1154 lines
40 KiB
Python
1154 lines
40 KiB
Python
"""I wanted to try to use the code in `parser.py` to do real work, and as you
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might expect the code did NOT work acceptibly.
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This version has some performance work done.
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2023
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"""
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import dataclasses
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import functools
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import typing
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###############################################################################
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# LR0
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#
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# We start with LR0 parsers, because they form the basis of everything else.
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###############################################################################
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class Configuration:
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"""A rule being tracked in a state.
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(Note: technically, lookahead isn't used until we get to LR(1) parsers,
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but if left at its default it's harmless. Ignore it until you get to
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the part about LR(1).)
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"""
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__slots__ = (
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'name',
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'symbols',
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'position',
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'lookahead',
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'next',
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'at_end',
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'_vals',
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'_hash',
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)
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name: int
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symbols: typing.Tuple[int, ...]
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position: int
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lookahead: typing.Tuple[int, ...]
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next: int | None
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at_end: bool
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_vals: typing.Tuple
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_hash: int
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def __init__(self, name, symbols, position, lookahead) -> None:
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self.name = name
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self.symbols = symbols
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self.position = position
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self.lookahead = lookahead
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at_end = position == len(symbols)
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self.at_end = at_end
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self.next = symbols[position] if not at_end else None
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self._vals = (name, symbols, position, lookahead)
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self._hash = hash(self._vals)
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@classmethod
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def from_rule(cls, name: int, symbols: typing.Tuple[int, ...], lookahead=()):
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return Configuration(
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name=name,
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symbols=symbols,
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position=0,
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lookahead=lookahead,
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)
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def __hash__(self) -> int:
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return self._hash
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def __eq__(self, value: object, /) -> bool:
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if value is self:
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return True
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if not isinstance(value, Configuration):
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return NotImplemented
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return (
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value._hash == self._hash and
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value.name == self.name and
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value.position == self.position and
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value.symbols == self.symbols and
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value.lookahead == self.lookahead
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)
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def __lt__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals < value._vals
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def __gt__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals > value._vals
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def __le__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals <= value._vals
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def __ge__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals >= value._vals
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def replace_position(self, new_position):
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return Configuration(
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name=self.name,
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symbols=self.symbols,
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position=new_position,
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lookahead=self.lookahead,
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)
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def clear_lookahead(self):
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return Configuration(
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name=self.name,
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symbols=self.symbols,
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position=self.position,
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lookahead=(),
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)
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@property
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def rest(self):
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return self.symbols[(self.position+1):]
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def format(self, alphabet: list[str]) -> str:
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la = ", " + str(tuple(alphabet[i] for i in self.lookahead)) if self.lookahead != () else ""
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return "{name} -> {bits}{lookahead}".format(
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name=self.name,
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bits=' '.join([
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'* ' + alphabet[sym] if i == self.position else alphabet[sym]
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for i, sym in enumerate(self.symbols)
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]) + (' *' if self.at_end else ''),
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lookahead=la,
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)
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ConfigSet = typing.Tuple[Configuration, ...]
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class TableBuilder(object):
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def __init__(self, alphabet: list[str]):
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self.errors = []
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self.table = []
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self.alphabet = alphabet
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self.row = None
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def flush(self):
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self._flush_row()
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if len(self.errors) > 0:
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raise ValueError("\n\n".join(self.errors))
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return self.table
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def new_row(self, config_set):
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self._flush_row()
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self.row = [(None, None) for _ in self.alphabet]
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self.current_config_set = config_set
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def _flush_row(self):
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if self.row:
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actions = {
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self.alphabet[k]: v[0]
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for k, v in enumerate(self.row)
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if v[0] is not None
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}
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self.table.append(actions)
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def set_table_reduce(self, symbol: int, config):
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action = ('reduce', self.alphabet[config.name], len(config.symbols))
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self._set_table_action(symbol, action, config)
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def set_table_accept(self, symbol: int, config: Configuration):
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action = ('accept',)
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self._set_table_action(symbol, action, config)
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def set_table_shift(self, symbol: int, index: int, config: Configuration):
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action = ('shift', index)
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self._set_table_action(symbol, action, config)
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def set_table_goto(self, symbol: int, index: int):
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action = ('goto', index)
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self._set_table_action(symbol, action, None)
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def _set_table_action(self, symbol_id: int, action, config):
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"""Set the action for 'symbol' in the table row to 'action'.
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This is destructive; it changes the table. It raises an error if
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there is already an action for the symbol in the row.
