part2.js

const path = require("path");
const fs = require("fs");

const data = fs.readFileSync(path.resolve(__dirname, '../input.txt'), 'utf-8');
const input = data.split("\r\n");


const leftWall = 0;
const rightWall = 8;
const shapes = [
    { points: [[0, 0], [1, 0], [2, 0], [3, 0]], left: [[0, 0]], right: [[3, 0]], down: [[0, 0], [1, 0], [2, 0], [3, 0]], width: 4, height: 1 },
    { points: [[0, 1], [1, 1], [2, 1], [1, 0], [1, 2]], left: [[0, 1], [1, 0], [1, 2]], right: [[1, 0], [2, 1], [1, 2]], down: [[0, 1], [1, 0], [2, 1]], width: 3, height: 3 },
    { points: [[0, 0], [1, 0], [2, 0], [2, 1], [2, 2]], left: [[0, 0], [2, 1], [2, 2]], right: [[2, 0], [2, 1], [2, 2]], down: [[0, 0], [1, 0], [2, 0]], width: 3, height: 3 },
    { points: [[0, 0], [0, 1], [0, 2], [0, 3]], left: [[0, 0], [0, 1], [0, 2], [0, 3]], right: [[0, 0], [0, 1], [0, 2], [0, 3]], down: [[0, 0]], width: 1, height: 4 },
    { points: [[0, 0], [0, 1], [1, 0], [1, 1]], left: [[0, 0], [0, 1]], right: [[1, 0], [1, 1]], down: [[0, 0], [1, 0]], width: 2, height: 2 },
]
const moves = input[0];
const tetris = new Set(["1,0", "2,0", "3,0", "4,0", "5,0", "6,0", "7,0"]);
let currentShape = 0;
let currentMove = 0;
let top = 0;

// only works for this input
// I found at which heights and number of rocks the cycle repeats manually looking through console.log

console.log(dropRocks(285)); // drop 285 rocks, height is 447
console.log(dropRocks(1715)); // after every 1715 rocks dropped, height increases by 2574

let dropRocksTimes = Math.floor((1000000000000 - 285) / 1715);
let leftover = (1000000000000 - 285) % 1715; // remainder at the end
let increaseHeight = 2574 * dropRocksTimes;

console.log(dropRocks(leftover)); // height increased by 2168 at the end

// final result
console.log(447 + increaseHeight + 3191);

function dropRocks(r) {
    let rocks = 0;
    while (rocks < r) {
        const shape = shapes[currentShape % shapes.length];
        const shapePos = [leftWall + 3, top + 4];

        falling: while (true) {
            // check right
            let move = 0;
            if (moves[currentMove % moves.length] === '>' && shapePos[0] + shape.width < rightWall) {
                move = 1;
                for (const point of shape.right) {
                    if (tetris.has(`${shapePos[0] + point[0] + 1},${shapePos[1] + point[1]}`)) {
                        move = 0;
                        break;
                    }
                }
            }
            // check left
            else if (moves[currentMove % moves.length] === '<' && shapePos[0] - 1 > leftWall) {
                move = -1;
                for (const point of shape.left) {
                    if (tetris.has(`${shapePos[0] + point[0] - 1},${shapePos[1] + point[1]}`)) {
                        move = 0;
                        break;
                    }
                }
            }
            shapePos[0] += move;
            currentMove++;

            // check down
            for (const point of shape.down) {
                if (tetris.has(`${shapePos[0] + point[0]},${shapePos[1] + point[1] - 1}`)) {
                    break falling;
                }
            }
            shapePos[1]--;
        }

        for (const point of shape.points) {
            tetris.add(`${shapePos[0] + point[0]},${shapePos[1] + point[1]}`);
        }
        if (shapePos[1] + shape.height - 1 > top) {
            top = shapePos[1] + shape.height - 1;
        }

        currentShape++;
        rocks++;

        // 285 rocks dropped to height 447, then 1715 rocks are dropped from height 448 to 3021
        // if(top === 447 || top === 448 || top === 3021 || top === 3040 || top === 5613) {
        //     console.log(`Height: ${top} - Rocks: ${rocks} - Shape: ${currentShape % shapes.length} - Move: ${moves[currentMove % moves.length]}`);
        // }
    }

    return top;
}