Add day 10 part 2

Author: lundgren2

src/2020/day10/part2.ts

@@ -0,0 +1,24 @@
+const countDistinctWays = (input: number[]) => {
+  const lastIndex = input.length - 1;
+  const distinctWays = input.reduce(
+    (distinctWays, adapter, i) => {
+      // Compare current adapter with the three next neighbours and
+      // check if the joltage difference less or equal to three.
+      for (let j = i + 1; j < i + 4; j++) {
+        if (input[j] - adapter <= 3) {
+          distinctWays[j] += distinctWays[i];
+        }
+      }
+      return distinctWays;
+    },
+    [1, ...Array(lastIndex).fill(0)] // First is always valid
+  );
+  return distinctWays[lastIndex];
+};
+
+export const part2 = (input: string) => {
+  const adapters = [0, ...input.split('\n')].map(Number).sort((a, b) => a - b);
+  adapters.push(adapters[adapters.length - 1] + 3); // 3 higher than the highest-rated adapter.
+
+  return countDistinctWays(adapters);
+};

src/2020/day10/test.spec.ts

@@ -1,43 +1,15 @@
 import {readInput} from '../../utils';
 
 import {part1} from './part1';
+import {part2} from './part2';
 
 describe('Advent of Code 2020 - Day 10', () => {
   let input: string;
-  const testInput = `28
-33
-18
-42
-31
-14
-46
-20
-48
-47
-24
-23
-49
-45
-19
-38
-39
-11
-1
-32
-25
-35
-8
-17
-7
-9
-4
-2
-34
-10
-3`;
+  let testInput: string;
 
   beforeAll(async () => {
     input = await readInput(__dirname + '/input');
+    testInput = await readInput(__dirname + '/test_input');
   });
 
   describe('part 1', () => {
@@ -63,4 +35,29 @@ describe('Advent of Code 2020 - Day 10', () => {
       expect(part1(input)).toBe(2112);
     });
   });
+
+  describe('part 2', () => {
+    it('should output 8', () => {
+      const input = `16
+10
+15
+5
+1
+11
+7
+19
+6
+12
+4`;
+      expect(part2(input)).toBe(8);
+    });
+
+    it('should output 19208 ', () => {
+      expect(part2(testInput)).toBe(19208);
+    });
+
+    it('should output 3022415986688 from input', () => {
+      expect(part2(input)).toBe(3022415986688);
+    });
+  });
 });

src/2020/day10/test_input

@@ -0,0 +1,31 @@
+28
+33
+18
+42
+31
+14
+46
+20
+48
+47
+24
+23
+49
+45
+19
+38
+39
+11
+1
+32
+25
+35
+8
+17
+7
+9
+4
+2
+34
+10
+3