2021.13

challenges/2021/13.js

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+// Part 1
+const solvePuzzle1 = (input) => {
+  let [paper, instructions] = getPaperAndInstructions(parseInput(input));
+
+  return fold(instructions[0], paper)
+    .flat()
+    .reduce((acc, val) => acc + val);
+};
+
+// Part 2
+const solvePuzzle2 = (input) => {
+  let [paper, instructions] = getPaperAndInstructions(parseInput(input));
+  instructions.forEach((ins) => (paper = fold(ins, paper)));
+
+  return "\n" + paper.map((y) => y.reduce((acc, x) => (x ? acc + "#" : acc + " "), "")).join("\n");
+};
+
+const fold = ([dir, pos], paper) => {
+  if (dir === "y") {
+    const firstHalf = paper.slice(0, pos);
+    const secondHalf = paper.slice(pos + 1).reverse();
+    paper = [];
+
+    while (firstHalf.length > secondHalf.length) {
+      secondHalf.unshift(Array(firstHalf[0].length).fill(0));
+    }
+    while (firstHalf.length < secondHalf.length) {
+      firstHalf.unshift(Array(firstHalf[0].length).fill(0));
+    }
+
+    for (let i = 0; i < firstHalf.length; i++) {
+      paper.push([]);
+      for (let j = 0; j < firstHalf[i].length; j++) {
+        paper[i][j] = firstHalf[i][j] || secondHalf[i][j];
+      }
+    }
+  } else {
+    const firstHalf = Array.from({ length: paper.length }, () => []);
+    const secondHalf = Array.from({ length: paper.length }, () => []);
+
+    for (let i = 0; i < paper.length; i++) {
+      for (let j = 0; j < pos; j++) {
+        firstHalf[i].push(paper[i][j]);
+      }
+      for (let j = paper[i].length - 1; j > pos; j--) {
+        secondHalf[i].push(paper[i][j]);
+      }
+    }
+
+    paper = Array.from({ length: paper.length }, () => []);
+    for (let i = 0; i < firstHalf.length; i++) {
+      for (let j = 0; j < Math.max(firstHalf[i].length, secondHalf[i].length); j++) {
+        paper[i].push(Math.max(firstHalf[i][j] || secondHalf[i][j]));
+      }
+    }
+  }
+  return paper;
+};
+
+const getPaperAndInstructions = (input) => {
+  const [dots, instructions] = [[], []];
+  let [maxX, maxY] = [0, 0];
+
+  for (let line of input) {
+    if (line[0] === "f") {
+      const [direction, pos] = line.replace("fold along ", "").split("=");
+      instructions.push([direction, Number(pos)]);
+    } else {
+      const [x, y] = line.split(",").map((n) => Number(n));
+      dots.push([x, y]);
+      [maxX, maxY] = [Math.max(maxX, x), Math.max(maxY, y)];
+    }
+  }
+
+  const paper = Array.from({ length: maxY + 1 }, () => Array(maxX + 1).fill(0));
+  dots.forEach(([x, y]) => (paper[y][x] = 1));
+  return [paper, instructions];
+};
+
+const parseInput = (input) => input.split("\n").filter((x) => x !== "");
+
+require(__dirname + "/../../utils/test.js").test(__filename, __dirname, solvePuzzle1);
+require(__dirname + "/../../utils/test.js").test(__filename, __dirname, solvePuzzle2);