Add day 9 part 2

Author: lundgren2

src/2020/day9/part2.ts

@@ -0,0 +1,23 @@
+import {part1} from './part1';
+
+export const part2 = (input: string, preamble: number) => {
+  const inputNumbers = input.split('\n').map(Number);
+  const invalidNumber = part1(input, preamble)!;
+  const contiguousNumbers = [];
+
+  let accumulator = 0;
+  for (const current of inputNumbers) {
+    accumulator = contiguousNumbers.reduce((a, b) => a + b, 0);
+
+    if (accumulator === invalidNumber) {
+      return Math.min(...contiguousNumbers) + Math.max(...contiguousNumbers);
+    }
+
+    // Make room for current number
+    while (accumulator + current > invalidNumber) {
+      accumulator -= contiguousNumbers.shift()!;
+    }
+
+    contiguousNumbers.push(current);
+  }
+};

src/2020/day9/test.spec.ts

@@ -1,6 +1,7 @@
 import {readInput} from '../../utils';
 
 import {part1} from './part1';
+import {part2} from './part2';
 
 describe('Advent of Code 2020 - Day 9', () => {
   let input: string;
@@ -37,4 +38,14 @@ describe('Advent of Code 2020 - Day 9', () => {
       expect(part1(input, 25)).toBe(50047984);
     });
   });
+
+  describe('part 2', () => {
+    it('should output 62', () => {
+      expect(part2(testInput, 5)).toBe(62);
+    });
+
+    it('should output 5407707 from input', () => {
+      expect(part2(input, 25)).toBe(5407707);
+    });
+  });
 });