AoC/2024/16/index.ts

const fs = require('fs');

const input = fs.readFileSync(__dirname + '/input.txt', 'utf8');

interface Coord {
    x: number,
    y: number,
};

enum Dir {
    Up,
    Right,
    Down,
    Left,
};

const map: Array<Array<string>> = input.split("\n").map((row: string) => row.split(''));

const width = map[0].length;
const height = map.length;

const snakeIndex = input.indexOf('S');
let snakePos = { x: snakeIndex % (width + 1), y: Math.floor(snakeIndex / height) };
let snakeDir = Dir.Right

function get(pos: Coord): string {
    return map[pos.y][pos.x];
}

function set(pos: Coord, val: string): void {
    map[pos.y][pos.x] = val;
}

function nextDir(dir: Dir): Dir {
    const directions = [Dir.Up, Dir.Right, Dir.Down, Dir.Left];
    const i = (directions.indexOf(dir) + 1) % directions.length;
    return directions[i];
}
function prevDir(dir: Dir): Dir {
    const directions = [Dir.Up, Dir.Right, Dir.Down, Dir.Left];
    const i = directions.indexOf(dir) - 1;
    if (i < 0) {
        return directions[directions.length - 1];
    }
    return directions[i];
}

function printMap() {
    const mapClone = map.slice().map((row: Array<string>) => row.slice());
    console.log(mapClone.map((row: Array<string>) => row.join('')).join("\n"));
}

function nextCoord(pos: Coord, direction: Dir): Coord {
    return direction === Dir.Up ? { x: pos.x, y: pos.y - 1 }
        : (direction === Dir.Right ? { x: pos.x + 1, y: pos.y }
            : (direction === Dir.Down ? { x: pos.x, y: pos.y + 1 } : { x: pos.x - 1, y: pos.y })
        );
}

function isDeadEnd(pos: Coord): boolean {
    const adjs = [Dir.Up, Dir.Right, Dir.Down, Dir.Left].map((dir) => get(nextCoord(pos, dir))).join('');
    return adjs === '###.'
        || adjs === '##.#'
        || adjs === '#.##'
        || adjs === '.###';
}

function isBranch(pos: Coord, dir: Dir): boolean {
    const leftDir = prevDir(dir);
    const rightDir = nextDir(dir);
    const next = nextCoord(pos, dir);
    const left = nextCoord(pos, leftDir);
    const right = nextCoord(pos, rightDir);
    const nextChar = get(next);
    const leftChar = get(left);
    const rightChar = get(right);
    return nextChar === '.' && leftChar === '.'
        || nextChar === '.' && rightChar === '.'
        || leftChar === '.' && rightChar === '.';
}

function fillDeadEnd(pos: Coord): void {
    set(pos, '#')
    for (let dir of [Dir.Up, Dir.Right, Dir.Down, Dir.Left]) {
        const next = nextCoord(pos, dir);
        if (get(next) === '.') {
            if (isDeadEnd(next)) {
                fillDeadEnd(next);
            }
        }
    };
}

for (let y = 1; y < height - 1; y++) {
    for (let x = 1; x < width - 1; x++) {
        const pos = { x, y };
        const char = get(pos);
        if (char !== '.') {
            continue;
        }
        if (isDeadEnd(pos)) {
            fillDeadEnd(pos);
        }
    }
}

let fastest: number|null = null;

const scoreCache: { [key: string]: [number, Coord, Dir] } = {};

const branchCache: { [key: string]: number } = {};

const scores: Array<[number, Array<string>]> = [];

function getScoresAfterPosition(pos: Coord, dir: Dir, currentScore = 0, path: Array<string> = [], lastBranch: [number, Coord]|null = null): void {
    if (fastest !== null && currentScore >= fastest) {
        return;
    }
    const branch = isBranch(pos, dir);
    if (branch) {
        const cacheKey = `${pos.x},${pos.y},${dir}`;
        const cache = branchCache[cacheKey];
        if (cache && cache < currentScore) {
            return;
        } else {
            branchCache[cacheKey] = currentScore;
        }
        if (lastBranch) {
            scoreCache[`${lastBranch[1].x},${lastBranch[1].y}`] = [currentScore - lastBranch[0], pos, dir];
        }
        lastBranch = [currentScore, pos];
    }
    [dir, nextDir(dir), prevDir(dir)].forEach((newDir, i) => {
        const next = nextCoord(pos, newDir);
        const nextStr = `${next.x},${next.y}`;
        if (!path.includes(nextStr)) {
            const add = i === 0 ? 1 : 1001;
            const score = currentScore + add;
            const nextChar = get(next);
            if (nextChar === 'E') {
                if (fastest === null || score < fastest) {
                    fastest = score;
                }
                lastBranch = null;
                scores.push([score, [...path, nextStr]]);
                return;
            } else if (nextChar === '.') {
                getScoresAfterPosition(next, newDir, score, [...path, nextStr], branch ? [score, next] : lastBranch);
            }
        }
    });

    return;
}

getScoresAfterPosition(snakePos, snakeDir);

const fastestScore = Math.min(...scores.map(([score]) => score));

let coordsOnPaths: Set<string> = new Set();
scores.forEach(([score, path]) => {
    if (score === fastestScore) {
        path.forEach((coord) => coordsOnPaths.add(coord));
    }
});

console.log(coordsOnPaths.size + 1);