diff --git a/2021/day-03/engineDiagnostics.js b/2021/day-03/engineDiagnostics.js
new file mode 100644
index 0000000..68748d7
--- /dev/null
+++ b/2021/day-03/engineDiagnostics.js
@@ -0,0 +1,86 @@
+/**
+ * Find the most common bit at a position
+ * @param [str] data
+ * @param int position
+ */
+const getMostCommon = (data, position) => {
+ const offs = data.filter((reading) => {
+ return reading[position] === '0'
+ }).length
+ // there can only be 2 values, so easy to check which has more as being above 50%
+ return (offs > data.length / 2) ? '0' : '1'
+}
+
+/**
+ * Find the least common bit at a position
+ * @param [str] data
+ * @param int position
+ */
+const getLeastCommon = (data, position) => {
+ return (getMostCommon(data, position) === '0') ? '1' : '0'
+}
+
+/**
+ * Calc the gamma value of the data set
+ * @param [str] data
+ */
+const getGamma = (data) => {
+ let gamma = ''
+ for (let x = 0; x < data[0].length; x++) {
+ gamma += getMostCommon(data, x)
+ }
+ return gamma
+}
+
+const getEpsilon = (data) => {
+ let epsilon = ''
+ for (let x = 0; x < data[0].length; x++) {
+ epsilon += getLeastCommon(data, x)
+ }
+ return epsilon
+}
+
+const getO2 = (data) => {
+ return getAir(data, getMostCommon)
+}
+
+const getCO2 = (data) => {
+ return getAir(data, getLeastCommon)
+}
+
+const getAir = (data, filterMethod) => {
+ let dataset = data
+ // Loop through each digit, find the most common bit for that digit, and filter
+ // out any readings that don't share that digit
+ //
+ // TODO: Probably faster with bitmap math, but .... ehh... runs fast enough
+ for (let x = 0; x < data[0].length; x++) {
+ if (dataset.length > 1) {
+ const bit = filterMethod(dataset, x)
+ dataset = dataset.filter((reading) => {
+ return reading[x] === bit
+ })
+ }
+ }
+
+ if (dataset.length > 1) {
+ throw new Error(`Found too many results ${dataset}`)
+ }
+
+ return dataset[0]
+}
+
+const calcPowerConsumption = (gamma, epsilon) => {
+ return parseInt(gamma, 2) * parseInt(epsilon, 2)
+}
+
+module.exports = {
+ getGamma,
+ getEpsilon,
+ getMostCommon,
+ getLeastCommon,
+ getO2,
+ getCO2,
+ calcPowerConsumption,
+ calcLifeSupport: calcPowerConsumption
+}
diff --git a/2021/day-03/engineDiagnostics.test.js b/2021/day-03/engineDiagnostics.test.js
new file mode 100644
index 0000000..af0f85b
--- /dev/null
+++ b/2021/day-03/engineDiagnostics.test.js
@@ -0,0 +1,73 @@
+/* eslint-env mocha */
+const { expect } = require('chai')
+const { getMostCommon, getLeastCommon, getEpsilon, getGamma, getO2, getCO2, calcPowerConsumption, calcLifeSupport } = require('./engineDiagnostics')
+
+const testData = [
+ '00100',
+ '11110',
+ '10110',
+ '10111',
+ '10101',
+ '01111',
+ '00111',
+ '11100',
+ '10000',
+ '11001',
+ '00010',
+ '01010'
+]
+
+describe('--- Day 3: Binary Diagnostic ---', () => {
+ describe('Part 1', () => {
+ describe('getMostCommon()', () => {
+ it('finds the most common bit at a position', () => {
+ expect(getMostCommon(testData, 0)).to.equal('1')
+ expect(getMostCommon(testData, 1)).to.equal('0')
+ expect(getMostCommon(testData, 2)).to.equal('1')
+ expect(getMostCommon(testData, 3)).to.equal('1')
+ expect(getMostCommon(testData, 4)).to.equal('0')
+ })
+ })
+ describe('getLeastCommon()', () => {
+ it('finds the least common bit at a position', () => {
