diff --git a/2021/day-06/fish.js b/2021/day-06/fish.js
new file mode 100644
index 0000000..f6f5cbc
--- /dev/null
+++ b/2021/day-06/fish.js
@@ -0,0 +1,75 @@
+
+let _fishes = []
+const NewFishAge = 8 // age of newly spawned fish
+const FishSpawnAge = 0 // age when the fish spawns
+const ResetFishAge = 6 // age of the original fish after spawning
+
+// allocate an empty big school
+const _newBigSchool = () => [...new Array(NewFishAge + 1)].map(() => 0)
+let _bigSchool = _newBigSchool()
+
+const ageFish = (age) => {
+ if (age > NewFishAge) { throw new Error('Fish is too young') }
+ if (age < FishSpawnAge) { throw new Error('Fish is too old') }
+ if (age === FishSpawnAge) { return ResetFishAge }
+ return age - 1
+}
+
+const spawn = (qty) => {
+ console.debug(`spawning ${qty} fish`)
+ const newFishes = [...new Array(qty)].map(() => NewFishAge)
+ _fishes.push(...newFishes)
+}
+
+const efficientSpawn = (qty) => {
+ console.debug(`spawning ${qty} fish`)
+ _bigSchool[NewFishAge] = qty
+}
+
+const school = {
+ get state () {
+ return _fishes
+ },
+ set state (state) {
+ _fishes = state
+ },
+
+ advance: () => {
+ // Calculate how many will spawn
+ const toSpawn = _fishes.filter((x) => x === FishSpawnAge).length
+ // Iterate each fish
+ _fishes = _fishes.map(ageFish)
+ // Spawn the new fish
+ spawn(toSpawn)
+ }
+}
+
+/**
+ * The efficient school doesn't track the position of the fish.
+ * It only cares about the total number of fish of each age
+ */
+const efficientSchool = {
+ get state () {
+ return _bigSchool
+ },
+ set state (state) {
+ _bigSchool = _newBigSchool()
+ state.forEach((fish) => { _bigSchool[fish]++ })
+ },
+ advance: () => {
+ // Calculate how many will spawn (age 0) and shift the age groups in one quick step
+ const toSpawn = _bigSchool.shift()
+ // Iterate old fish back to young since they're spawning
+ _bigSchool[ResetFishAge] += toSpawn
+ // Spawn the new fish
+ efficientSpawn(toSpawn)
+ }
+}
+
+module.exports = {
+ school,
+ efficientSchool,
+ ageFish,
+ spawn,
+ efficientSpawn
+}
diff --git a/2021/day-06/fish.test.js b/2021/day-06/fish.test.js
new file mode 100644
index 0000000..bfdb0be
--- /dev/null
+++ b/2021/day-06/fish.test.js
@@ -0,0 +1,141 @@
+/* eslint-env mocha */
+const { expect } = require('chai')
+const { school, ageFish, spawn, efficientSchool, efficientSpawn } = require('./fish')
+
+describe('--- Day 6: Lanternfish ---', () => {
+ describe('Part 1', () => {
+ beforeEach(() => {
+ // ensure flushed state
+ school.state = [3, 4, 3, 1, 2]
+ expect(school.state).to.deep.equal([3, 4, 3, 1, 2])
+ })
+ describe('spawn()', () => {
+ it('adds new fish to the end of the list', () => {
+ spawn(4)
+ expect(school.state).to.deep.equal([3, 4, 3, 1, 2, 8, 8, 8, 8])
+ })
+ })
+ describe('ageFish()', () => {
+ it('ages a particular fish', () => {
+ expect(ageFish(6)).to.equal(5)
+ expect(ageFish(5)).to.equal(4)
+ expect(ageFish(4)).to.equal(3)
+ expect(ageFish(3)).to.equal(2)
+ expect(ageFish(2)).to.equal(1)
+ expect(ageFish(1)).to.equal(0)
+ expect(ageFish(0)).to.equal(6)
+ expect(ageFish(8)).to.equal(7)
+ expect(ageFish(7)).to.equal(6)
+ })
+ it('throws an error if the fish is out of range', () => {
+ expect(() => { ageFish(9) }).to.throw('Fish is too young')
+ expect(() => { ageFish(-1) }).to.throw('Fish is too old')
+ })
+ })
+ describe('advance()', () => {
