class Array

def combination(num)
  num = Backports.coerce_to_int(num)
  return to_enum(:combination, num) unless block_given?
  return self unless (0..size).include? num
  # Implementation note: slightly tricky.
                                           # Example: self = 1..7, num = 3
  picks = (0...num).to_a                   # picks start at 0, 1, 2
  max_index = ((size-num)...size).to_a           # max (index for a given pick) is [4, 5, 6]
  pick_max_pairs = picks.zip(max_index).reverse  # pick_max_pairs = [[2, 6], [1, 5], [0, 4]]
  lookup = pick_max_pairs.find(Proc.new{return self})
  loop do
    yield values_at(*picks)
    move = lookup.each{|pick, max| picks[pick] < max}.first
    new_index = picks[move] + 1
    picks[move...num] = (new_index...(new_index+num-move)).to_a
  end
end unless method_defined? :combination

def cycle(n = nil)
  return to_enum(:cycle, n) unless block_given?
  if n.nil?
    each{|e| yield e } until false
  else
    n = Backports.coerce_to_int(n)
    n.times{each{|e| yield e }}
  end
  nil
end unless method_defined? :cycle

def extract_options!
  last.is_a?(::Hash) ? pop : {}
end unless method_defined? :extract_options!

def flatten_with_optional_argument(level=-1)
  dup.flatten!(level) || self
end

def flatten_with_optional_argument!(level=-1)
  level = Backports.coerce_to_int(level)
  return flatten_without_optional_argument! unless level >= 0
  ret, out = nil, []
  ret = recursively_flatten_finite(self, out, level)
  replace(out) if ret
  ret
end

def index_with_block(*arg)
  return index_without_block(*arg) unless block_given? && arg.empty?
  each_with_index{|o,i| return i if yield o}
  return nil
end

def keep_if
  return to_enum(:keep_if) unless block_given?
  delete_if{|elem| !yield elem}
end unless method_defined? :keep_if

def permutation(num = Backports::Undefined)
  return to_enum(:permutation, num) unless block_given?
  num = num.equal?(Backports::Undefined) ?
        size :
        Backports.coerce_to_int(num)
  return self unless (0..size).include? num
  final_lambda = lambda do |partial, remain|
    yield partial
  end
  outer_lambda = num.times.inject(final_lambda) do |proc, ignore|
    lambda do |partial, remain|
      remain.each_with_index do |val, i|
        new_remain = remain.dup
        new_remain.delete_at(i)
        proc.call(partial.dup << val, new_remain)
      end
    end
  end
  outer_lambda.call([], self)
end unless method_defined? :permutation

def pop_with_optional_argument(n = Backports::Undefined)
  return pop_without_optional_argument if n == Backports::Undefined
  n = Backports.coerce_to_int(n)
  raise ArgumentError, "negative array size" if n < 0
  first = size - n
  first = 0 if first < 0
  slice!(first..size).to_a
end

def product(*arg)
  # Implementation notes: We build a block that will generate all the combinations
  # by building it up successively using "inject" and starting with one
  # responsible to append the values.
  #
  result = []
  arg.map!{|ary| Backports.coerce_to_ary(ary)}
  arg.reverse! # to get the results in the same order as in MRI, vary the last argument first
  arg.push self
  outer_lambda = arg.inject(result.method(:push)) do |proc, values|
    lambda do |partial|
      values.each do |val|
        proc.call(partial.dup << val)
      end
    end
  end
  outer_lambda.call([])
  result
end unless method_defined? :product

def product_with_block(*arg, &block)
  return product_without_block(*arg) unless block_given?
  # Same implementation as 1.8.7, but yielding
  arg.map!{|ary| Backports.coerce_to_ary(ary)}
  arg.reverse! # to get the results in the same order as in MRI, vary the last argument first
  arg.push self
  outer_lambda = arg.inject(block) do |proc, values|
    lambda do |partial|
      values.each do |val|
        proc.call(partial.dup << val)
      end
    end
  end
  outer_lambda.call([])
  self
end

def recursively_flatten_finite(array, out, level)
  ret = nil
  if level <= 0
    out.concat(array)
  else
    array.each do |o|
      if ary = Backports.is_array?(o)
        recursively_flatten_finite(ary, out, level - 1)
        ret = self
      else
        out << o
      end
    end
  end
  ret
end

def repeated_combination(num)
  return to_enum(:repeated_combination, num) unless block_given?
  num = Backports.coerce_to_int(num)
  if num <= 0
    yield [] if num == 0
  else
    indices = Array.new(num, 0)
    indices[-1] = size
    while dec = indices.find_index(&:nonzero?)
      indices[0..dec] = Array.new dec+1, indices[dec]-1
      yield values_at(*indices)
    end
  end
  self
end unless method_defined? :repeated_combination

def repeated_permutation(num)
  return to_enum(:repeated_permutation, num) unless block_given?
  num = Backports.coerce_to_int(num)
  if num <= 0
    yield [] if num == 0
  else
    indices = Array.new(num, 0)
    indices[-1] = size
    while dec = indices.find_index(&:nonzero?)
      indices[0...dec] = Array.new dec, size-1
      indices[dec] -= 1
      yield values_at(*indices)
    end
  end
  self
end unless method_defined? :repeated_permutation

def rindex_with_block(*arg)
  return rindex_without_block(*arg) unless block_given? && arg.empty?
  reverse_each.each_with_index{|o,i| return size - 1 - i if yield o}
  return nil
end

def rotate(n=1)
  Array.new(self).rotate!(n)
end unless method_defined? :rotate

def rotate!(n=1)
  n = Backports.coerce_to_int(n) % (empty? ? 1 : size)
  concat(shift(n))
end unless method_defined? :rotate!

def sample(n = Backports::Undefined)
  return self[Kernel.rand(size)] if n == Backports::Undefined
  n = Backports.coerce_to_int(n)
  raise ArgumentError, "negative array size" if n < 0
  n = size if n > size
  result = Array.new(self)
  n.times do |i|
    r = i + Kernel.rand(size - i)
    result[i], result[r] = result[r], result[i]
  end
  result[n..size] = []
  result
end unless method_defined? :sample

def select!
  return to_enum(:select!) unless block_given?
  reject!{|elem| ! yield elem}
end unless method_defined? :select!

def shift_with_optional_argument(n = Backports::Undefined)
  return shift_without_optional_argument if n == Backports::Undefined
  n = Backports.coerce_to_int(n)
  raise ArgumentError, "negative array size" if n < 0
  slice!(0, n)
end

def shuffle
  dup.shuffle!
end unless method_defined? :shuffle

def shuffle!
  raise TypeError, "can't modify frozen array" if frozen?
  size.times do |i|
    r = i + Kernel.rand(size - i)
    self[i], self[r] = self[r], self[i]
  end
  self
end unless method_defined? :shuffle!

def sort_by!
  return to_enum(:sort_by!) unless block_given?
  raise "can't modify frozen array" if frozen?
  replace sort_by{|e| yield e}
end unless method_defined? :sort_by!

def try_convert(obj)
  Backports.try_convert(obj, Array, :to_ary)
end unless method_defined? :try_convert

def uniq_with_block
  return uniq_without_block unless block_given?
  h = {}
  each do |elem|
    h[yield(elem)] ||= elem
  end
  h.values
end

def uniq_with_block!
  return uniq_without_block! unless block_given?
  replace self if frozen? # force error
  u = uniq{|e| yield e}
  replace u unless u.size == size
end