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

nasty nasty hackish solution. but it works :(

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())