public String addSymbol(String buffer, int offset, int len, int hash) {
        return addSymbol(buffer, offset, len, hash, false);
    }
Adds the specified symbol to the symbol table and returns a reference to the unique symbol. If the symbol already
exists, the previous symbol reference is returned instead, in order guarantee that symbol references remain
unique.

@param buffer The buffer containing the new symbol.
@param offset The offset into the buffer of the new symbol.
@param len The length of the new symbol in the buffer.
src/main/java/com/alibaba/fastjson/parser/SymbolTable.java