+ expect(getLeastCommon(testData, 0)).to.equal('0')
+ expect(getLeastCommon(testData, 1)).to.equal('1')
+ expect(getLeastCommon(testData, 2)).to.equal('0')
+ expect(getLeastCommon(testData, 3)).to.equal('0')
+ expect(getLeastCommon(testData, 4)).to.equal('1')
+ })
+ })
+ describe('getGamma()', () => {
+ it('finds the gamma rate from the provided data using the most common bits in each position', () => {
+ expect(getGamma(testData)).to.equal('10110')
+ })
+ })
+ describe('getEpsilon()', () => {
+ it('finds the epsilon rate from the provided data using the least common bits in each position', () => {
+ expect(getEpsilon(testData)).to.equal('01001')
+ })
+ })
+ describe('calcPowerConsumption', () => {
+ it('calculates the power consumption by multiplying the gamma and epsilon rates as decimals', () => {
+ expect(calcPowerConsumption('10110', '01001')).to.equal(198)
+ })
+ })
+ })
+ describe('Part 2', () => {
+ describe('getO2()', () => {
+ it('calculates the oxygen generator rating from the provided data', () => {
+ expect(getO2(testData)).to.equal('10111')
+ })
+ })
+ describe('getCO2()', () => {
+ it('calculates the carbon dioxide scrubber rating from the provided data', () => {
+ expect(getCO2(testData)).to.equal('01010')
+ })
+ })
+ describe('calcLifeSupport', () => {
+ it('calculates the life support rating by multiplying the O2 and C02 rates rates as decimals', () => {
+ expect(calcLifeSupport('10110', '01001')).to.equal(198)
+ })
+ })
+ })
+})
diff --git a/2021/day-03/index.js b/2021/day-03/index.js
new file mode 100644
index 0000000..af7e035
--- /dev/null
+++ b/2021/day-03/index.js
@@ -0,0 +1,3 @@
+// eslint-disable-next-line no-unused-vars
+const console = require('../helpers')
+require('./solution')
diff --git a/2021/day-03/input.txt b/2021/day-03/input.txt
new file mode 100644
index 0000000..79a8fb8
--- /dev/null
+++ b/2021/day-03/input.txt
@@ -0,0 +1,1000 @@
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diff --git a/2021/day-03/solution.js b/2021/day-03/solution.js
new file mode 100644
index 0000000..6ec52eb
--- /dev/null
+++ b/2021/day-03/solution.js
@@ -0,0 +1,40 @@
+const fs = require('fs')
+const path = require('path')
+const filePath = path.join(__dirname, 'input.txt')
+const { inputToArray } = require('../../2018/inputParser')
+const { getGamma, getEpsilon, calcPowerConsumption, calcLifeSupport, getO2, getCO2 } = require('./engineDiagnostics')
+
+fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
+ if (err) throw err
+
+ initData = inputToArray(initData.trim())
+
+ const resetInput = () => {
+ // Deep copy to ensure we aren't mutating the original data
+ return JSON.parse(JSON.stringify(initData))
+ }
+
+ const part1 = () => {
+ const data = resetInput()
+ return calcPowerConsumption(
+ getGamma(data),
+ getEpsilon(data)
+ )
+ }
+
+ const part2 = () => {
+ const data = resetInput()
+ return calcLifeSupport(
+ getO2(data),
+ getCO2(data)
+ )
+ }
+ const answers = []
+ answers.push(part1())
+ answers.push(part2())
+
+ answers.forEach((ans, idx) => {
+ console.info(`-- Part ${idx + 1} --`)
+ console.info(`Answer: ${ans}`)
+ })
+})
diff --git a/index.js b/index.js
index fe32819..b752549 100644
--- a/index.js
+++ b/index.js
@@ -1 +1 @@
-require('./2021/day-02/solution')
+require('./2021/day-03/solution')