feat(2020-day-09): find a weakness range in the encrypted data stream

Author: developher-net

2020/day-09/xmasEncryption.js

@@ -1,3 +1,26 @@
+const arrSum = arr => arr.reduce((a, b) => a + b, 0)
+
+const findWeaknessRange = (data, target) => {
+  let counter = 0
+  while (counter < data.length) {
+    const range = []
+    let rangeCounter = counter
+    // Tally up the next set of values trying to hit target
+    while (arrSum(range) < target && rangeCounter < data.length) {
+      range.push(data[rangeCounter])
+
+      if (arrSum(range) === target) {
+        console.debug(`Found range totally ${target}`, range)
+        return range
+      }
+      rangeCounter++
+    }
+    counter++
+  }
+
+  throw new Error('No range found matching target')
+}
+
 const findInvalid = (data, preambleSize) => {
   let match = true
   let counter = preambleSize
@@ -23,6 +46,9 @@ const findInvalid = (data, preambleSize) => {
 const isValid = (value, preamble) => {
   let valid = false
 
+  // TODO: Speed this up by breaking out of while() loops instead
+  // of forEach
+  //
   // Search for a combination of 2 entries that add up to `value`
   preamble.forEach((outer) => {
     preamble.forEach((inner) => {
@@ -39,5 +65,6 @@ const isValid = (value, preamble) => {
 
 module.exports = {
   findInvalid,
+  findWeaknessRange,
   isValid
 }

2020/day-09/xmasEncryption.test.js

@@ -1,6 +1,6 @@
 /* eslint-env mocha */
 const { expect } = require('chai')
-const { isValid, findInvalid } = require('./xmasEncryption')
+const { isValid, findInvalid, findWeaknessRange } = require('./xmasEncryption')
 
 const range = (i) => { return i ? range(i - 1).concat(i) : [] }
 
@@ -38,5 +38,16 @@ describe('--- Day 9: Encoding Error ---', () => {
         expect(() => { findInvalid([1, 2, 3, 5, 8, 13], 2) }).to.throw('No invalid values found')
       })
     })
+    describe('findWeaknessRange()', () => {
+      it('locates a range of congtiguous values that sum up to the provided input', () => {
+        const data = [35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299, 277, 309, 576]
+        expect(
+          findWeaknessRange(data, 127)
+        ).to.deep.equal([15, 25, 47, 40])
+      })
+      it('throws an error if no matching range found', () => {
+        expect(() => { findWeaknessRange([1, 2, 3, 5, 8, 13], 50) }).to.throw('No range found matching target')
+      })
+    })
   })
 })