+ it('advances one day', () => {
+ school.state = [3, 4, 3, 1, 2]
+ school.advance()
+ expect(school.state).to.deep.equal([2, 3, 2, 0, 1])
+ school.advance()
+ expect(school.state).to.deep.equal([1, 2, 1, 6, 0, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([0, 1, 0, 5, 6, 7, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([6, 0, 6, 4, 5, 6, 7, 8, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([5, 6, 5, 3, 4, 5, 6, 7, 7, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([4, 5, 4, 2, 3, 4, 5, 6, 6, 7])
+ school.advance()
+ expect(school.state).to.deep.equal([3, 4, 3, 1, 2, 3, 4, 5, 5, 6])
+ school.advance()
+ expect(school.state).to.deep.equal([2, 3, 2, 0, 1, 2, 3, 4, 4, 5])
+ school.advance()
+ expect(school.state).to.deep.equal([1, 2, 1, 6, 0, 1, 2, 3, 3, 4, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([0, 1, 0, 5, 6, 0, 1, 2, 2, 3, 7, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([6, 0, 6, 4, 5, 6, 0, 1, 1, 2, 6, 7, 8, 8, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([5, 6, 5, 3, 4, 5, 6, 0, 0, 1, 5, 6, 7, 7, 7, 8, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([4, 5, 4, 2, 3, 4, 5, 6, 6, 0, 4, 5, 6, 6, 6, 7, 7, 8, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([3, 4, 3, 1, 2, 3, 4, 5, 5, 6, 3, 4, 5, 5, 5, 6, 6, 7, 7, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([2, 3, 2, 0, 1, 2, 3, 4, 4, 5, 2, 3, 4, 4, 4, 5, 5, 6, 6, 7])
+ school.advance()
+ expect(school.state).to.deep.equal([1, 2, 1, 6, 0, 1, 2, 3, 3, 4, 1, 2, 3, 3, 3, 4, 4, 5, 5, 6, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([0, 1, 0, 5, 6, 0, 1, 2, 2, 3, 0, 1, 2, 2, 2, 3, 3, 4, 4, 5, 7, 8])
+ school.advance()
+ expect(school.state).to.deep.equal([6, 0, 6, 4, 5, 6, 0, 1, 1, 2, 6, 0, 1, 1, 1, 2, 2, 3, 3, 4, 6, 7, 8, 8, 8, 8])
+ })
+ })
+ })
+ describe('Part 2', () => {
+ beforeEach(() => {
+ // ensure flushed state
+ efficientSchool.state = [3, 4, 3, 1, 2]
+ expect(efficientSchool.state).to.deep.equal([0, 1, 1, 2, 1, 0, 0, 0, 0])
+ })
+ describe('efficientSpawn()', () => {
+ it('efficiently adds new fish to the school', () => {
+ efficientSpawn(4)
+ expect(efficientSchool.state).to.deep.equal([0, 1, 1, 2, 1, 0, 0, 0, 4])
+ })
+ })
+ describe('efficientAdvance', () => {
+ it('advances one day following the same pattern without tracking unique position', () => {
+ const sum = (x, y) => x + y
+ efficientSchool.state = [3, 4, 3, 1, 2]
+
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([2, 3, 2, 0, 1].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([1, 2, 1, 6, 0, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([0, 1, 0, 5, 6, 7, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([6, 0, 6, 4, 5, 6, 7, 8, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([5, 6, 5, 3, 4, 5, 6, 7, 7, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([4, 5, 4, 2, 3, 4, 5, 6, 6, 7].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([3, 4, 3, 1, 2, 3, 4, 5, 5, 6].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([2, 3, 2, 0, 1, 2, 3, 4, 4, 5].