kea/src/lib/eval/eval.dox

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/**
  @page dhcpEval libeval - Expression evaluation and client classification

  @section dhcpEvalIntroduction Introduction

  The core of the libeval library is a parser that is able to parse an
  expression (e.g. option[123].text == 'APC'). This is currently used for
  client classification, but in the future may be also used for other
  applications.

  The external interface to the library is the @ref isc::eval::EvalContext
  class.  Once instantiated, it offers a major method:
  @ref isc::eval::EvalContext::parseString, which parses the specified
  string.  Once the expression is parsed, it is converted to a collection of
  tokens that are stored in Reverse Polish Notation in
  EvalContext::expression.

  Internally, the parser code is generated by flex and bison. These two
  tools convert lexer.ll and parser.yy files into a number of .cc and .hh files.
  To avoid a build of Kea depending on the presence of flex and bison, the
  result of the generation is checked into the github repository and is
  distributed in the tarballs.

  @section dhcpEvalLexer Lexer generation using flex

  Flex is used to generate the lexer, a piece of code that converts input
  data into a series of tokens. It contains a small number of directives,
  but the majority of the code consists of the definitions of tokens. These
  definitions are regular expressions that define various tokens, e.g. strings,
  numbers, parentheses, etc. Once the expression is matched, the associated
  action is executed. In the majority of the cases a generator method from
  @ref isc::eval::EvalParser is called, which returns returns a newly created
  bison token. The purpose of the lexer is to generate a stream
  of tokens that are consumed by the parser.

  lexer.cc and lexer.hh must not be edited. If there is a need
  to introduce changes, lexer.ll must be updated and the .cc and .hh files
  regenerated.

  @section dhcpEvalParser Parser generation using bison

  Bison is used to generate the parser, a piece of code that consumes a
  stream of tokens and attempts to match it against a defined grammar.
  The bison parser is created from parser.yy. It contains
  a number of directives, but the two most important sections are:
  a list of tokens (for each token defined here, bison will generate the
  make_NAMEOFTOKEN method in the @ref isc::eval::EvalParser class) and
  the grammar. The Grammar is a tree like structure with possible loops.

  Here is an over-simplified version of the grammar:

@code
01. %start expression;
02.
03. expression : token EQUAL token
04.            | token
05.            ;
06.
07. token : STRING
08.             {
09.                 TokenPtr str(new TokenString($1));
10.                 ctx.expression.push_back(str);
11.             }
12.       | HEXSTRING
13.             {
14.                 TokenPtr hex(new TokenHexString($1));
15.                 ctx.expression.push_back(hex);
16.             }
17.       | OPTION '[' INTEGER ']' DOT TEXT
18.             {
19.                 TokenPtr opt(new TokenOption($3, TokenOption::TEXTUAL));
20.                 ctx.expression.push_back(opt);
21.             }
22.       | OPTION '[' INTEGER ']' DOT HEX
23.             {
24.                 TokenPtr opt(new TokenOption($3, TokenOption::HEXADECIMAL));
25.                 ctx.expression.push_back(opt);
26.              }
27.       ;
@endcode

This code determines that the grammar starts from expression (line 1).
The actual definition of expression (lines 3-5) may either be a
single token or an expression "token == token" (EQUAL has been defined as
"==" elsewhere). Token is further
defined in lines 7-22: it may either be a string (lines 7-11),
a hex string (lines 12-16), option in the textual format (lines 17-21)
or option in a hexadecimal format (lines 22-26).
When the actual case is determined, the respective C++ action
is executed. For example, if the token is a string, the TokenString class is
instantiated with the appropriate value and put onto the expression vector.

@section dhcpEvalMakefile Generating parser files

 In the general case, we want to avoid generating parser files, so an
 average user interested in just compiling Kea would not need flex or
 bison. Therefore the generated files are already included in the
 git repository and will be included in the tarball releases.

 However, there will be cases when one of the developers would want
 to tweak the lexer.ll and parser.yy files and then regenerate
 the code. For this purpose, two makefile targets are defined:
 @code
 make parser
 @endcode
 will generate the parsers and
 @code
 make parser-clean
 @endcode
 will remove the files. Generated files removal was also hooked
 into the maintainer-clean target.

@section dhcpEvalConfigure Configure options

 Since the flex/bison tools are not necessary for a regular compilation,
 checks conducted during configure, but the lack of flex or
 bison tools does not stop the configure process. There is a flag
 (--enable-generate-parser) that tells configure script that the
 parser will be generated. With this flag, the checks for flex/bison
 are mandatory. If either tool is missing or at too early a version, the
 configure process will terminate with an error.

*/