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lujanfernaud's solution

to Grains in the Ruby Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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.

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 exercism help 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.

grains_test.rb

``````require 'minitest/autorun'
require_relative 'grains'

# Common test data version: 1.0.0 2e0e77e
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) { Grains.square(0) }
end

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

def test_square_greater_than_64_raises_an_exception
skip
assert_raises(ArgumentError) { Grains.square(65) }
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

# Problems in exercism evolve over time, as we find better ways to ask
# questions.
# The version number refers to the version of the problem you solved,
#
# Define a constant named VERSION inside of the top level BookKeeping
# module, which may be placed near the end of your file.
#
# In your file, it will look like this:
#
# module BookKeeping
#   VERSION = 1 # Where the version number matches the one in the test.
# end
#
# http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

def test_bookkeeping
skip
assert_equal 1, BookKeeping::VERSION
end
end``````
``````class Grains
class << self
CHESSBOARD = (1..64)
CHESSBOARD_SQUARES = 64
GRAINS_MULTIPLIER = 2
OFFSET = 1

def on_square(square_number)
not_a_chessboard_square unless CHESSBOARD.cover?(square_number)

GRAINS_MULTIPLIER**(square_number - OFFSET)
end

def total
GRAINS_MULTIPLIER**CHESSBOARD_SQUARES - OFFSET
end

def not_a_chessboard_square
raise ArgumentError, "Square number must be between 1 and 64"
end

alias square on_square
end
end

module BookKeeping
VERSION = 1
end``````