feat(2020-day-10): count combinations to solve part 2

nasty nasty hackish solution. but it works :(

Author: developher-net

2020/day-10/jolts.js

@@ -22,15 +22,83 @@ const countDifferences = (data) => {
  console.debug(`Joltage difference between ${curr} and ${next} is ${delta}.`)
  tallies[delta]++
  })
-
+ console.debug('Tallied voltage differences:', tallies)
  return tallies
 }
 
 const countCombinations = (data) => {
- if (data.length > 15) {
- return 19208
+ const tallies = Array(5).fill(0)
+ const delta = (idx) => {
+ return data[idx] - data[idx - 1]
  }
- return 8
+
+ // Always account for the outlet
+ data.push(0)
+ data = data.sort((a, b) => a - b)
+
+ const deltas = data.reduce((res, el, idx) => {
+ console.debug(idx, el, delta(idx))
+ if (idx <= 0) {
+ return res
+ }
+ res.push(delta(idx))
+ return res
+ }, [])
+ console.debug('joltage deltas', deltas)
+
+ // I'm really not proud of this solution. It hardcodes too much logic with magic constants
+ // and only works because there are no joltage differences of 2, and the max allowed
+ // skip is 3.
+ //
+ // Since the rules say adapters can support 1, 2, or 3 jolt diferences,
+ // that means if the difference between n and n+2 is 3 or less, n+1 can be safely
+ // skipped. Potentially we can skip two.
+ // Every time we skip a number, the total amount of variations doubles
+
+ // This logic would be a LOT messier if we had diffs of 2 in the data set
+
+ // When we have 2 skips in a row, we need to leave one combo in case
+ // skipping both exceeds the max difference
+ // TODO: we aren't implementing this because our data set doesn't have
+ // any diffs of 2, which means we never have a 1 + 2 skip to worry about
+
+ // When we have 3 skips in a row, we're definitely exceeding the max difference
+ // if the next is also a skip so we have to leave at least one in place
+
+ // When we have 5 skips in a row.... etc..
+ // TODO: we aren't implementing this because dataset doesn't have any examples
+
+ deltas.forEach((d, idx, arr) => {
+ if (d === 1 && arr[idx + 1] === 1 && arr[idx + 2] === 1 && arr[idx + 3] === 1) {
+ console.debug('Found 4 in a row')
+ tallies[4]++
+ deltas.splice(idx, 4)
+ } else if (d === 1 && arr[idx + 1] === 1 && arr[idx + 2] === 1) {
+ console.debug('Found 3 in a row')
+ tallies[3]++
+ deltas.splice(idx, 3)
+ } else if (d === 1 && arr[idx + 1] === 1) {
+ console.debug('Found 2 in a row')
+ tallies[2]++
+ deltas.splice(idx, 2)
+ } else if (d === 1) {
+ console.debug('Found 1 in a row')
+ tallies[1]++
+ deltas.splice(idx, 1)
+ }
+ })
+
+ console.debug('skippable ranges', tallies)
+ console.debug([1, 1 ** tallies[1], 2 ** tallies[2], 4 ** tallies[3], 7 ** tallies[4]])
+ return (
+ 1 ** tallies[1]
+ ) * (
+ 2 ** tallies[2]
+ ) * (
+ 4 ** tallies[3]
+ ) * (
+ 7 ** tallies[4] // 4 in a row is special case because we can't skip more than 3
+ )
 }
 
 module.exports = {

2020/day-10/jolts.test.js

@@ -2,12 +2,17 @@
 const { expect } = require('chai')
 const { countDifferences, countCombinations } = require('./jolts')
 
-const adapters = [
+const srcAdapters = [
  [16, 10, 15, 5, 1, 11, 7, 19, 6, 12, 4],
  [28, 33, 18, 42, 31, 14, 46, 20, 48, 47, 24, 23, 49, 45, 19, 38, 39, 11, 1, 32, 25, 35, 8, 17, 7, 9, 4, 2, 34, 10, 3]
 ]
 
 describe('--- Day 10: Adapter Array ---', () => {
+ let adapters
+ beforeEach(() => {
+ // reset test data since arrays get mutated using a quick-and-dirty deep copy
+ adapters = JSON.parse(JSON.stringify(srcAdapters))
+ })
  describe('Part 1', () => {
  describe('countDifferences()', () => {
  it('tabulates the amoount of joltage differences in the adapter set', () => {
@@ -21,19 +26,19 @@ describe('--- Day 10: Adapter Array ---', () => {
  it('throws an error if any joltage differences exceed 3', () => {
  expect(() => countDifferences([5, 40])).to.throw()
  })
+ it('throws an error if any joltage differences is less than 1', () => {
+ expect(() => countDifferences([5, 5])).to.throw()
+ })
  })
  })
  describe('Part 2', () => {
  describe('countCombinations()', () => {
- it('tabulates the amoount of adapter combinations in the set', () => {
+ it('tabulates the amount of adapter combinations in the set', () => {
  const result = countCombinations(adapters[0])
  expect(result).to.equal(8)
  const result2 = countCombinations(adapters[1])
  expect(result2).to.equal(19208)
  })
- it('throws an error if any joltage differences exceed 3', () => {
- expect(() => countDifferences([5, 40])).to.throw()
- })
  })
  })
 })

2020/day-10/solution.js

@@ -2,7 +2,7 @@ const fs = require('fs')
 const path = require('path')
 const filePath = path.join(__dirname, 'input.txt')
 const { inputToArray } = require('../../2018/inputParser')
-const { countDifferences } = require('./jolts')
+const { countDifferences, countCombinations } = require('./jolts')
 
 fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
  if (err) throw err
@@ -22,8 +22,7 @@ fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
 
  const part2 = () => {
  const data = resetInput()
- console.debug(data)
- return 'No answer yet'
+ return countCombinations(data)
  }
  const answers = []
  answers.push(part1())