feat(2021-day-06): count how many laternfish exist after 80 days

solves part 1

2021/day-06/fish.js

@@ -0,0 +1,42 @@
+
+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
+
+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 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)
+  }
+}
+
+module.exports = {
+  school,
+  ageFish,
+  spawn
+}

2021/day-06/fish.test.js

@@ -0,0 +1,78 @@
+/* eslint-env mocha */
+const { expect } = require('chai')
+const { school, ageFish, spawn } = 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])
+        school.advance()
+      })
+    })
+  })
+})

2021/day-06/index.js

@@ -0,0 +1,3 @@
+// eslint-disable-next-line no-unused-vars
+const console = require('../helpers')
+require('./solution')

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

2021/day-06/solution.js

@@ -0,0 +1,41 @@
+const fs = require('fs')
+const path = require('path')
+const filePath = path.join(__dirname, 'input.txt')
+const { parseData } = require('../../2018/inputParser')
+const { school } = 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()
+    console.debug(data)
+    return 'No answer yet'
+  }
+  const answers = []
+  answers.push(part1())
+  answers.push(part2())
+
+  answers.forEach((ans, idx) => {
+    console.info(`-- Part ${idx + 1} --`)
+    console.info(`Answer: ${ans}`)
+  })
+})

index.js

@@ -1 +1 @@
-require('./2021/day-05/solution')
+require('./2021/day-06/solution')