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"""
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assert isinstance(symbol_id, int)
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assert self.row is not None
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existing, existing_config = self.row[symbol_id]
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if existing is not None and existing != action:
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config_old = str(existing_config)
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config_new = str(config)
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max_len = max(len(config_old), len(config_new)) + 1
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error = (
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"Conflicting actions for token '{symbol}':\n"
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" {config_old: <{max_len}}: {old}\n"
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" {config_new: <{max_len}}: {new}\n".format(
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config_old=config_old,
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config_new=config_new,
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max_len=max_len,
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old=existing,
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new=action,
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symbol=self.alphabet[symbol_id],
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)
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)
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self.errors.append(error)
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self.row[symbol_id] = (action, config)
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class ConfigurationSetInfo:
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config_set_key: dict[ConfigSet, int]
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sets: list[ConfigSet]
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successors: list[dict[int, int]]
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def __init__(self):
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self.config_set_key = {}
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self.sets = []
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self.successors = []
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def register_config_set(self, c: ConfigSet) -> typing.Tuple[int, bool]:
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existing = self.config_set_key.get(c)
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if existing is not None:
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return existing, False
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index = len(self.sets)
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self.sets.append(c)
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self.successors.append({})
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self.config_set_key[c] = index
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return index, True
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def add_successor(self, c_id: int, symbol: int, successor: int):
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self.successors[c_id][symbol] = successor
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class GenerateLR0(object):
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"""Generate parser tables for an LR0 parser.
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The input grammars are of the form:
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grammar_simple = [
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('E', ['E', '+', 'T']),
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('E', ['T']),
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('T', ['(', 'E', ')']),
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('T', ['id']),
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]
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Which is to say, they are a list of productions. Each production is a
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tuple where the first element of the tuple is the name of the
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non-terminal being added, and the second elment of the tuple is the
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list of terminals and non-terminals that make up the production.
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There is currently no support for custom actions or alternation or
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anything like that. If you want alternations that you'll have to lower
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the grammar by hand into the simpler form first.
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Don't name anything with double-underscores; those are reserved for
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the generator. Don't add '$' either, as it is reserved to mean
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end-of-stream. Use an empty list to indicate nullability, that is:
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('O', []),
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means that O can be matched with nothing.
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Implementation notes:
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- This is implemented in the dumbest way possible, in order to be the
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most understandable it can be. I built this to learn, and I want to
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make sure I can keep learning with it.
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- We tend to use tuples everywhere. This is because tuples can be
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compared for equality and put into tables and all that jazz. They might
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be a little bit slower in places but like I said, this is for
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learning. (Also, if we need this to run faster we can probably go a
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long way by memoizing results, which is much easier if we have tuples
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everywhere.)
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"""
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alphabet: list[str]
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grammar: list[list[typing.Tuple[int, ...]]]
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nonterminals: typing.Tuple[bool, ...]
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terminals: typing.Tuple[bool, ...]
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symbol_key: dict[str, int]
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start_symbol: int
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end_symbol: int
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config_sets_key: dict[ConfigSet, int]
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successors: list[set[int]]
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def __init__(self, start: str, grammar: list[typing.Tuple[str, list[str]]]):
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"""Initialize the parser generator with the specified grammar and
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start symbol.
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"""
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# Work out the alphabet.
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alphabet = set()
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for name, rule in grammar:
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alphabet.add(name)
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alphabet.update(symbol for symbol in rule)
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# Check to make sure they didn't use anything that will give us
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# heartburn later.
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reserved = [a for a in alphabet if a.startswith('__') or a == '$']
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if reserved:
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raise ValueError(
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"Can't use {symbols} in grammars, {what} reserved.".format(
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symbols=' or '.join(reserved),
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what="it's" if len(reserved) == 1 else "they're",
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)
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)
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alphabet.add('__start')
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alphabet.add('$')
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self.alphabet = list(sorted(alphabet))
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symbol_key = {
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symbol: index
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for index, symbol in enumerate(self.alphabet)
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}
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start_symbol = symbol_key['__start']
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end_symbol = symbol_key['$']
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assert self.alphabet[start_symbol] == '__start'
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assert self.alphabet[end_symbol] == '$'
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# Turn the incoming grammar into a dictionary, indexed by nonterminal.
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#
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# We count on python dictionaries retaining the insertion order, like
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# it or not.
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full_grammar = [list() for _ in self.alphabet]
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terminals = [True for _ in self.alphabet]
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assert terminals[end_symbol]
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nonterminals = [False for _ in self.alphabet]
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for name, rule in grammar:
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name_symbol = symbol_key[name]
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terminals[name_symbol] = False
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nonterminals[name_symbol] = True
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rules = full_grammar[name_symbol]
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rules.append(tuple(symbol_key[symbol] for symbol in rule))
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self.grammar = full_grammar
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self.grammar[start_symbol].append((symbol_key[start],))
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terminals[start_symbol] = False
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nonterminals[start_symbol] = True
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self.terminals = tuple(terminals)
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self.nonterminals = tuple(nonterminals)
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assert self.terminals[end_symbol]
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assert self.nonterminals[start_symbol]
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self.symbol_key = symbol_key
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self.start_symbol = start_symbol
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self.end_symbol = end_symbol
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@functools.cache
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def gen_closure_next(self, config: Configuration):
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"""Return the next set of configurations in the closure for
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config.