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([1, 2, 1, 6, 0, 1, 2, 3, 3, 4, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([0, 1, 0, 5, 6, 0, 1, 2, 2, 3, 7, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([6, 0, 6, 4, 5, 6, 0, 1, 1, 2, 6, 7, 8, 8, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([5, 6, 5, 3, 4, 5, 6, 0, 0, 1, 5, 6, 7, 7, 7, 8, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([4, 5, 4, 2, 3, 4, 5, 6, 6, 0, 4, 5, 6, 6, 6, 7, 7, 8, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([3, 4, 3, 1, 2, 3, 4, 5, 5, 6, 3, 4, 5, 5, 5, 6, 6, 7, 7, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([2, 3, 2, 0, 1, 2, 3, 4, 4, 5, 2, 3, 4, 4, 4, 5, 5, 6, 6, 7].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([1, 2, 1, 6, 0, 1, 2, 3, 3, 4, 1, 2, 3, 3, 3, 4, 4, 5, 5, 6, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([0, 1, 0, 5, 6, 0, 1, 2, 2, 3, 0, 1, 2, 2, 2, 3, 3, 4, 4, 5, 7, 8].length)
+ efficientSchool.advance()
+ expect(efficientSchool.state.reduce(sum)).to.equal([6, 0, 6, 4, 5, 6, 0, 1, 1, 2, 6, 0, 1, 1, 1, 2, 2, 3, 3, 4, 6, 7, 8, 8, 8, 8].length)
+ })
+ it('advances efficiently for a large number of days', () => {
+ efficientSchool.state = [3, 4, 3, 1, 2]
+ for (let d = 1; d <= 256; d++) {
+ efficientSchool.advance()
+ }
+ const sum = (x, y) => x + y
+ efficientSchool.state.reduce(sum)
+ })
+ })
+ })
+})
diff --git a/2021/day-06/index.js b/2021/day-06/index.js
new file mode 100644
index 0000000..af7e035
--- /dev/null
+++ b/2021/day-06/index.js
@@ -0,0 +1,3 @@
+// eslint-disable-next-line no-unused-vars
+const console = require('../helpers')
+require('./solution')
diff --git a/2021/day-06/input.txt b/2021/day-06/input.txt
new file mode 100644
index 0000000..ff0b23a
--- /dev/null
+++ b/2021/day-06/input.txt
@@ -0,0 +1 @@
+1,1,3,5,1,3,2,1,5,3,1,4,4,4,1,1,1,3,1,4,3,1,2,2,2,4,1,1,5,5,4,3,1,1,1,1,1,1,3,4,1,2,2,5,1,3,5,1,3,2,5,2,2,4,1,1,1,4,3,3,3,1,1,1,1,3,1,3,3,4,4,1,1,5,4,2,2,5,4,5,2,5,1,4,2,1,5,5,5,4,3,1,1,4,1,1,3,1,3,4,1,1,2,4,2,1,1,2,3,1,1,1,4,1,3,5,5,5,5,1,2,2,1,3,1,2,5,1,4,4,5,5,4,1,1,3,3,1,5,1,1,4,1,3,3,2,4,2,4,1,5,5,1,2,5,1,5,4,3,1,1,1,5,4,1,1,4,1,2,3,1,3,5,1,1,1,2,4,5,5,5,4,1,4,1,4,1,1,1,1,1,5,2,1,1,1,1,2,3,1,4,5,5,2,4,1,5,1,3,1,4,1,1,1,4,2,3,2,3,1,5,2,1,1,4,2,1,1,5,1,4,1,1,5,5,4,3,5,1,4,3,4,4,5,1,1,1,2,1,1,2,1,1,3,2,4,5,3,5,1,2,2,2,5,1,2,5,3,5,1,1,4,5,2,1,4,1,5,2,1,1,2,5,4,1,3,5,3,1,1,3,1,4,4,2,2,4,3,1,1
diff --git a/2021/day-06/solution.js b/2021/day-06/solution.js
new file mode 100644
index 0000000..5fe66a1
--- /dev/null
+++ b/2021/day-06/solution.js
@@ -0,0 +1,47 @@
+const fs = require('fs')
+const path = require('path')
+const filePath = path.join(__dirname, 'input.txt')
+const { parseData } = require('../../2018/inputParser')
+const { school, efficientSchool } = require('./fish')
+
+fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
+ if (err) throw err
+
+ initData = parseData(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()
+ school.state = data
+ // Advance the designated time
+ for (let x = 0; x < 80; x++) {
+ school.advance()
+ }
+ // Count how many fish we have
+ return school.state.length
+ }
+
+ const part2 = () => {
+ const data = resetInput()
+ efficientSchool.state = data
+ // Advance the designated time
+ for (let x = 0; x < 256; x++) {
+ efficientSchool.advance()
+ }
+ // Count how many fish we have
+ const sum = (x, y) => x + y
+ return efficientSchool.state.reduce(sum)
+ }
+ 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 ed39a00..0d94d20 100644
--- a/index.js
+++ b/index.js
@@ -1 +1 @@
-require('./2021/day-05/solution')
+require('./2021/day-06/solution')
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