Assignment: Data Packer

packer_spec.rb

require 'minitest/autorun'
require './short_string_packer'

def random_string
  [*'a'..'z'].sample(rand(1..12)).join
end

test_cases = %w(a z asdf abcdefghijkl aaaaaazzzzzz)

describe 'Test whether packing methods are valid' do
  it 'should pack strings into numbers' do
    packed = ShortStringPacker.pack(random_string)
    packed.must_be_instance_of Fixnum
  end

  it 'should unpack numbers back into strings' do
    rand_packed = (rand * 10000000).to_i
    str = ShortStringPacker.unpack(rand_packed)
    str.must_be_instance_of String
  end
end

describe 'Test whether roundtripping of packing works' do
  test_cases.each do |str|
    it "should pack and unpack \'#{str}\' correctly" do
      packed = ShortStringPacker.pack(str)
      unpacked = ShortStringPacker.unpack(packed)
      unpacked.must_equal str
    end
  end
end

short_string_packer.rb

class ShortStringPacker
  ## Packs a short string into a Fixnum
  # Arguments:
  #   str - String object
  # Returns: a Fixnum object
  def self.pack(str)
    puts 'Packing string: ' + str
    ret = []
    char_len = str.length

    str.each_char do |c|
      bits = ((c.ord - 96) << ((char_len-1) * 5))
      ret << bits
      char_len -= 1
    end
    inject_or = ret.inject(0, :|)

    # showing
    # ret.each do |p|
    #   puts p.to_s(2)
    # end
    # p ret

    inject_or
  end

  ## Unpacks a Fixnum from pack() method into a short string
  # Arguments:
  #   packed - a Fixnum object
  # Returns: a String object
  def self.unpack(packed)
    puts 'Unpacking'

    len = packed.to_s(2).length

    r = len % 5
    q = len / 5

    if r == 0
      if q > 0
        actual_len = len
      else
        actual_len = 0
      end
    else
      actual_len = len + (5-r)
    end

    # puts 'length: ' + actual_len.to_s
    mask = 0b11111 << (actual_len - 5)
    # puts 'mask:' + mask.to_s(2)

    ret = []
    until mask == 0
      bits = (packed & mask)
      ret << bits
      mask = mask >> 5
    end

    # showing
    # ret.each do |p|
    #   puts p.to_s(2)
    # end
    # p ret

    str = []
    i = 0
    ret.each do |p|
      i += 1
      seg = p >> (actual_len - (5 * i))
      char = (seg.to_i + 96).chr
      str << char
    end
    str.join
  end
end