lines.length;
+  const width = lines[0].length;
+
+  const horizontalReflections = getReflections(lines);
+  const fullHorizontalReflections = Object.entries(
+    horizontalReflections
+  ).filter(([_, count]) => count === height);
+
+  for (const horizontalReflection of fullHorizontalReflections) {
+    result += +horizontalReflection[0];
+  }
+
+  const verticalReflections = getReflections(transpose(lines));
+  const fullVerticalReflections = Object.entries(verticalReflections).filter(
+    ([_, count]) => count === width
+  );
+
+  for (const verticalReflection of fullVerticalReflections) {
+    result += +verticalReflection[0] * 100;
+  }
+}
+
+console.log(result);
+
+function getReflections(lines) {
+  const height = lines.length;
+  const width = lines[0].length;
+
+  const reflections = {};
+  for (let y = 0; y < height; y++) {
+    const line = lines[y];
+
+    for (let x = 1; x < width; ++x) {
+      const left = line.slice(0, x);
+      const right = line.slice(x);
+
+      const minLength = Math.min(left.length, right.length);
+
+      const leftTrimmed = left.slice(-minLength);
+      const rightTrimmed = right.slice(0, minLength).reverse();
+
+      if (isEqual(leftTrimmed, rightTrimmed)) {
+        reflections[x] ??= 0;
+        reflections[x]++;
+      }
+    }
+  }
+
+  return reflections;
+}
+
+function isEqual(a, b) {
+  return a.length === b.length && a.every((c, i) => c === b[i]);
+}
+
+function transpose(arr) {
+  return arr[0].map((_, colIndex) => arr.map((row) => row[colIndex]));
+}

13a/input0.txt

@@ -0,0 +1,15 @@
+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#