day 4 initial solution

src/2023/day04.js

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+function parse(input) {
+  return input.split('\n').map(line => {
+    const [, numbers] = line.split(':');
+    const [left, right] = numbers
+      .split('|')
+      .map(s => s.trim().split(/\s+/).map(Number));
+    return right.filter(n => left.includes(n)).length;
+  });
+}
+
+export function part1(input) {
+  const cards = parse(input);
+  return cards
+    .map(x => (x > 0 ? Math.pow(2, x - 1) : 0))
+    .reduce((a, b) => a + b, 0);
+}
+
+export function part2(input) {
+  const cards = parse(input);
+  const seen = {};
+  for (let i = 0; i < cards.length; i++) {
+    seen[i] = 1;
+  }
+  for (let i = 0; i < cards.length; i++) {
+    for (let j = 1; j <= cards[i]; j++) {
+      seen[i + j] += seen[i];
+    }
+  }
+  return Object.values(seen).reduce((a, b) => a + b, 0);
+}

src/2023/day04.spec.js

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+import { part1, part2 } from './day04.js';
+import readInput from '../utils/read-input.js';
+
+const input = readInput(import.meta.url);
+
+describe('day04 2023', () => {
+  describe('part1', () => {
+    it('should work for part 1 examples', () => {
+      expect(
+        part1(
+          [
+            'Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53',
+            'Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19',
+            'Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1',
+            'Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83',
+            'Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36',
+            'Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11',
+          ].join('\n'),
+        ),
+      ).toEqual(13);
+    });
+
+    it('should work for part 1 input', () => {
+      expect(part1(input)).toEqual(15205);
+    });
+  });
+
+  describe('part2', () => {
+    it('should work for part 2 examples', () => {
+      expect(
+        part2(
+          [
+            'Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53',
+            'Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19',
+            'Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1',
+            'Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83',
+            'Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36',
+            'Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11',
+          ].join('\n'),
+        ),
+      ).toEqual(30);
+    });
+
+    it('should work for part 2 input', () => {
+      expect(part2(input)).toEqual(6189740);
+    });
+  });
+});