Ruby: Squaring numbers and the rice checkerboard problem

grains.rb

# 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 a program 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?

class Grains
  def square(num)
    # more readable - square the number! Gotta start at 2, so we have to take away 1 from our number
    # 2**(num - 1)

    # less readable - voodoo bitshifting magicks
    # 0001 becomes 0010 after bitshifting by 1
    #    1            2
    # 0001 becomes 0100 after bitshifting by 2
    #    1            4
    # ALU is happy!
    1 << num - 1
  end

  def total
    # inject will let us keep a running total in the total param
    # adding in the square of each number
    # then the return value of the block, becomes the new total - thus, running total
    total = (1..64).inject { |total, num| total += square(num) }
  end
end

RSpec.configure do |config|
  # Use color in STDOUT
  config.color_enabled = true
 
  # Use color not only in STDOUT but also in pagers and files
  config.tty = true
 
  # Use the specified formatter
  config.formatter = :documentation # :progress, :html, :textmate
end

describe Grains do

  it 'has square_1' do
    Grains.new.square(1).should == 1
  end

  it 'has square_2' do
    Grains.new.square(2).should == 2
  end

  it 'has square_3' do
    Grains.new.square(3).should == 4
  end

  it 'has square_4' do
    Grains.new.square(4).should == 8
  end

  it 'has square_16' do
    Grains.new.square(16).should == 32768
  end

  it 'has square_32' do
    Grains.new.square(32).should == 2147483648
  end

  it 'has square_64' do
    Grains.new.square(64).should == 9223372036854775808
  end

  it 'has total_grains' do
    Grains.new.total.should == 18446744073709551615
  end

end