libs/spirit/repository/doc/qi/advance.qbk

[/==============================================================================
    Copyright (C) 2001-2011 Hartmut Kaiser
    Copyright (C) 2001-2011 Joel de Guzman
    Copyright (C) 2011 Aaron Graham

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section:advance Qi advance Parser]

[heading Description]

The __qi__ `advance` is a primitive parser component allowing the parser to
skip (advance) through a specified number of iterations without performing
unnecessary work:

    advance(distance)

The most obvious existing alternative to this, the `repeat` directive, will
cause the parser to advance one iterator at a time while usually performing
operations at each step. In some cases that work is unnecessary, as in the case
where large binary objects are being parsed. Take, for example, the following
binary data:

[pre
    00 00 00 01 77 fc b4 51 0a b3 b7 ... 1e 60 70 b6 00 00 01 00
]

If the first 4 bytes are a little-endian 32-bit integer describing the length
of the subsequent data, but the data itself is not relevant to parsing, then the
repeat directive would cause all of the subsequent 16 MB of data to be consumed
one byte at a time while generating page faults or other superfluous I/O. If the
value is large, as it is in this case, the parser becomes very slow.

    little_dword[_a = _1] >> repeat(_a)[byte_] >> little_dword...

The `advance` parser component solves this problem by performing as little work
as possible to advance the parser's iterator, and will optimize for the case of
random-access iterators by advancing directly to the desired relative iterator
position.

    little_dword[_a = _1] >> advance(_a) >> little_dword...

[heading Header]

    // forwards to <boost/spirit/repository/home/qi/primitive/advance.hpp>
    #include <boost/spirit/repository/include/qi_advance.hpp>

[heading Synopsis]

    advance(distance)

[heading Parameters]

[table
    [[Parameter]            [Description]]
    [['distance']           [The distance that the iterator shall be advanced]]
]

[heading Attribute]

The `advance` component exposes no attribute (the exposed attribute type is
`unused_type`):

    advance --> unused

[heading Example]

The following example shows simple use cases of the `advance` component. We will
illustrate its usage by generating parsers for some binary data (for the full
example code see
[@../../example/qi/advance.cpp advance.cpp])

[import ../example/qi/advance.cpp]

[heading Prerequisites]

In addition to the main header file needed to include the core components 
implemented in __qi__ we add the header file needed for the new `advance`
component.

[qi_advance_includes]

In order to make the examples below more readable we introduce or use the
following namespaces.

[qi_advance_namespaces]

[heading Setting up the Grammar]

This is a very simple grammar that recognizes several fields of a binary stream
of data. There are two fields explicitly delimited by a field indicating the
number of bytes that are spanned. They are separated by a literal string.

[qi_advance_grammar]

Note that the second binary field may either contain the number of specified
bytes, or the word "qi". If the `advance` parser component fails to advance the
specified number of bytes before reaching the end of input, it will fail and
the parser will attempt to descend into alternatives.

[heading Parsing a Correctly-delimited String of Data]

The data below is correctly delimited and will thus result in a valid parse.
Note that both random-access and bidirectional iterators are used here.

[qi_advance_example1]

[heading Parsing the Alternative Representation]

The data below is not correctly delimited, but will correctly parse because the
alternative word "qi" is available.

[qi_advance_example2]

[heading Notes]

The `advance` parser component will fail unconditionally on negative values.
It will never attempt to advance the iterator in the reverse direction.

[endsect]