Iterative Parsing (for issue #35)

[thebusytypist]

• Implementation Overview
  This pull request provides an iterative(non-recursive) implementation of JSON parsing.
  It tries to resolve the issue Research on using custom stack instead of call-stack in Reader #35 by:

  • saving parser state in an internal stack rather than using function local stack. In this way the complexity of function stack size is constant.
  • providing a stack size limit to the parsing function.

• Implementation Details
  This pull request adds a new flag kParseIterativeFlag to ParseFlag.
  Parsing work with this flag will be directed to GenericReader::IterativeParse, which is the entry point of this iterative parser.
  Then the parser will traverse the given token stream and make transitions on a state machine.
  The state transition table is manually encoded in a static array, which is in function GenericReader::Predict.
  And the action code for every transition is in function GenericReader::Transit.

• Performance Report
  Performance tests have been taken among:

  • kParseDefaultFlags (recursive parser, without in-situ)
  • kParseIterativeFlag (iterative parser, without in-situ)
  • kParseInsituFlag (recursive parser, with in-situ)
  • kParseIterativeFlag | kParseInsituFlag (iterative parser, with in-situ)

  I expected the parsers with kParseIterativeFlag are either slower or quicker than their counterparts.
  But the iterative parser without in-situ flag runs exceptionally slower than other parsers, while the iterative parser with in-situ being the most quick one.
  @pah pointed out this may be compiler-specific in Research on using custom stack instead of call-stack in Reader #35 (comment).

[pah]

The build currently fails (due to -Werror) with the warning:

../../include/rapidjson/reader.h:1233:29: error: ‘rapidjson::GenericReader<rapidjson::UTF8<>, rapidjson::UTF8<> >::stack_’ will be initialized after [-Werror=reorder]
../../include/rapidjson/reader.h:1232:15: error: ‘const size_t rapidjson::GenericReader<rapidjson::UTF8<>, rapidjson::UTF8<> >::kStackSizeLimit’ [-Werror=reorder]
../../include/rapidjson/reader.h:280:2: error: when initialized here [-Werror=reorder]

You should move the kStackSizeLimit member after the stack_ member to fix it. Secondly, following the current naming conventions, the member should probably be named stackSizeLimit_ instead.