Recursively flatten an array by flattening each half of the input array and concatenating the results.

flatten_array.rb

module ArrayTransform

  # Recursively flatten an array by flattening each half of the input array and concatenating the results.  
  # Special cases: 
  # 1) A zero-length array is returned unchanged
  # 2) An array nested inside a single-element array is flattened and returned. 
  # Terminating condition: An array containing a single non-array-element is returned without modification
  def self.flatten(obj)
    raise "flatten: Array required; #{obj.class} provided" unless obj.is_a?(Array)
    olen = obj.length # because we use it multiple times
    return obj if olen==0 # Special case: zero-length array

    # Flatten array inside nested single element array,
    # or return single element array containing non-array
    if olen == 1
      return obj[0].is_a?(Array) ? flatten(obj[0]) : obj
    end 

    # head and tail will be at least one element in length
    head = obj[0, olen/2] # about half
    tail = obj[head.length, olen - head.length] # remainder
    flatten(head).concat( flatten(tail) )
  end

end

test_cases = [ 
  ['non-array object', 1 ], 
  ['empty array', [] ], 
  ['single-obj array', [2] ], 
  ['single-obj nested array', [[2]] ], 
  ['two-obj flat array', [2, 3] ], 
  ['three-obj nested array', [[2], 3, 4] ], 
  ['three-obj nested array', [2, 3, [4]] ], 
  ['four-obj nested array', [2, 3, [[4]]] ], 
  ['complex nested array', [[2, 3], [[4], [5, [6, [[ [7, [[8]]] ]], 9, [10, 11]]]]] ] 
]

test_cases.each do |params|
  begin
    puts "Input: #{params[0]} = #{params[1]}"
    puts "Output: #{ArrayTransform.flatten(params[1])}"
  rescue Exception => err
   puts "The test suite for #{params[0]} raised '#{err}'"
   next
  end

end

# To run from command line: `ruby flatten_array.rb`