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Merge pull request #207 from amclin/feat/2021-day-09
Feat/2021 day 09
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‎2021/day-09/basins.js

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const isLow = (val, col, row, rows) => {
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// Collect points in each cardinal direction
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const points = []
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// TODO: If supporting diagonal checks, use this logic instead to loop
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// for (let x = -1; x <= 1; x++) {
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// for (let y = -1; y <= 1; y++) {
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// if(x != 0 && y != 0)
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// if(rows[row + y] && rows[row + y][col + x]
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// }
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// }
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if (rows[row - 1] && rows[row - 1][col]) { points.push(parseInt(rows[row - 1][col])) }
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if (rows[row + 1] && rows[row + 1][col]) { points.push(parseInt(rows[row + 1][col])) }
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if (rows[row] && rows[row][col - 1]) { points.push(parseInt(rows[row][col - 1])) }
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if (rows[row] && rows[row][col + 1]) { points.push(parseInt(rows[row][col + 1])) }
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// NOTE - if the value is the same as a neighbor,
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// that isn't counted as a low (even though together, they can be a low)
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// ... this might be a concern for part 2 ....
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return (val < Math.min(...points)) // value should be lower than any other points
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}
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const findLocalLows = (data) => {
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const lows = []
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const rows = data.split('\n')
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let checked = 0
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rows.forEach((row, rowIdx) => {
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for (let c = 0; c < row.length; c++) {
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const cell = parseInt(row[c])
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if (isLow(cell, c, rowIdx, rows)) {
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lows.push(cell)
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console.debug(`Found low at ${c},${rowIdx}: ${cell}`)
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}
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checked++
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}
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})
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console.debug(`Checked ${checked} cells`)
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return lows
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}
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const flow = (col, row, map, data, source) => {
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// Don't test invalid points
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if (col < 0 || col >= map.coords[0].length) {
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console.debug(`${col},${row} is out of bounds`)
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// Exceeds map horizontally
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return {
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map,
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result: false
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}
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}
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if (row < 0 || row >= map.coords.length) {
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console.debug(`${col},${row} is out of bounds`)
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// Exceeds map vertically
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return {
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map,
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result: false
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}
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}
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// If the point is a peak, register and don't continue
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if (parseInt(data[row][col]) === 9) {
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console.debug(`${col},${row} is a peak.`)
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// Peaks aren't part of basins
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map.coords[row][col] = 'p'
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return {
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map,
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result: false
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}
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}
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// If the point is higher than the source, we can't drain
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// BIG ASSUMPTION here about equal-height points
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if (data[row][col] >= source) {
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console.debug(`${col},${row} is higher (${data[row][col]} >= ${source}). Water can't flow uphill.`)
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return {
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map,
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result: false
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}
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}
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// If the point already mapped to a basin, don't recalculate its flow
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if (map.coords[row] && map.coords[row][col]) {
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console.debug(`${col},${row} is already known to be in basin ${map.coords[row][col]}`)
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return {
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map,
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result: map.coords[row][col]
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}
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}
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// If we've reached a low point, stop tracing
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if (isLow(data[row][col], col, row, data)) {
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console.debug(`${col},${row} is a low point in basin.`)
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// register a basin with an area of 1
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map.basins.push(1)
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// mark the low point to the basin
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map.coords[row][col] = map.basins.length - 1
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console.debug(`registered basin ${map.basins.length - 1}`)
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return {
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map,
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result: map.coords[row][col]
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}
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// HUGE ASSUMPTION that each basin only has 1 low point
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}
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console.debug(`checking where point ${col},${row} drains to`)
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// Check the next points in each cardinal direction
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const drains = []
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let result = false
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result = flow(col + 1, row, map, data, data[row][col]) // right
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map = result.map
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drains.push(result.result)
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result = flow(col - 1, row, map, data, data[row][col]) // left
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map = result.map
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drains.push(result.result)
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result = flow(col, row - 1, map, data, data[row][col]) // up
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map = result.map
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drains.push(result.result)
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result = flow(col, row + 1, map, data, data[row][col]) // down
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map = result.map
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drains.push(result.result)
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const results = drains.filter((c) => c !== false)
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if (results.length > 1) {
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console.warn('Point has more than one possilbe drain.')
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const uniqueDrains = [...new Set(results)]
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if (uniqueDrains.length > 1) {
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console.debug(drains)
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throw new Error('Point drains into multiple drains. Data might be bad.')
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}
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// Otherwise, all drains go to the same basin, so that's the same as having 1 drain
133+
}
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if (results.length === 0) {
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console.debug(drains)
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throw new Error('Point is not the low, but has no drains. Data might be bad.')
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}
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const basin = parseInt(results[0])
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// Mark the point as belonging to the basin it drains into
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map.coords[row][col] = basin
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// Track the area of the basin so we don't have to recalculate it later
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map.basins[basin]++
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// return the findings recursively
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return {
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map,
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result: map.coords[row][col]
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}
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}
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module.exports = {
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findLocalLows,
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flow
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}