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If the position for config is just before a non-terminal, then the
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next set of configurations is configurations for all of the
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productions for that non-terminal, with the position at the
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beginning. (If the position for config is just before a terminal,
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or at the end of the production, then the next set is empty.)
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"""
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next = config.next
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if next is None:
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return ()
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else:
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return tuple(
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Configuration.from_rule(next, rule)
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for rule in self.grammar[next]
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)
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@functools.cache
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def gen_closure(self, seeds: typing.Iterable[Configuration]) -> ConfigSet:
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"""Compute the closure for the specified configs. The closure is all
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of the configurations we could be in. Specifically, if the position
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for a config is just before a non-terminal then we must also consider
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configurations where the rule is the rule for the non-terminal and
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the position is just before the beginning of the rule.
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(We have replaced a recursive version with an iterative one.)
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"""
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closure = set()
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pending = list(seeds)
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while len(pending) > 0:
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config = pending.pop()
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if config in closure:
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continue
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closure.add(config)
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for next_config in self.gen_closure_next(config):
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pending.append(next_config)
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return tuple(sorted(closure)) # TODO: Why tuple?
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@functools.cache
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def gen_successor(self, config_set: typing.Iterable[Configuration], symbol: str) -> ConfigSet:
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"""Compute the successor state for the given config set and the
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given symbol.
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The successor represents the next state of the parser after seeing
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the symbol.
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"""
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seeds = tuple(
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config.replace_position(config.position + 1)
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for config in config_set
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if config.next == symbol
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)
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closure = self.gen_closure(seeds)
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return closure
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def gen_all_successors(self, config_set: typing.Iterable[Configuration]) -> list[typing.Tuple[int, ConfigSet]]:
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"""Return all of the non-empty successors for the given config set."""
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possible = tuple(sorted({
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config.next
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for config in config_set
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if config.next is not None
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}))
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next = []
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for symbol in possible:
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successor = self.gen_successor(config_set, symbol)
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if len(successor) > 0:
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next.append((symbol, successor))
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return next
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def gen_sets(self, config_set: typing.Tuple[Configuration,...]) -> ConfigurationSetInfo:
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"""Generate all configuration sets starting from the provided set."""
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result = ConfigurationSetInfo()
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successors = []
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pending = [config_set]
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while len(pending) > 0:
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config_set = pending.pop()
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id, is_new = result.register_config_set(config_set)
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if is_new:
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for symbol, successor in self.gen_all_successors(config_set):
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successors.append((id,symbol,successor))
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pending.append(successor)
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for id,symbol,successor in successors:
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result.add_successor(id, symbol, result.config_set_key[successor])
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return result
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def gen_all_sets(self) -> ConfigurationSetInfo:
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"""Generate all of the configuration sets for the grammar."""
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seeds = tuple(
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Configuration.from_rule(self.start_symbol, rule)
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for rule in self.grammar[self.start_symbol]
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)
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initial_set = self.gen_closure(seeds)
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return self.gen_sets(initial_set)
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def build_set_index(self, sets: typing.Tuple[ConfigSet, ...]) -> dict[ConfigSet, int]:
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return { s: index for index, s in enumerate(sets) }
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def find_set_index(self, sets: dict[ConfigSet, int], s: ConfigSet) -> int | None:
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"""Find the specified set in the set of sets, and return the
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index, or None if it is not found.
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"""
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return sets.get(s)
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def gen_reduce_set(self, config: Configuration) -> typing.Iterable[int]:
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"""Return the set of symbols that indicate we should reduce the given
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configuration.
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In an LR0 parser, this is just the set of all terminals."""
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del(config)
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return [index for index, value in enumerate(self.terminals) if value]
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def gen_table(self):
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"""Generate the parse table.
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The parse table is a list of states. The first state in the list is
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the starting state. Each state is a dictionary that maps a symbol to an
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action. Each action is a tuple. The first element of the tuple is a
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string describing what to do:
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- 'shift': The second element of the tuple is the state
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number. Consume the input and push that state onto the stack.
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- 'reduce': The second element is the name of the non-terminal being
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reduced, and the third element is the number of states to remove
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from the stack. Don't consume the input; just remove the specified
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number of things from the stack, and then consult the table again,
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this time using the new top-of-stack as the current state and the
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name of the non-terminal to find out what to do.
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- 'goto': The second element is the state number to push onto the
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stack. In the literature, these entries are treated distinctly from
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the actions, but we mix them here because they never overlap with the
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other actions. (These are always associated with non-terminals, and
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the other actions are always associated with terminals.)
