adventofcode/2021/09/part2.mjs

import fs from "fs-extra";
import fp from "lodash/fp.js";
import _ from "lodash";

import * as p1 from "./part1.mjs";

function getBasinSize([x, y], grid, seen) {
    if (seen[x][y]) return 0;

    seen[x][y] = true;

    if (grid[x][y] == 9) return 0;

    const neighbours = p1.genNeighbours(grid.length, grid[0].length);

    return _.sum([
        1,
        ...neighbours(x, y).map((n) => getBasinSize(n, grid, seen)),
    ]);
}

export function solution(grid) {
    const seen = grid.map(() => []);
    let basinSizes = [];

    for (const x of _.range(grid.length)) {
        for (const y of _.range(grid[x].length)) {
            basinSizes.push(getBasinSize([x, y], grid, seen));
        }
    }

    return basinSizes
        .sort((a, b) => (a > b ? 1 : -1))
        .splice(-3)
        .reduce(_.multiply);
}