‎2021/day-09/basins.test.js

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/* eslint-env mocha */
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const { expect } = require('chai')
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const { findLocalLows } = require('./basins')
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const testData = `2199943210
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3987894921
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9856789892
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8767896789
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9899965678`
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describe('--- Day 9: Smoke Basin ---', () => {
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describe('Part 1', () => {
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describe('findLocalLows()', () => {
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it('finds the local low points in the terrain', () => {
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// low points in the testData are 1, 0 on row 0
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// 5 on row 2
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// 5 on row 5
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const results = findLocalLows(testData)
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expect(results.length).to.equal(4)
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expect(results.reduce((a, b) => a + b)).to.equal(11)
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})
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})
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})
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})

‎2021/day-09/index.js

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// eslint-disable-next-line no-unused-vars
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const console = require('../helpers')
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require('./solution')

‎2021/day-09/input.txt

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‎2021/day-09/solution.js

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const fs = require('fs')
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const path = require('path')
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const filePath = path.join(__dirname, 'input.txt')
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const { linesToArray } = require('../../2018/inputParser')
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const { findLocalLows, flow } = require('./basins')
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fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
8+
if (err) throw err
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initData = linesToArray(initData.trim())
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const resetInput = () => {
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// Deep copy to ensure we aren't mutating the original data
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return JSON.parse(JSON.stringify(initData))
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}
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const part1 = () => {
18+
const data = resetInput().join('\n')
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const lows = findLocalLows(data)
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console.debug(lows.length)
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// risk profiles is the sum of each height + 1, which is the same as adding the count of lows
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return lows.reduce((a, b) => a + b) + lows.length
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}
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const part2 = () => {
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const data = resetInput()
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let map = {
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coords: [...Array(data.length)].map(() => Array(data[0].length)), // allocate a map
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basins: [0] // fake first basin with no area to avoid having to figure out JS loose-type false/truthy and <> bullshit
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}
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for (let x = 0; x < data[0].length; x++) {
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for (let y = 0; y < data.length; y++) {
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console.debug(`Starting flow trace at ${x},${y}`)
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map = flow(x, y, map, data, 9).map // 9 is a magic number for assuming we start with fresh drains
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}
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}
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console.debug(`Found ${map.basins.length} basins.`)
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// Multiply the area of the 3 largest basins
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return map.basins.sort((a, b) => a - b)
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.slice(map.basins.length - 3)
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.reduce((a, b) => a * b)
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}
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const answers = []
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answers.push(part1())
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answers.push(part2())
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answers.forEach((ans, idx) => {
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console.info(`-- Part ${idx + 1} --`)
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console.info(`Answer: ${ans}`)
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})
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})

‎index.js

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1 +1 @@
1-
require('./2021/day-08/solution')
1+
require('./2021/day-09/solution')

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