Hash::reverse and Array::summarize

array_hash_extras.rb

require 'test/unit'

Hash::class_eval do
  
  # Flip the Hash so that former keys become values and
  # former values become keys. The difference however is
  # that if one value is assigned to multiple keys, then
  # in the reversed Hash you get an array of values under
  # that key.
  def reverse
    new_hash = {}
    self.each do |key,val|
      if new_hash.has_key?(val)
        new_hash[val].push(key)
      else
        new_hash[val] = [key]
      end
    end
    return new_hash
  end
  
end

Array::class_eval do
  
  # Count occurrences of elements in an Array so that the
  # result is a Hash with unique elements as keys and their
  # respective frequencies as values.
  def summarize
    hash = {}
    self.each do |el|
      if hash.has_key?(el)
        hash[el] += 1
      else
        hash[el] = 1
      end
    end
    return hash
  end
  
end

class HashReverseTest < Test::Unit::TestCase
  
  def setup
    @hash = { 'a' => 1, 'b' => 1 }
  end
  
  def test_invert
    result = { 1 => 'b' }
    assert_equal(result, @hash.invert)
  end
  
  def test_reverse
    result = { 1 => ['a', 'b'] }
    assert_equal(result, @hash.reverse)
  end
  
end

class ArraySummarizeTest < Test::Unit::TestCase
  
  def setup
    @array = [1,1,1,2,2,3]
  end
  
  def test_summarize
    expected_result = { 1 => 3, 2 => 2, 3 => 1 }
    assert_equal @array.summarize, expected_result
  end
  
end