diff --git a/2021/day-09/basins.js b/2021/day-09/basins.js
new file mode 100644
index 0000000..3e2a4c1
--- /dev/null
+++ b/2021/day-09/basins.js
@@ -0,0 +1,156 @@
+const isLow = (val, col, row, rows) => {
+ // Collect points in each cardinal direction
+ const points = []
+ // TODO: If supporting diagonal checks, use this logic instead to loop
+ // for (let x = -1; x <= 1; x++) {
+ // for (let y = -1; y <= 1; y++) {
+ // if(x != 0 && y != 0)
+ // if(rows[row + y] && rows[row + y][col + x]
+ // }
+ // }
+ if (rows[row - 1] && rows[row - 1][col]) { points.push(parseInt(rows[row - 1][col])) }
+ if (rows[row + 1] && rows[row + 1][col]) { points.push(parseInt(rows[row + 1][col])) }
+ if (rows[row] && rows[row][col - 1]) { points.push(parseInt(rows[row][col - 1])) }
+ if (rows[row] && rows[row][col + 1]) { points.push(parseInt(rows[row][col + 1])) }
+
+ // NOTE - if the value is the same as a neighbor,
+ // that isn't counted as a low (even though together, they can be a low)
+ // ... this might be a concern for part 2 ....
+ return (val < Math.min(...points)) // value should be lower than any other points
+}
+
+const findLocalLows = (data) => {
+ const lows = []
+ const rows = data.split('\n')
+ let checked = 0
+
+ rows.forEach((row, rowIdx) => {
+ for (let c = 0; c < row.length; c++) {
+ const cell = parseInt(row[c])
+ if (isLow(cell, c, rowIdx, rows)) {
+ lows.push(cell)
+ console.debug(`Found low at ${c},${rowIdx}: ${cell}`)
+ }
+ checked++
+ }
+ })
+
+ console.debug(`Checked ${checked} cells`)
+ return lows
+}
+
+const flow = (col, row, map, data, source) => {
+ // Don't test invalid points
+ if (col < 0 || col >= map.coords[0].length) {
+ console.debug(`${col},${row} is out of bounds`)
+ // Exceeds map horizontally
+ return {
+ map,
+ result: false
+ }
+ }
+ if (row < 0 || row >= map.coords.length) {
+ console.debug(`${col},${row} is out of bounds`)
+ // Exceeds map vertically
+ return {
+ map,
+ result: false
+ }
+ }
+
+ // If the point is a peak, register and don't continue
+ if (parseInt(data[row][col]) === 9) {
+ console.debug(`${col},${row} is a peak.`)
+ // Peaks aren't part of basins
+ map.coords[row][col] = 'p'
+ return {
+ map,
+ result: false
+ }
+ }
+
+ // If the point is higher than the source, we can't drain
+ // BIG ASSUMPTION here about equal-height points
+ if (data[row][col] >= source) {
+ console.debug(`${col},${row} is higher (${data[row][col]} >= ${source}). Water can't flow uphill.`)
+ return {
+ map,
+ result: false
+ }
+ }
+
+ // If the point already mapped to a basin, don't recalculate its flow
+ if (map.coords[row] && map.coords[row][col]) {
+ console.debug(`${col},${row} is already known to be in basin ${map.coords[row][col]}`)
+ return {
+ map,
+ result: map.coords[row][col]
+ }
+ }
+
+ // If we've reached a low point, stop tracing
+ if (isLow(data[row][col], col, row, data)) {
+ console.debug(`${col},${row} is a low point in basin.`)
+ // register a basin with an area of 1
+ map.basins.push(1)
+ // mark the low point to the basin
+ map.coords[row][col] = map.basins.length - 1
+ console.debug(`registered basin ${map.basins.length - 1}`)
+ return {
+ map,
+ result: map.coords[row][col]
+ }
+ // HUGE ASSUMPTION that each basin only has 1 low point
+ }
+
+ console.debug(`checking where point ${col},${row} drains to`)
+
+ // Check the next points in each cardinal direction
+ const drains = []
+ let result = false
+ result = flow(col + 1, row, map, data, data[row][col]) // right
+ map = result.map
+ drains.push(result.result)
+ result = flow(col - 1, row, map, data, data[row][col]) // left
+ map = result.map
+ drains.push(result.result)
+ result = flow(col, row - 1, map, data, data[row][col]) // up
+ map = result.map
+ drains.push(result.result)
+ result = flow(col, row + 1, map, data, data[row][col]) // down
+ map = result.map
+ drains.push(result.result)
+
+ const results = drains.filter((c) => c !== false)
+ if (results.length > 1) {
+ console.warn('Point has more than one possilbe drain.')
