PatrickMcSweeny's solution

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

• how many grains were on each square, and
• the total number of grains

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

• Optimize for speed.
• Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

---

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby grains_test.rb

To include color from the command line:

ruby -r minitest/pride grains_test.rb

Source

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

Submitting Incomplete Solutions

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

require 'minitest/autorun'
require_relative 'grains'

# Common test data version: 1.2.0 2ec42ab
class GrainsTest < Minitest::Test
  def test_1
    # skip
    assert_equal 1, Grains.square(1)
  end

  def test_2
    skip
    assert_equal 2, Grains.square(2)
  end

  def test_3
    skip
    assert_equal 4, Grains.square(3)
  end

  def test_4
    skip
    assert_equal 8, Grains.square(4)
  end

  def test_16
    skip
    assert_equal 32_768, Grains.square(16)
  end

  def test_32
    skip
    assert_equal 2_147_483_648, Grains.square(32)
  end

  def test_64
    skip
    assert_equal 9_223_372_036_854_775_808, Grains.square(64)
  end

  def test_square_0_raises_an_exception
    skip
    assert_raises(ArgumentError) do
      Grains.square(0)
    end
  end

  def test_negative_square_raises_an_exception
    skip
    assert_raises(ArgumentError) do
      Grains.square(-1)
    end
  end

  def test_square_greater_than_64_raises_an_exception
    skip
    assert_raises(ArgumentError) do
      Grains.square(65)
    end
  end

  def test_returns_the_total_number_of_grains_on_the_board
    skip
    assert_equal 18_446_744_073_709_551_615, Grains.total
  end
end

class Grains
  SQUARES = 64

  def self.square(number)
    unless number.between?(1, SQUARES)
      raise ArgumentError, "Number must be between 1 and #{SQUARES}."
    end
    2**(number - 1)
  end

  def self.total
    (1..SQUARES).sum { |number| square(number) }
  end
end