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- 'accept': Accept the result of the parse, it worked.
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Anything missing from the row indicates an error.
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"""
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builder = TableBuilder(self.alphabet)
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config_sets = self.gen_all_sets()
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# WHAT.
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# set_index = self.build_set_index(config_sets)
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for config_set_id, config_set in enumerate(config_sets.sets):
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builder.new_row(config_set)
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successors = config_sets.successors[config_set_id]
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for config in config_set:
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config_next = config.next
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if config_next is None:
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if config.name != self.start_symbol:
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for a in self.gen_reduce_set(config):
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builder.set_table_reduce(a, config)
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else:
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builder.set_table_accept(self.end_symbol, config)
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elif self.terminals[config_next]:
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index = successors[config_next]
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builder.set_table_shift(config_next, index, config)
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# Gotos
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for symbol, index in successors.items():
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if self.nonterminals[symbol]:
|
|
builder.set_table_goto(symbol, index)
|
|
|
|
return builder.flush()
|
|
|
|
|
|
def parse(table, input, trace=False):
|
|
"""Parse the input with the generated parsing table and return the
|
|
concrete syntax tree.
|
|
|
|
The parsing table can be generated by GenerateLR0.gen_table() or by any
|
|
of the other generators below. The parsing mechanism never changes, only
|
|
the table generation mechanism.
|
|
|
|
input is a list of tokens. Don't stick an end-of-stream marker, I'll stick
|
|
one on for you.
|
|
"""
|
|
assert '$' not in input
|
|
input = input + ['$']
|
|
input_index = 0
|
|
|
|
# Our stack is a stack of tuples, where the first entry is the state number
|
|
# and the second entry is the 'value' that was generated when the state was
|
|
# pushed.
|
|
stack : list[typing.Tuple[int, typing.Any]] = [(0, None)]
|
|
while True:
|
|
current_state = stack[-1][0]
|
|
current_token = input[input_index]
|
|
|
|
action = table[current_state].get(current_token, ('error',))
|
|
if trace:
|
|
print("{stack: <20} {input: <50} {action: <5}".format(
|
|
stack=repr([s[0] for s in stack]),
|
|
input=repr(input[input_index:]),
|
|
action=repr(action)
|
|
))
|
|
|
|
if action[0] == 'accept':
|
|
return stack[-1][1]
|
|
|
|
elif action[0] == 'reduce':
|
|
name = action[1]
|
|
size = action[2]
|
|
|
|
value = (name, tuple(s[1] for s in stack[-size:]))
|
|
stack = stack[:-size]
|
|
|
|
goto = table[stack[-1][0]].get(name, ('error',))
|
|
assert goto[0] == 'goto' # Corrupt table?
|
|
stack.append((goto[1], value))
|
|
|
|
elif action[0] == 'shift':
|
|
stack.append((action[1], (current_token, ())))
|
|
input_index += 1
|
|
|
|
elif action[0] == 'error':
|
|
raise ValueError(
|
|
'Syntax error: unexpected symbol {sym}'.format(
|
|
sym=current_token,
|
|
),
|
|
)
|
|
|
|
|
|
###############################################################################
|
|
# SLR(1)
|
|
###############################################################################
|
|
def add_changed(items: set, item)->bool:
|
|
old_len = len(items)
|
|
items.add(item)
|
|
return old_len != len(items)
|
|
|
|
def update_changed(items: set, other: set) -> bool:
|
|
old_len = len(items)
|
|
items.update(other)
|
|
return old_len != len(items)
|
|
|
|
@dataclasses.dataclass(frozen=True)
|
|
class FirstInfo:
|
|
firsts: list[set[int]]
|
|
is_epsilon: list[bool]
|
|
|
|
@classmethod
|
|
def from_grammar(
|
|
cls,
|
|
grammar: list[list[typing.Tuple[int,...]]],
|
|
terminals: typing.Tuple[bool, ...],
|
|
):
|
|
firsts = [set() for _ in grammar]
|
|
|
|
# Add all terminals to their own firsts
|
|
for index, is_terminal in enumerate(terminals):
|
|
if is_terminal:
|
|
firsts[index].add(index)
|
|
|
|
epsilons = [False] * len(grammar)
|
|
changed = True
|
|
while changed:
|
|
changed = False
|
|
for name, rules in enumerate(grammar):
|
|
f = firsts[name]
|
|
for rule in rules:
|
|
if len(rule) == 0:
|
|
changed = changed or not epsilons[name]
|
|
epsilons[name] = True
|
|
continue
|
|
|
|
for index, symbol in enumerate(rule):
|
|
if terminals[symbol]:
|
|
changed = add_changed(f, symbol) or changed
|
|
else:
|
|
other_firsts = firsts[symbol]
|
|
changed = update_changed(f, other_firsts) or changed
|
|
|
|
is_last = index == len(rule) - 1
|
|
if is_last and epsilons[symbol]:
|
|
# If this is the last symbol and the last
|
|
# symbol can be empty then I can be empty
|
|
# too! :P
|
|
changed = changed or not epsilons[name]
|
|
epsilons[name] = True
|
|
|
|
if not epsilons[symbol]:
