Day Eight: Part Two

src/day_eight.test.ts

@@ -1,4 +1,4 @@
-import {DesertMap} from "./day_eight";
+import {DesertMap, lcm} from "./day_eight";
 
 describe('Day Eight', () => {
 	const input = `RL
@@ -17,6 +17,17 @@ AAA = (BBB, BBB)
 BBB = (AAA, ZZZ)
 ZZZ = (ZZZ, ZZZ)`
 
+	const ghostInput = `LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)`
+
 	it('should calculate the number of steps needed to reach ZZZ', () => {
 		const map = new DesertMap(input);
 		expect(map.stepsTo('ZZZ')).toEqual(2);
@@ -25,5 +36,16 @@ ZZZ = (ZZZ, ZZZ)`
 	it('should repeat the pattern', () => {
 		const map = new DesertMap(repeatedInput);
 		expect(map.stepsTo('ZZZ')).toEqual(6);
-	})
+	});
+
+	it('should count how many steps it takes to get from every node beginning with A, to every node ending in Z simultaneously', () => {
+		const map = new DesertMap(ghostInput);
+		expect(map.ghostStepsToZ()).toEqual(6);
+	});
+
+	describe('lcm', () => {
+		it('should get the correct lowest common multiple', () => {
+			expect(lcm(12, 30)).toEqual(60);
+		});
+	});
 });

src/day_eight.ts

@@ -1,10 +1,11 @@
-import {anyCharOf, newline, uniLetter, whitespace} from "parjs";
+import {anyCharOf, int, newline, uniDecimal, uniLetter, whitespace} from "parjs";
-import {between, exactly, manySepBy, manyTill, stringify, then} from "parjs/combinators";
+import {between, exactly, manySepBy, manyTill, or, stringify, then} from "parjs/combinators";
 import fs from "fs";
+import { Set } from 'immutable';
 
 const patternParser = anyCharOf("LR").pipe(manyTill(newline().pipe(exactly(2))));
 
-const nodeNameParser = uniLetter().pipe(exactly(3), stringify());
+const nodeNameParser = uniLetter().pipe(or(uniDecimal()), exactly(3), stringify());
 const childParser = nodeNameParser.pipe(manySepBy(", "), exactly(2), between("(", ")"));
 const nodeParser = nodeNameParser.pipe(then(childParser.pipe(between(" = ", whitespace()))))
 
@@ -16,6 +17,16 @@ type Instruction = "L" | "R";
 type NodeName = string;
 type NodeChildren = [Maybe<NodeName>, Maybe<NodeName>];
 
+const gcd = (a: number, b: number): number => {
+	if (b === 0) return a;
+	return gcd(b, a % b);
+}
+
+export const lcm = (a: number, b: number): number => {
+	const product = a * b;
+	return product / gcd(a, b);
+}
+
 export class DesertMap {
 	private readonly pattern: Instruction[];
 
@@ -30,7 +41,7 @@ export class DesertMap {
 			if (!this.map[name]) {
 				this.map[name] = [undefined, undefined];
 			}
-			const children = [leftNode !== name ? leftNode : undefined, rightNode !== name ? rightNode : undefined];
+			const children = [leftNode, rightNode];
 			this.map[name] = children as NodeChildren;
 		}
 	}
@@ -56,11 +67,46 @@ export class DesertMap {
 		}
 		return step;
 	}
+
+	public stepsToZ(from: string): number {
+		let step = 0;
+		let curr = from;
+
+		while (!curr.endsWith('Z')) {
+			const instruction = this.pattern[step % this.pattern.length];
+
+			const [left, right] = this.map[curr];
+
+			if (instruction === "L" && left) {
+				curr = left;
+			} else if (instruction === "R" && right) {
+				curr = right;
 			}
 
+			if (!curr) return 0;
+
+			step++;
+		}
+		return step;
+	}
+
+	private isComplete(keys: string[]) {
+		return keys.every(key => key.endsWith('Z'));
+	}
+
+	public ghostStepsToZ(): number {
+		let keys = Object.keys(this.map).filter(key => key.endsWith('A'));
+
+		return keys.map(key => this.stepsToZ(key)).reduce(lcm, 1);
+	}
+}
+
+
 export const runDayEight = () => {
 	const input = fs.readFileSync('./inputs/day_eight_input.txt', 'utf-8').trimEnd();
 
 	const map = new DesertMap(input);
 	console.log(map.stepsTo('ZZZ'));
+
+	console.log(map.ghostStepsToZ());
 }