aoc21/09/main.js

const fs = require("fs")

const input = fs.readFileSync("input.txt", "utf-8", err => {
    err && console.error(err)
}).split("\n").filter(str => str.length).map(str => str.split("").map(num => parseInt(num)))

const getNeighbours = (rowNum, colNum) => {
    return [
        input[rowNum][colNum - 1] < 9 && [rowNum, colNum - 1],
        input[rowNum][colNum + 1] < 9 && [rowNum, colNum + 1],
        input[rowNum - 1] && input[rowNum - 1][colNum] < 9 && [rowNum - 1, colNum],
        input[rowNum + 1] && input[rowNum + 1][colNum] < 9 && [rowNum + 1, colNum],
    ].filter(point => point)
}

const equalArr = (a, b) => a.length === b.length && a.every((v, i) => v === b[i])

const basins = []
let sum = 0

for (const [rowNum, row] of input.entries()){
    for (const [colNum, height] of row.entries()){
        let previousColHeight = row[colNum-1]
        if (typeof(previousColHeight) === "undefined") {
            previousColHeight = 9
        }
        if (height < previousColHeight){
            const nextColHeight = row[colNum+1]
            const previousRowHeight = input[rowNum - 1] && input[rowNum - 1][colNum]
            const nextRowHeight = input[rowNum + 1] && input[rowNum + 1][colNum]

            if (   (typeof(nextColHeight) != "undefined" && nextColHeight < height)
                || (typeof(previousRowHeight) != "undefined" && previousRowHeight < height)
                || (typeof(nextRowHeight) != "undefined" && nextRowHeight < height))
                continue

            basins.push([rowNum, colNum])
            sum += height
        }
    }
}

const basinSizes = []

for (const basin of basins) {
    let neighbours = [basin]
    for (const neighbour of neighbours){
        const next = getNeighbours(neighbour[0], neighbour[1])
        const nonDupes = next.filter(n => {
            for (let d of neighbours){
                if (equalArr(d, n))
                    return false
            }
            return true
        })
        neighbours.push(...nonDupes);
    }
    basinSizes.push(neighbours.length)
}

basinSizes.sort((a, b) => b - a)
console.log(sum + basins.length)
console.log(basinSizes[0] * basinSizes[1] * basinSizes[2])