feat(2020-day-09): find the first invalid value in a data set

Author: developher-net

2020/day-09/xmasEncryption.js

@@ -1,3 +1,25 @@
+const findInvalid = (data, preambleSize) => {
+  let match = true
+  let counter = preambleSize
+
+  while (match === true && counter < data.length) {
+    console.debug(`Looking for mismatch ${data[counter]} in:`)
+    console.debug(data.slice(counter - preambleSize, counter))
+    console.debug('-----------------------------------------')
+    match = isValid(
+      data[counter],
+      data.slice(counter - preambleSize, counter)
+    )
+    if (!match) {
+      console.debug(`${data[counter]} doesn't match.`)
+      return data[counter]
+    }
+    counter++
+  }
+
+  throw new Error('No invalid values found')
+}
+
 const isValid = (value, preamble) => {
   let valid = false
 
@@ -16,5 +38,6 @@ const isValid = (value, preamble) => {
 }
 
 module.exports = {
+  findInvalid,
   isValid
 }

2020/day-09/xmasEncryption.test.js

@@ -1,6 +1,6 @@
 /* eslint-env mocha */
 const { expect } = require('chai')
-const { isValid } = require('./xmasEncryption')
+const { isValid, findInvalid } = require('./xmasEncryption')
 
 const range = (i) => { return i ? range(i - 1).concat(i) : [] }
 
@@ -25,5 +25,18 @@ describe('--- Day 9: Encoding Error ---', () => {
         expect(isValid(66, preamble), '65 is valid').to.equal(true)
       })
     })
+    describe('findInvalid()', () => {
+      it('searches a data set for the first invalid data based on the defined preamble size', () => {
+        const data = [35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299, 277, 309, 576]
+        const preambleSize = 5
+        expect(
+          findInvalid(data, preambleSize),
+          `127 cannot be summed from the previous ${preambleSize} numbers`
+        ).to.equal(127)
+      })
+      it('throws an error if no invalid value is found', () => {
+        expect(() => { findInvalid([1, 2, 3, 5, 8, 13], 2) }).to.throw('No invalid values found')
+      })
+    })
   })
 })