+ const uniqueDrains = [...new Set(results)]
+ if (uniqueDrains.length > 1) {
+ console.debug(drains)
+ throw new Error('Point drains into multiple drains. Data might be bad.')
+ }
+ // Otherwise, all drains go to the same basin, so that's the same as having 1 drain
+ }
+ if (results.length === 0) {
+ console.debug(drains)
+ throw new Error('Point is not the low, but has no drains. Data might be bad.')
+ }
+
+ const basin = parseInt(results[0])
+
+ // Mark the point as belonging to the basin it drains into
+ map.coords[row][col] = basin
+ // Track the area of the basin so we don't have to recalculate it later
+ map.basins[basin]++
+
+ // return the findings recursively
+ return {
+ map,
+ result: map.coords[row][col]
+ }
+}
+
+module.exports = {
+ findLocalLows,
+ flow
+}
diff --git a/2021/day-09/basins.test.js b/2021/day-09/basins.test.js
new file mode 100644
index 0000000..e3486a7
--- /dev/null
+++ b/2021/day-09/basins.test.js
@@ -0,0 +1,24 @@
+/* eslint-env mocha */
+const { expect } = require('chai')
+const { findLocalLows } = require('./basins')
+
+const testData = `2199943210
+3987894921
+9856789892
+8767896789
+9899965678`
+
+describe('--- Day 9: Smoke Basin ---', () => {
+ describe('Part 1', () => {
+ describe('findLocalLows()', () => {
+ it('finds the local low points in the terrain', () => {
+ // low points in the testData are 1, 0 on row 0
+ // 5 on row 2
+ // 5 on row 5
+ const results = findLocalLows(testData)
+ expect(results.length).to.equal(4)
+ expect(results.reduce((a, b) => a + b)).to.equal(11)
+ })
+ })
+ })
+})
diff --git a/2021/day-09/index.js b/2021/day-09/index.js
new file mode 100644
index 0000000..af7e035
--- /dev/null
+++ b/2021/day-09/index.js
@@ -0,0 +1,3 @@
+// eslint-disable-next-line no-unused-vars
+const console = require('../helpers')
+require('./solution')
diff --git a/2021/day-09/input.txt b/2021/day-09/input.txt
new file mode 100644
index 0000000..44432ed
--- /dev/null
+++ b/2021/day-09/input.txt
@@ -0,0 +1,100 @@
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diff --git a/2021/day-09/solution.js b/2021/day-09/solution.js
new file mode 100644
index 0000000..b80022f
--- /dev/null
+++ b/2021/day-09/solution.js
@@ -0,0 +1,54 @@
+const fs = require('fs')
+const path = require('path')
+const filePath = path.join(__dirname, 'input.txt')
+const { linesToArray } = require('../../2018/inputParser')
+const { findLocalLows, flow } = require('./basins')
+
+fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
+ if (err) throw err
+
+ initData = linesToArray(initData.trim())
+
+ const resetInput = () => {
+ // Deep copy to ensure we aren't mutating the original data
+ return JSON.parse(JSON.stringify(initData))
+ }
+
+ const part1 = () => {
+ const data = resetInput().join('\n')
+ const lows = findLocalLows(data)
+ console.debug(lows.length)
+ // risk profiles is the sum of each height + 1, which is the same as adding the count of lows
+ return lows.reduce((a, b) => a + b) + lows.length
+ }
+
+ const part2 = () => {
+ const data = resetInput()
+ let map = {
+ coords: [...Array(data.length)].map(() => Array(data[0].length)), // allocate a map
+ basins: [0] // fake first basin with no area to avoid having to figure out JS loose-type false/truthy and <> bullshit
+ }
+
+ for (let x = 0; x < data[0].length; x++) {
+ for (let y = 0; y < data.length; y++) {
+ console.debug(`Starting flow trace at ${x},${y}`)
+ map = flow(x, y, map, data, 9).map // 9 is a magic number for assuming we start with fresh drains
+ }
+ }
+
+ console.debug(`Found ${map.basins.length} basins.`)
+
+ // Multiply the area of the 3 largest basins
+ return map.basins.sort((a, b) => a - b)
+ .slice(map.basins.length - 3)
+ .reduce((a, b) => a * b)
+ }
+ const answers = []
+ answers.push(part1())
+ answers.push(part2())
+
+ answers.forEach((ans, idx) => {
+ console.info(`-- Part ${idx + 1} --`)
+ console.info(`Answer: ${ans}`)
+ })
+})
diff --git a/index.js b/index.js
index 23dbf0d..f4abd84 100644
--- a/index.js
+++ b/index.js
@@ -1 +1 @@
-require('./2021/day-08/solution')
+require('./2021/day-09/solution')
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