|
|
# If we believe that there is at least one
|
|
# terminal in the first set of this
|
|
# nonterminal then I don't have to keep
|
|
# looping through the symbols in this rule.
|
|
break
|
|
|
|
return FirstInfo(firsts=firsts, is_epsilon=epsilons)
|
|
|
|
@dataclasses.dataclass(frozen=True)
|
|
class FollowInfo:
|
|
follows: list[set[int]]
|
|
|
|
@classmethod
|
|
def from_grammar(
|
|
cls,
|
|
grammar: list[list[typing.Tuple[int,...]]],
|
|
terminals: typing.Tuple[bool, ...],
|
|
start_symbol: int,
|
|
end_symbol: int,
|
|
firsts: FirstInfo,
|
|
):
|
|
follows = [set() for _ in grammar]
|
|
follows[start_symbol].add(end_symbol)
|
|
|
|
changed = True
|
|
while changed:
|
|
changed = False
|
|
for name, rules in enumerate(grammar):
|
|
for rule in rules:
|
|
epsilon = True
|
|
prev_symbol = None
|
|
for symbol in reversed(rule):
|
|
f = follows[symbol]
|
|
if terminals[symbol]:
|
|
# This particular rule can't produce epsilon.
|
|
epsilon = False
|
|
prev_symbol = symbol
|
|
continue
|
|
|
|
# While epsilon is still set, update the follow of
|
|
# this nonterminal with the follow of the production
|
|
# we're processing. (This also means that the follow
|
|
# of the last symbol in the production is the follow
|
|
# of the entire production, as it should be.)
|
|
if epsilon:
|
|
changed = update_changed(f, follows[name]) or changed
|
|
|
|
# If we're not at the end of the list then the follow
|
|
# of the current symbol contains the first of the
|
|
# next symbol.
|
|
if prev_symbol is not None:
|
|
changed = update_changed(f, firsts.firsts[prev_symbol]) or changed
|
|
|
|
# Now if there's no epsilon in this symbol there's no
|
|
# more epsilon in the rest of the sequence.
|
|
if not firsts.is_epsilon[symbol]:
|
|
epsilon = False
|
|
|
|
prev_symbol = symbol
|
|
|
|
return FollowInfo(follows=follows)
|
|
|
|
|
|
|
|
class GenerateSLR1(GenerateLR0):
|
|
"""Generate parse tables for SLR1 grammars.
|
|
|
|
SLR1 parsers can recognize more than LR0 parsers, because they have a
|
|
little bit more information: instead of generating reduce actions for a
|
|
production on all possible inputs, as LR0 parsers do, they generate
|
|
reduce actions only for inputs that are in the 'follow' set of the
|
|
non-terminal.
|
|
|
|
That means SLR1 parsers need to know how to generate 'follow(A)', which
|
|
means they need to know how to generate 'first(A)', which is most of the
|
|
code in this class.
|
|
"""
|
|
_firsts: FirstInfo
|
|
|
|
def __init__(self, *args, **kwargs):
|
|
super().__init__(*args, **kwargs)
|
|
self._firsts = FirstInfo.from_grammar(self.grammar, self.terminals)
|
|
self._follows = FollowInfo.from_grammar(
|
|
self.grammar,
|
|
self.terminals,
|
|
self.start_symbol,
|
|
self.end_symbol,
|
|
self._firsts,
|
|
)
|
|
|
|
def gen_first(self, symbols: typing.Iterable[int]) -> typing.Tuple[set[int], bool]:
|
|
"""Return the first set for a sequence of symbols.
|
|
|
|
Build the set by combining the first sets of the symbols from left to
|
|
right as long as epsilon remains in the first set. If we reach the end
|
|
and every symbol has had epsilon, then this set also has epsilon.
|
|
|
|
Otherwise we can stop as soon as we get to a non-epsilon first(), and
|
|
our result does not have epsilon.
|
|
"""
|
|
result = set()
|
|
for s in symbols:
|
|
result.update(self._firsts.firsts[s])
|
|
if s not in self._firsts.is_epsilon:
|
|
return (result, False)
|
|
|
|
return (result, True)
|
|
|
|
def gen_follow(self, symbol: int) -> set[int]:
|
|
"""Generate the follow set for the given nonterminal.
