gitmirror/lark/parsers/lalr_parser.py

"""This module implements a LALR(1) Parser
"""
# Author: Erez Shinan (2017)
# Email : erezshin@gmail.com
from ..exceptions import UnexpectedToken
from ..lexer import Token
from ..utils import Enumerator, Serialize
from ..grammar import END

from .lalr_analysis import LALR_Analyzer, Shift, IntParseTable


###{standalone
class LALR_Parser(object):
    def __init__(self, parser_conf, debug=False):
        assert all(r.options is None or r.options.priority is None
                   for r in parser_conf.rules), "LALR doesn't yet support prioritization"
        analysis = LALR_Analyzer(parser_conf, debug=debug)
        analysis.compute_lookahead()
        callbacks = parser_conf.callbacks

        self._parse_table = analysis.parse_table
        self.parser_conf = parser_conf
        self.parser = _Parser(analysis.parse_table, callbacks)

    @classmethod
    def deserialize(cls, data, memo, callbacks):
        inst = cls.__new__(cls)
        inst._parse_table = IntParseTable.deserialize(data, memo)
        inst.parser = _Parser(inst._parse_table, callbacks)
        return inst

    def serialize(self, memo):
        return self._parse_table.serialize(memo)

    def parse(self, *args):
        return self.parser.parse(*args)


class _Parser:
    def __init__(self, parse_table, callbacks):
        self.states = parse_table.states
        self.start_states = parse_table.start_states
        self.end_states = parse_table.end_states
        self.callbacks = callbacks

    def parse(self, seq, start, set_state=None):
        token = None
        stream = iter(seq)
        states = self.states

        start_state = self.start_states[start]
        end_state = self.end_states[start]

        state_stack = [start_state]
        value_stack = []

        if set_state: set_state(start_state)

        def get_action(token):
            state = state_stack[-1]
            try:
                return states[state][token.type]
            except KeyError:
                expected = [s for s in states[state].keys() if s.isupper()]
                raise UnexpectedToken(token, expected, state=state)

        def reduce(rule):
            size = len(rule.expansion)
            if size:
                s = value_stack[-size:]
                del state_stack[-size:]
                del value_stack[-size:]
            else:
                s = []

            value = self.callbacks[rule](s)

            _action, new_state = states[state_stack[-1]][rule.origin.name]
            assert _action is Shift
            state_stack.append(new_state)
            value_stack.append(value)

        # Main LALR-parser loop
        for token in stream:
            while True:
                action, arg = get_action(token)
                assert arg != end_state

                if action is Shift:
                    state_stack.append(arg)
                    value_stack.append(token)
                    if set_state: set_state(arg)
                    break # next token
                else:
                    reduce(arg)

        token = Token.new_borrow_pos(END, None, token) if token else Token(END, None, 0, 1, 1)
        while True:
            _action, arg = get_action(token)
            if _action is Shift:
                if arg == end_state:
                    val ,= value_stack
                    return val
                state_stack.append(arg)
            else:
                reduce(arg)

###}