Add document for implementation of iterative parser.

doc/internals.md

@@ -19,3 +19,101 @@ This section records some design and implementation details.
 ## Pow10()
 
 ## Local Stream Copy
+
+# Parser
+
+## Iterative Parser
+
+The iterative parser is a recursive descent LL(1) parser
+implemented in a non-recursive manner.
+
+### Grammar
+
+The grammar used for this parser is based on strict JSON syntax:
+~~~~~~~~~~
+S -> array | object
+array -> [ values ]
+object -> { members }
+values -> non-empty-values | ε
+non-empty-values -> value addition-values
+addition-values -> ε | , non-empty-values
+members -> non-empty-members | ε
+non-empty-members -> member addition-members
+addition-members -> ε | , non-empty-members
+member -> STRING : value
+value -> STRING | NUMBER | NULL | BOOLEAN | object | array
+~~~~~~~~~~
+
+Note that left factoring is applied to non-terminals `values` and `members`
+to make the grammar be LL(1).
+
+### Parsing Table
+
+Based on the grammar, we can construct the FIRST and FOLLOW set.
+
+The FIRST set of non-terminals is listed below:
+
+|    NON-TERMINAL   |               FIRST              |
+|:-----------------:|:--------------------------------:|
+|       array       |                 [                |
+|       object      |                 {                |
+|       values      | ε STRING NUMBER NULL BOOLEAN { [ |
+|  addition-values  |              ε COMMA             |
+|      members      |             ε STRING             |
+|  addition-members |              ε COMMA             |
+|       member      |              STRING              |
+|       value       |  STRING NUMBER NULL BOOLEAN { [  |
+|         S         |                [ {               |
+| non-empty-members |              STRING              |
+|  non-empty-values |  STRING NUMBER NULL BOOLEAN { [  |
+
+The FOLLOW set is listed below:
+
+|    NON-TERMINAL   |  FOLLOW |
+|:-----------------:|:-------:|
+|         S         |    $    |
+|       array       | , $ } ] |
+|       object      | , $ } ] |
+|       values      |    ]    |
+|  non-empty-values |    ]    |
+|  addition-values  |    ]    |
+|      members      |    }    |
+| non-empty-members |    }    |
+|  addition-members |    }    |
+|       member      |   , }   |
+|       value       |  , } ]  |
+
+Finally the parsing table can be constructed from FIRST and FOLLOW set:
+
+|    NON-TERMINAL   |           [           |           {           |          ,          | : | ] | } |          STRING         |         NUMBER        |          NULL         |        BOOLEAN        |
+|:-----------------:|:---------------------:|:---------------------:|:-------------------:|:-:|:-:|:-:|:-----------------------:|:---------------------:|:---------------------:|:---------------------:|
+|         S         |         array         |         object        |                     |   |   |   |                         |                       |                       |                       |
+|       array       |       [ values ]      |                       |                     |   |   |   |                         |                       |                       |                       |
+|       object      |                       |      { members }      |                     |   |   |   |                         |                       |                       |                       |
+|       values      |    non-empty-values   |    non-empty-values   |                     |   | ε |   |     non-empty-values    |    non-empty-values   |    non-empty-values   |    non-empty-values   |
+|  non-empty-values | value addition-values | value addition-values |                     |   |   |   |  value addition-values  | value addition-values | value addition-values | value addition-values |
+|  addition-values  |                       |                       |  , non-empty-values |   | ε |   |                         |                       |                       |                       |
+|      members      |                       |                       |                     |   |   | ε |    non-empty-members    |                       |                       |                       |
+| non-empty-members |                       |                       |                     |   |   |   | member addition-members |                       |                       |                       |
+|  addition-members |                       |                       | , non-empty-members |   |   | ε |                         |                       |                       |                       |
+|       member      |                       |                       |                     |   |   |   |      STRING : value     |                       |                       |                       |
+|       value       |         array         |         object        |                     |   |   |   |          STRING         |         NUMBER        |          NULL         |        BOOLEAN        |
+
+There is a great [tool](http://hackingoff.com/compilers/predict-first-follow-set) for above grammar analysis.
+
+### Implementation
+
+Based on the parsing table, a direct(or conventional) implementation
+that pushes the production body in reverse order
+while generating a production could work.
+
+In RapidJSON, several modifications(or adaptations to current design) are made to a direct implementation.
+
+First, the parsing table is encoded in a state machine in RapidJSON.
+Extra states are added for productions involved with `array` and `object`.
+In this way the generation of array values or object members would be a single state transition,
+rather than several pop/push operations in the direct implementation.
+This also makes the estimation of stack size more easier.
+
+Second, the iterative parser also keeps track of array's value count and object's member count
+in its internal stack, which may be different from a conventional implementation.