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to Two Bucket in the TypeScript Track

Published at Dec 11 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Given two buckets of different size, demonstrate how to measure an exact number of liters by strategically transferring liters of fluid between the buckets.

Since this mathematical problem is fairly subject to interpretation / individual approach, the tests have been written specifically to expect one overarching solution.

To help, the tests provide you with which bucket to fill first. That means, when starting with the larger bucket full, you are NOT allowed at any point to have the smaller bucket full and the larger bucket empty (aka, the opposite starting point); that would defeat the purpose of comparing both approaches!

Your program will take as input:

  • the size of bucket one
  • the size of bucket two
  • the desired number of liters to reach
  • which bucket to fill first, either bucket one or bucket two

Your program should determine:

  • the total number of "moves" it should take to reach the desired number of liters, including the first fill
  • which bucket should end up with the desired number of liters (let's say this is bucket A) - either bucket one or bucket two
  • how many liters are left in the other bucket (bucket B)

Note: any time a change is made to either or both buckets counts as one (1) move.

Example: Bucket one can hold up to 7 liters, and bucket two can hold up to 11 liters. Let's say bucket one, at a given step, is holding 7 liters, and bucket two is holding 8 liters (7,8). If you empty bucket one and make no change to bucket two, leaving you with 0 liters and 8 liters respectively (0,8), that counts as one "move". Instead, if you had poured from bucket one into bucket two until bucket two was full, leaving you with 4 liters in bucket one and 11 liters in bucket two (4,11), that would count as only one "move" as well.

To conclude, the only valid moves are:

  • pouring from one bucket to another
  • emptying one bucket and doing nothing to the other
  • filling one bucket and doing nothing to the other

Written with <3 at Fullstack Academy by Lindsay.


Go through the setup instructions for TypeScript to install the necessary dependencies:



Install assignment dependencies:

$ yarn install

Making the test suite pass

Execute the tests with:

$ yarn test


Water Pouring Problem http://demonstrations.wolfram.com/WaterPouringProblem/

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import {TwoBucket, Bucket} from './two-bucket'

describe('TwoBucket', () => {
  describe('works for input of 3, 5, 1', () => {
    const buckOne = 3
    const buckTwo = 5
    const goal = 1

    test('starting with bucket one', () => {
      const starterBuck = Bucket.One // indicates which bucket to fill first
      const twoBucket = new TwoBucket(buckOne, buckTwo, goal, starterBuck)
      expect(twoBucket.moves()).toEqual(4) // includes the first fill
      expect(twoBucket.goalBucket).toEqual('one') // which bucket should end up with the desired # of liters
      expect(twoBucket.otherBucket).toEqual(5) // leftover value in the "other" bucket once the goal has been reached

    xtest('starting with bucket two', () => {
      const starterBuck = Bucket.Two
      const twoBucket = new TwoBucket(buckOne, buckTwo, goal, starterBuck)

  describe('works for input of 7, 11, 2', () => {
    const buckOne = 7
    const buckTwo = 11
    const goal = 2

    xtest('starting with bucket one', () => {
      const starterBuck = Bucket.One
      const twoBucket = new TwoBucket(buckOne, buckTwo, goal, starterBuck)

    xtest('starting with bucket two', () => {
      const starterBuck = Bucket.Two
      const twoBucket = new TwoBucket(buckOne, buckTwo, goal, starterBuck)
export class Bucket {
  size: number
  amount: number = 0
  readonly name: string

  constructor(name: string) { this.name = name }

  private static _one: Bucket
  static get One() { return this._one || (this._one = new this('one')) }

  private static _two: Bucket
  static get Two() { return this._two || (this._two = new this('two')) }

  get capacity(): number { return this.size - this.amount }
  fill(): void { this.amount = this.size }
  empty(): void { this.amount = 0 }
  isFull(): boolean { return this.amount === this.size }
  isEmpty(): boolean { return this.amount === 0 }

  pour(other: Bucket): void {
    const toPour = Math.min(this.amount, other.capacity)
    this.amount -= toPour
    other.amount += toPour

const GCD = (a: number, b: number): number => (b ? GCD(b, a % b) : a)

export class TwoBucket {
  readonly one: Bucket = Bucket.One
  readonly two: Bucket = Bucket.Two

  private _moves: number = 0
  goalBucket: string
  otherBucket: number = 0

  constructor(size1: number, size2: number, goal: number, startBucket: Bucket) {
    if (goal > Math.max(size1, size2)) {
      throw new Error('No solution: goal bigger than largest bucket')

    const gcd = GCD(size1, size2)
    if (gcd !== 1 && goal % gcd !== 0) {
      // buckets are not relatively prime. Goal must be a multiple of the gcd
      throw new Error('No solution possible')

    this.one.size = size1
    this.two.size = size2
    const other = startBucket.name === 'one' ? this.two : this.one;
    [this.goalBucket, this.otherBucket, this._moves] = this.solve(startBucket, other, goal)

  moves(): number { return this._moves }

  private solve(start: Bucket, other: Bucket, goal: number): [string, number, number] {
    let moves = 0

    while (true) {
      if (start.amount === goal) {
        return [start.name, other.amount, moves]
      if (other.amount === goal) {
        return [other.name, start.amount, moves]

      if (start.isEmpty()) {
      else if (other.isFull()) {
      else {

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