[/==============================================================================
    Copyright (C) 2001-2012 Joel de Guzman
    Copyright (C) 2001-2012 Hartmut Kaiser
    Copyright (C) 2011-2012 Thomas Bernard

    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:kwd Qi Keyword Parser Directive ]

[heading Description]

The `kwd[]`, `dkwd[]` and `ikwd[]`, `idkwd[]` provide a powerful and flexible mechanism for parsing keyword
based input. It works in conjunction with the / operator to create an effective
keyword parsing loop. The keyword parsing loop doesn't require the 
keywords to appear in a defined order in the input but also provides the possibility
to check how many times a keyword appears in the input.

The kwd directive will parse the keywords respecting case sensitivity whereas the ikwd
direcive is case insensitive. You can mix the kwd and ikwd directives inside a set of 
keywords, but be aware that this has a small overhead. It should be preferred not to 
mix the kwd and ikwd directives.

The dkwd and idkwd provide a mechanism to pase distinct keywords. These directives require
that the skipper successeds parsing input right after the keyword part. 

dkwd("keyword1")['='>>int_]

is equivalent to:

lit("keyword1") >> skipper+ >> '=' >> int_

All the keyword directives can be mixed inside a keyword list.

The kwd directive is very similar to the repeat directive in that it enables to enforce
keyword occurrence constraints but also provides very interesting speed improvement
over the pure EBNF syntax or the Nabialek-Trick. 

[heading Header]

    // forwards to <boost/spirit/repository/home/qi/directive/kwd.hpp>
    #include <boost/spirit/repository/include/qi_kwd.hpp>

[heading Synopsis]

[table
    [[Expression]               [Semantics]]
    [[`kwd(keyword)[subject]`]              [Parse ( `"keyword"` > `subject`) zero or more times.]]
    [[`kwd(keyword,n)[subject]`]           [Parse ( `"keyword"` > `subject`) exactly `n` times.]]
    [[`kwd(keyword,min, max)[subject]`]    [Parse ( `"keyword"` > `subject`) at least `min` times and at most `max` times.]]
    [[`kwd(keyword,min, inf)[subject]`]    [Parse ( `"keyword"` > `subject`) at least `min` or more. ]]
]

For non case sensitive keywords use the ikwd directive.
If distinct keyword parsing is required, use the dkwd and idkwd directive instead.

[heading Parameters]

[table
    [[Parameter]            [Description]]
    [[`keyword`]             [The parser for the opening (the prefix).]]    
    [[`subject`]            [The parser for the input sequence following the keyword part.]]
    [[`n`]            [Int representing the exact number of times the keyword must be repeated.]]
    [[`min`]            [Int representing the minimum number of times the keyword must be repeated.]]
    [[`max`]            [Int representing the maximum number of times the keyword must be repeated.]]
]

The keyword as well as the subject parameters can be any valid spirit parser.
The parameter n, min and max are integer constants.

[heading Attributes]

[table
    [[Expression]             [Attribute]]
    [[`kwd(k1)[a]`]
[``a: A --> kwd(k1)[a]: optional<A> or vector<A>
a: Unused --> kwd(k1)[a]: Unused``]]
    [[`kwd(k1,n)[a]`]
[``a: A --> kwd(k1,n)[a]: optional<A> or vector<A>
a: Unused --> kwd(k1,n)[a]: Unused``]]
    [[`kwd(k1,min, max)[a]`]
[``a: A --> kwd(k1,min, max)[a]: optional<A> or vector<A>
a: Unused --> kwd(k1,min, max)[a]: Unused``]]
    [[`kwd(k1,min, inf)[a]`]
[``a: A --> kwd(k1,min, inf)[a]: optional<A> or vector<A>
a: Unused --> kwd(k1,min, inf)[a]: Unused``]]
]

[heading Complexity]

[:The overall complexity is defined by the complexity of its subject
parser. The complexity of the keyword list construct `kwd` itself is O(N), where N is the number
of repetitions executed.

In the case where all the keywords are strings, the complexity of the keyword list itself determined by the complexity of the internal TST contents :

O(log n+k)

Where k is the length of the string to be searched in a TST with n strings.

When the keywords used are complex parsers, then the complexity is the sum of the sub parser complexities.
 
]

[heading Example]

Please refer to keyword_list.

[endsect]