|
|
|
|
The follow set for a nonterminal is the set of terminals that can
|
|
follow the nonterminal in a valid sentence. The resulting set never
|
|
contains epsilon and is never empty, since we should always at least
|
|
ground out at '$', which is the end-of-stream marker.
|
|
"""
|
|
return self._follows.follows[symbol]
|
|
|
|
def gen_reduce_set(self, config: Configuration) -> typing.Iterable[int]:
|
|
"""Return the set of symbols that indicate we should reduce the given
|
|
config.
|
|
|
|
In an SLR1 parser, this is the follow set of the config nonterminal."""
|
|
return self.gen_follow(config.name)
|
|
|
|
|
|
class GenerateLR1(GenerateSLR1):
|
|
"""Generate parse tables for LR1, or "canonical LR" grammars.
|
|
|
|
LR1 parsers can recognize more than SLR parsers. Like SLR parsers, they
|
|
are choosier about when they reduce. But unlike SLR parsers, they specify
|
|
the terminals on which they reduce by carrying a 'lookahead' terminal in
|
|
the configuration. The lookahead of a configuration is computed as the
|
|
closure of a configuration set is computed, so see gen_closure_next for
|
|
details. (Except for the start configuration, which has '$' as its
|
|
lookahead.)
|
|
"""
|
|
def gen_reduce_set(self, config: Configuration) -> typing.Iterable[int]:
|
|
"""Return the set of symbols that indicate we should reduce the given
|
|
config.
|
|
|
|
In an LR1 parser, this is the lookahead of the configuration."""
|
|
return config.lookahead
|
|
|
|
@functools.cache
|
|
def gen_closure_next(self, config: Configuration):
|
|
"""Return the next set of configurations in the closure for
|
|
config.
|
|
|
|
In LR1 parsers, we must compute the lookahead for the configurations
|
|
we're adding to the closure. The lookahead for the new configurations
|
|
is the first() of the rest of this config's production. If that
|
|
contains epsilon, then the lookahead *also* contains the lookahead we
|
|
already have. (This lookahead was presumably generated by the same
|
|
process, so in some sense it is a 'parent' lookahead, or a lookahead
|
|
from an upstream production in the grammar.)
|
|
|
|
(See the documentation in GenerateLR0 for more information on how
|
|
this function fits into the whole process.)
|
|
"""
|
|
config_next = config.next
|
|
if config_next is None:
|
|
return ()
|
|
else:
|
|
next = []
|
|
for rule in self.grammar[config_next]:
|
|
lookahead, epsilon = self.gen_first(config.rest)
|
|
if epsilon:
|
|
lookahead.update(config.lookahead)
|
|
lookahead = tuple(sorted(lookahead))
|
|
next.append(Configuration.from_rule(config_next, rule, lookahead=lookahead))
|
|
|
|
return tuple(next)
|
|
|
|
def gen_all_sets(self):
|
|
"""Generate all of the configuration sets for the grammar.
|
|
|
|
In LR1 parsers, we must remember to set the lookahead of the start
|
|
symbol to '$'.
|
|
"""
|
|
seeds = tuple(
|
|
Configuration.from_rule(self.start_symbol, rule, lookahead=(self.end_symbol,))
|
|
for rule in self.grammar[self.start_symbol]
|
|
)
|
|
initial_set = self.gen_closure(seeds)
|
|
return self.gen_sets(initial_set)
|
|
|
|
|
|
class GenerateLALR(GenerateLR1):
|
|
"""Generate tables for LALR.
|
|
|
|
LALR is smaller than LR(1) but bigger than SLR(1). It works by generating
|
|
the LR(1) configuration sets, but merging configuration sets which are
|
|
equal in everything but their lookaheads. This works in that it doesn't
|
|
generate any shift/reduce conflicts that weren't already in the LR(1)
|
|
grammar. It can, however, introduce new reduce/reduce conflicts, because
|
|
it does lose information. The advantage is that the number of parser
|
|
states is much much smaller in LALR than in LR(1).
|
|
|
|
(Note that because we use immutable state everywhere this generator does
|
|
a lot of copying and allocation.)
|
|
"""
|
|
def merge_sets(self, config_set_a, config_set_b):
|
|
"""Merge the two config sets, by keeping the item cores but merging
|
|
the lookahead sets for each item.
|
|
"""
|
|
assert len(config_set_a) == len(config_set_b)
|
|
merged = []
|
|
for index, a in enumerate(config_set_a):
|
|
b = config_set_b[index]
|
|
assert a.clear_lookahead() == b.clear_lookahead()
|
|
|
|
new_lookahead = a.lookahead + b.lookahead
|
|
new_lookahead = tuple(sorted(set(new_lookahead)))
|
|
merged.append(a.clear_lookahead())
|
|
|
|
return tuple(merged)
|
|
|
|
def sets_equal(self, a, b):
|
|
a_no_la = tuple(s.clear_lookahead() for s in a)
|
|
b_no_la = tuple(s.clear_lookahead() for s in b)
|
|
return a_no_la == b_no_la
|
|
|
|
def gen_sets(self, config_set) -> ConfigurationSetInfo:
|
|
"""Recursively generate all configuration sets starting from the
|
|
provided set, and merge them with the provided set 'F'.
