count_occurrences.rb

class Array
  # Count occurences of values in the array.
  # Return it as a hash.
  # 
  #   [1, 1, "foo", :bar, 1, :bar].count_occurrences  # => {1 => 3, "foo" => 1, :bar => 2}
  def count_occurrences
    h = self.group_by { |x| x }
    Hash[h.map { |k, v| [ k, v.length ] } ]
  end
end

tests.rb

class ArrayCountOccurrencesTests < Test::Unit::TestCase
  def test_count_occurrences_with_scalar
    source = [ 1, 1, "foo", :bar, 1, :bar ]
    result = { 1 => 3, "foo" => 1, :bar => 2 }
    assert_equal result, source.count_occurrences
  end

  def test_count_occurrences_with_hash
    source = [ { "foo" => "bar" }, { "foo" => "bar" }, { :bar => :foo } ]
    result = { { "foo" => "bar" } => 2, { :bar => :foo } => 1 }
    assert_equal result, source.count_occurrences
  end

  def test_count_occurrences_with_hash_and_arrays
    source = [ { "foo" => "bar" }, { "foo" => "bar" }, { :bar => :foo }, [ 1, 2, 3 ], [ 1, 2, 3 ], [ 4 ] ]
    result = { { "foo" => "bar" } => 2, { :bar => :foo } => 1, [ 1, 2, 3 ] => 2, [ 4 ] => 1 }
    assert_equal result, source.count_occurrences
  end
  
  def test_count_occurrences_with_everything
    source = [ 8, { "foo" => "bar" }, { "foo" => "bar" }, 8, { :bar => :foo }, [ 1, 2, 3 ], [ 1, 2, 3 ], [ 4 ], :foo ]
    result = { { "foo" => "bar" } => 2, { :bar => :foo } => 1, [ 1, 2, 3 ] => 2, [ 4 ] => 1, 8 => 2, :foo => 1 }
    assert_equal result, source.count_occurrences
  end
end