feat(2021-day-06): efficient methods for tracking lampfish schools at…

… scale solves part 2
amclin · Dec 14, 2021 · f46f23a · f46f23a
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diff --git a/2021/day-06/fish.js b/2021/day-06/fish.js
@@ -4,6 +4,10 @@ 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') }
@@ -17,6 +21,11 @@ const spawn = (qty) => {
   _fishes.push(...newFishes)
 }
 
+const efficientSpawn = (qty) => {
+  console.debug(`spawning ${qty} fish`)
+  _bigSchool[NewFishAge] = qty
+}
+
 const school = {
   get state () {
     return _fishes
@@ -35,8 +44,32 @@ const school = {
   }
 }
 
+/**
+ * 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
+  spawn,
+  efficientSpawn
 }
diff --git a/2021/day-06/fish.test.js b/2021/day-06/fish.test.js
@@ -1,6 +1,6 @@
 /* eslint-env mocha */
 const { expect } = require('chai')
-const { school, ageFish, spawn } = require('./fish')
+const { school, ageFish, spawn, efficientSchool, efficientSpawn } = require('./fish')
 
 describe('--- Day 6: Lanternfish ---', () => {
   describe('Part 1', () => {
@@ -71,7 +71,70 @@ describe('--- Day 6: Lanternfish ---', () => {
         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()
+      })
+    })
+  })
+  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/solution.js b/2021/day-06/solution.js
@@ -2,7 +2,7 @@ const fs = require('fs')
 const path = require('path')
 const filePath = path.join(__dirname, 'input.txt')
 const { parseData } = require('../../2018/inputParser')
-const { school } = require('./fish')
+const { school, efficientSchool } = require('./fish')
 
 fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
   if (err) throw err
@@ -27,8 +27,14 @@ fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
 
   const part2 = () => {
     const data = resetInput()
-    console.debug(data)
-    return 'No answer yet'
+    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())