|
|
|
|
The difference between this method and the one in GenerateLR0, where
|
|
this comes from, is in the part that stops recursion. In LALR we
|
|
compare for set equality *ignoring lookahead*. If we find a match,
|
|
then instead of returning F unchanged, we merge the two equal sets
|
|
and replace the set in F, returning the modified set.
|
|
"""
|
|
F = {}
|
|
successors = []
|
|
pending = [config_set]
|
|
while len(pending) > 0:
|
|
config_set = pending.pop()
|
|
config_set_no_la = tuple(s.clear_lookahead() for s in config_set)
|
|
|
|
existing = F.get(config_set_no_la)
|
|
if existing is not None:
|
|
F[config_set_no_la] = self.merge_sets(config_set, existing)
|
|
else:
|
|
F[config_set_no_la] = config_set
|
|
for symbol, successor in self.gen_all_successors(config_set):
|
|
successor_no_la = tuple(s.clear_lookahead() for s in successor)
|
|
successors.append((config_set_no_la, symbol, successor_no_la))
|
|
pending.append(successor)
|
|
|
|
# Register all the actually merged, final config sets.
|
|
result = ConfigurationSetInfo()
|
|
for config_set in F.values():
|
|
result.register_config_set(config_set)
|
|
|
|
# Now record all the successors that we found. Of course, the actual
|
|
# sets that wound up in the ConfigurationSetInfo don't match anything
|
|
# we found during the previous phase.
|
|
#
|
|
# *Fortunately* we recorded the no-lookahead keys in the successors
|
|
# so we can find the final sets, then look them up in the registered
|
|
# sets, and actually register the successor.
|
|
for config_set_no_la, symbol, successor_no_la in successors:
|
|
actual_config_set = F[config_set_no_la]
|
|
from_index = result.config_set_key[actual_config_set]
|
|
|
|
actual_successor = F[successor_no_la]
|
|
to_index = result.config_set_key[actual_successor]
|
|
|
|
result.add_successor(from_index, symbol, to_index)
|
|
|
|
return result
|
|
|
|
def set_without_lookahead(self, config_set: ConfigSet) -> ConfigSet:
|
|
return tuple(sorted(set(c.clear_lookahead() for c in config_set)))
|
|
|
|
def build_set_index(self, sets: typing.Tuple[ConfigSet, ...]) -> dict[ConfigSet, int]:
|
|
index = {}
|
|
for s in sets:
|
|
s_no_la = self.set_without_lookahead(s)
|
|
if s_no_la not in index:
|
|
index[s_no_la] = len(index)
|
|
return index
|
|
|
|
def find_set_index(self, sets: dict[ConfigSet, int], s: ConfigSet) -> int | None:
|
|
"""Find the specified set in the set of sets, and return the
|
|
index, or None if it is not found.
|
|
"""
|
|
s_no_la = self.set_without_lookahead(s)
|
|
return sets.get(s_no_la)
|
|
|
|
|
|
###############################################################################
|
|
# Formatting
|
|
###############################################################################
|
|
def format_node(node):
|
|
"""Print out an indented concrete syntax tree, from parse()."""
|
|
lines = [
|
|
'{name}'.format(name=node[0])
|
|
] + [
|
|
' ' + line
|
|
for child in node[1]
|
|
for line in format_node(child).split('\n')
|
|
]
|
|
return '\n'.join(lines)
|
|
|
|
|
|
def format_table(generator, table):
|
|
"""Format a parser table so pretty."""
