HParser

A simple parser supports LL(1) (finished) and LR(1)(in the plan) parser..

principle

LL(1) Parser:

1. Calculate First-set and follow-set and nullable-set by fixed point algorithm (production.cpp)

2. Calculate every productions' first-set, and generate the LL(1) parsing table.

3. Parsing procedure.

   • Firstly push the destination non-terminal symbol to the parsing stack

   • If the top of the parsing stack is non-terminal symbol, get the matched production by LL(1) table(By cur non-terminal symbol on the stack and current token to find the production). Push the correspondent symbols to a stack.

   • If the top of the parsing stack is terminal, read a token and check if they are same type and pop it (if they are different the throw an error).

4. Some production ajustments algorithm

   • Because LL(1) parsing cannot process some productions such as left recursive productions, left common factors productions. We need to transform them as the equivalent productions at first.

   • Kill left common factors is very simple and intuitive, for example

     =========INPUT========
     A	-> a b c
     	-> a b
         -> a n
     =========OUTPUT==========
     A  -> a A'
     A' -> b A''
        -> n
     A'' -> c
         -> epision
     

     The recovery is also every simple:d push the generated nodes to his parent..

   • Kill left recursive : read ./docs/alg/kill-left-recursive.md (Chinese) to learn more. and recovery the AST after kill the left recursive