feat(2020-day-09): check for valid data entry in XMAS encryption method

Author: developher-net

2020/day-09/xmasEncryption.js

@@ -0,0 +1,20 @@
+const isValid = (value, preamble) => {
+  let valid = false
+
+  // Search for a combination of 2 entries that add up to `value`
+  preamble.forEach((outer) => {
+    preamble.forEach((inner) => {
+      // match cannot use the same number twice
+      if (outer !== inner && outer + inner === value) {
+        console.debug(`Found matches ${outer} + ${inner} = ${value}`)
+        valid = true
+      }
+    })
+  })
+
+  return valid
+}
+
+module.exports = {
+  isValid
+}

2020/day-09/xmasEncryption.test.js

@@ -0,0 +1,29 @@
+/* eslint-env mocha */
+const { expect } = require('chai')
+const { isValid } = require('./xmasEncryption')
+
+const range = (i) => { return i ? range(i - 1).concat(i) : [] }
+
+describe('--- Day 9: Encoding Error ---', () => {
+  describe('Part 1', () => {
+    describe('isValid()', () => {
+      it('checks if an entry is valid', () => {
+        // first example
+        const preamble = range(25)
+        expect(isValid(26, preamble), '26 is valid').to.equal(true)
+        expect(isValid(49, preamble), '49 is valid').to.equal(true)
+        expect(isValid(100, preamble), '100 is larger than any sum').to.equal(false)
+        expect(isValid(50, preamble), '50 not valid because cannot use the same number (25) twice').to.equal(false)
+        // 2nd example
+        // move 20 to start of the list, and then pop it off
+        preamble.splice(preamble.indexOf(20), 1)
+        // add 45 to the list
+        preamble.push(45)
+        expect(isValid(26, preamble), '26 is valid').to.equal(true)
+        expect(isValid(65, preamble), '65 doesnt match any 2 values').to.equal(false)
+        expect(isValid(64, preamble), '64 is valid').to.equal(true)
+        expect(isValid(66, preamble), '65 is valid').to.equal(true)
+      })
+    })
+  })
+})