|
|
def format_action(state, terminal):
|
|
action = state.get(terminal, ('error',))
|
|
if action[0] == 'accept':
|
|
return 'accept'
|
|
elif action[0] == 'shift':
|
|
return 's' + str(action[1])
|
|
elif action[0] == 'error':
|
|
return ''
|
|
elif action[0] == 'reduce':
|
|
return 'r' + str(action[1])
|
|
|
|
terminals = [
|
|
generator.alphabet[i]
|
|
for i,v in enumerate(generator.terminals)
|
|
if v
|
|
]
|
|
nonterminals = [
|
|
generator.alphabet[i]
|
|
for i,v in enumerate(generator.nonterminals)
|
|
if v
|
|
]
|
|
header = " | {terms} | {nts}".format(
|
|
terms=' '.join(
|
|
'{0: <6}'.format(terminal)
|
|
for terminal in sorted(terminals)
|
|
),
|
|
nts=' '.join(
|
|
'{0: <5}'.format(nt)
|
|
for nt in sorted(nonterminals)
|
|
),
|
|
)
|
|
|
|
lines = [
|
|
header,
|
|
'-' * len(header),
|
|
] + [
|
|
"{index: <3} | {actions} | {gotos}".format(
|
|
index=i,
|
|
actions=' '.join(
|
|
'{0: <6}'.format(format_action(row, terminal))
|
|
for terminal in sorted(terminals)
|
|
),
|
|
gotos=' '.join(
|
|
'{0: <5}'.format(row.get(nt, ('error', ''))[1])
|
|
for nt in sorted(nonterminals)
|
|
),
|
|
)
|
|
for i, row in enumerate(table)
|
|
]
|
|
return '\n'.join(lines)
|
|
|
|
|
|
###############################################################################
|
|
# Examples
|
|
###############################################################################
|
|
def examples():
|
|
# OK, this is a very simple LR0 grammar.
|
|
print("grammar_simple:")
|
|
grammar_simple = [
|
|
('E', ['E', '+', 'T']),
|
|
('E', ['T']),
|
|
('T', ['(', 'E', ')']),
|
|
('T', ['id']),
|
|
]
|
|
|
|
gen = GenerateLR0('E', grammar_simple)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
tree = parse(table, ['id', '+', '(', 'id', ')'])
|
|
print(format_node(tree) + "\n")
|
|
print()
|
|
|
|
# This one doesn't work with LR0, though, it has a shift/reduce conflict.
|
|
print("grammar_lr0_shift_reduce (LR0):")
|
|
grammar_lr0_shift_reduce = grammar_simple + [
|
|
('T', ['id', '[', 'E', ']']),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_lr0_shift_reduce)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Nor does this: it has a reduce/reduce conflict.
|
|
print("grammar_lr0_reduce_reduce (LR0):")
|
|
grammar_lr0_reduce_reduce = grammar_simple + [
|
|
('E', ['V', '=', 'E']),
|
|
('V', ['id']),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_lr0_reduce_reduce)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Nullable symbols just don't work with constructs like this, because you can't
|
|
# look ahead to figure out if you should reduce an empty 'F' or not.
|
|
print("grammar_nullable (LR0):")
|
|
grammar_nullable = [
|
|
('E', ['F', 'boop']),
|
|
('F', ['beep']),
|
|
('F', []),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_nullable)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
|
|
print("grammar_lr0_shift_reduce (SLR1):")
|
|
gen = GenerateSLR1('E', grammar_lr0_shift_reduce)
|
|
first, epsilon=gen.gen_first((gen.symbol_key['E'],))
|
|
print(f"First: {str(first)} (epsilon={epsilon})")
|
|
print(f"Follow: {str(gen.gen_follow(gen.symbol_key['E']))}")
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
tree = parse(table, ['id', '+', '(', 'id', '[', 'id', ']', ')'], trace=True)
|
|
print(format_node(tree) + "\n")
|
|
print()
|
|
|
|
# SLR1 can't handle this.
|
|
print("grammar_aho_ullman_1 (SLR1):")
|
|
grammar_aho_ullman_1 = [
|
|
('S', ['L', '=', 'R']),
|
|
('S', ['R']),
|
|
('L', ['*', 'R']),
|
|
('L', ['id']),
|
|
('R', ['L']),
|
|
]
|
|
try:
|
|
gen = GenerateSLR1('S', grammar_aho_ullman_1)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Here's an example with a full LR1 grammar, though.
|
|
print("grammar_aho_ullman_2 (LR1):")
|
|
grammar_aho_ullman_2 = [
|
|
('S', ['X', 'X']),
|
|
('X', ['a', 'X']),
|
|
('X', ['b']),
|
|
]
|
|
gen = GenerateLR1('S', grammar_aho_ullman_2)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
parse(table, ['b', 'a', 'a', 'b'], trace=True)
|
|
print()
|
|
|
|
# What happens if we do LALR to it?
|
|
print("grammar_aho_ullman_2 (LALR):")
|
|
gen = GenerateLALR('S', grammar_aho_ullman_2)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
print()
|
|
|
|
# A fun LALAR grammar.
|
|
print("grammar_lalr:")
|
|
grammar_lalr = [
|
|
('S', ['V', 'E']),
|
|
|
|
('E', ['F']),
|
|
('E', ['E', '+', 'F']),
|
|
|
|
('F', ['V']),
|
|
('F', ['int']),
|
|
('F', ['(', 'E', ')']),
|
|
|
|
('V', ['id']),
|
|
]
|
|
gen = GenerateLALR('S', grammar_lalr)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
print()
|
|
|
|
if __name__=="__main__":
|
|
examples()
|