Solutions to Advent of Code 2020 Day 12 Part 2

AOC_2020_Day12_Part2.js

const { parseInputFile } = require('../utils/parser');

const inputSteps = parseInputFile('day12-input.txt', /^(\w)(\d+)$/gm)
    .map(([, action, num]) => ({
        action,
        amount: parseInt(num)
    }));

// +east -west
// +north -south
// 0north 90east 180south 270 west
let shipX = 0;
let shipY = 0;
let wayX = 10;
let wayY = 1;

function rotateWaypoint(amount) {
    while (amount > 360) {
        amount -= 360;
    }

    while (amount < 0) {
        amount += 360;
    }
    
    switch (amount) {
        case 0:
        case 360:
            return;
        case 90: {
            const tmpY = wayY;
            wayY = -1 * wayX;
            wayX = tmpY;
            break;
        }
        case 180: {
            wayY = -1 * wayY;
            wayX = -1 * wayX;
            break;
        }
        case 270: {
            const tmpY = wayY;
            wayY = wayX;
            wayX = -1 * tmpY;
            break;
        }
        default:
            throw new Error(`Unknown rotation: ${ amount }`);
    }
}

for (const step of inputSteps) {
    switch (step.action) {
        case 'N':
            wayY += step.amount;
            break;
        case 'S':
            wayY -= step.amount;
            break;
        case 'E':
            wayX += step.amount;
            break;
        case 'W':
            wayX -= step.amount;
            break;
        case 'L':
            rotateWaypoint(-1 * step.amount);
            break;
        case 'R':
            rotateWaypoint(step.amount);
            break;
        case 'F':
            shipX += (wayX * step.amount);
            shipY += (wayY * step.amount);
            break;
        default:
            throw new Error('Invalid step');
    }
}

const manhattanDistance = Math.abs(shipX) + Math.abs(shipY);
console.log(`Part 2: Manhattan distance of trip is ${ manhattanDistance }`);