.. _ref-deque:

------
Deque
------

The ``Deque`` type implements a double-ended queue using a list of blocks. This data structure supports constant-time insertion/removal of elements at both ends of a sequence.

Usage::

  a = Deque{Int}()
  isempty(a)          # test whether the dequeue is empty
  length(a)           # get the number of elements
  push!(a, 10)        # add an element to the back
  pop!(a)             # remove an element from the back
  unshift!(a, 20)     # add an element to the front
  shift!(a)           # remove an element from the front
  front(a)            # get the element at the front
  back(a)             # get the element at the back

*Note:* Julia's ``Vector`` type also provides this interface, and thus can be used as a deque. However, the ``Deque`` type in this package is implemented as a list of contiguous blocks (default size = 2K). As a deque grows, new blocks may be created and linked to existing blocks. This way avoids the copying when growing a vector.

Benchmark shows that the performance of ``Deque`` is comparable to ``Vector`` on ``push!``, and is noticeably faster on ``unshift!`` (by about 30% to 40%).

