Merge pull request #65 from ehudt/master

Adds a CYK parser to lark.

lark/lark.py

@@ -59,7 +59,7 @@ class LarkOptions(object):
         self.propagate_positions = o.pop('propagate_positions', False)
         self.earley__predict_all = o.pop('earley__predict_all', False)
 
-        assert self.parser in ('earley', 'lalr', None)
+        assert self.parser in ('earley', 'lalr', 'cyk', None)
 
         if self.parser == 'earley' and self.transformer:
             raise ValueError('Cannot specify an embedded transformer when using the Earley algorithm.'
@@ -131,6 +131,8 @@ class Lark:
                 self.options.lexer = 'standard'
             elif self.options.parser == 'earley':
                 self.options.lexer = 'dynamic'
+            elif self.options.parser == 'cyk':
+                self.options.lexer = 'standard'
             else:
                 assert False, self.options.parser
         lexer = self.options.lexer
@@ -140,7 +142,9 @@ class Lark:
             if self.options.parser == 'earley':
                 self.options.ambiguity = 'resolve'
         else:
-            assert self.options.parser == 'earley', "Only Earley supports disambiguation right now"
+            disambig_parsers = ['earley', 'cyk']
+            assert self.options.parser in disambig_parsers, (
+                'Only %s supports disambiguation right now') % ', '.join(disambig_parsers)
         assert self.options.ambiguity in ('resolve', 'explicit', 'auto', 'resolve__antiscore_sum')
 
         # Parse the grammar file and compose the grammars (TODO)

lark/parser_frontends.py

@@ -1,10 +1,13 @@
 import re
 from .utils import get_regexp_width
 
+from .parsers.grammar_analysis import GrammarAnalyzer
 from .lexer import Lexer, ContextualLexer, Token
 
-from .common import is_terminal, GrammarError, ParserConf
-from .parsers import lalr_parser, earley, xearley, resolve_ambig
+from .common import GrammarError
+from .common import is_terminal, GrammarError
+from .parsers import lalr_parser, earley, xearley, resolve_ambig, cyk
+from .tree import Tree
 
 class WithLexer:
     def init_traditional_lexer(self, lexer_conf):
@@ -134,6 +137,55 @@ class XEarley:
     def parse(self, text):
         return self.parser.parse(text)
 
+
+class Earley(WithLexer):
+    def __init__(self, lexer_conf, parser_conf, options=None):
+        self.init_traditional_lexer(lexer_conf)
+
+        self.parser = earley.Parser(parser_conf, self.match,
+                                    resolve_ambiguity=get_ambiguity_resolver(options))
+
+    def match(self, term, token):
+        return term == token.type
+
+    def parse(self, text):
+        tokens = self.lex(text)
+        return self.parser.parse(tokens)
+
+class CYK(WithLexer):
+
+  def __init__(self, lexer_conf, parser_conf, options=None):
+    self.init_traditional_lexer(lexer_conf)
+
+    self._analysis = GrammarAnalyzer(parser_conf)
+    self._parser = cyk.Parser(self._analysis.rules, parser_conf.start)
+
+    self._postprocess = {}
+    for rule in self._analysis.rules:
+        a = rule.alias
+        self._postprocess[a] = a if callable(a) else (a and getattr(parser_conf.callback, a))
+
+  def parse(self, text):
+    tokens = list(self.lex(text))
+    parse = self._parser.parse(tokens)
+    parse = self._transform(parse)
+    return parse
+
+  def _transform(self, tree):
+    subtrees = list(tree.iter_subtrees())
+    for subtree in subtrees:
+      subtree.children = [self._apply_callback(c) if isinstance(c, Tree) else c for c in subtree.children]
+
+    return self._apply_callback(tree)
+
+  def _apply_callback(self, tree):
+    children = tree.children
+    callback = self._postprocess[tree.rule.alias]
+    assert callback, tree.rule.alias
+    r = callback(children)
+    return r
+
+
 def get_frontend(parser, lexer):
     if parser=='lalr':
         if lexer is None:
@@ -155,6 +207,11 @@ def get_frontend(parser, lexer):
             raise ValueError('The Earley parser does not support the contextual parser')
         else:
             raise ValueError('Unknown lexer: %s' % lexer)
+    elif parser == 'cyk':
+        if lexer == 'standard':
+            return CYK
+        else:
+            raise ValueError('CYK parser requires using standard parser.')
     else:
         raise ValueError('Unknown parser: %s' % parser)
 

lark/parsers/cyk.py

@@ -0,0 +1,361 @@
+"""This module implements a CYK parser."""
+
+from collections import defaultdict
+import itertools
+
+from ..common import ParseError, is_terminal
+from ..lexer import Token
+from ..tree import Tree
+
+try:
+    xrange
+except NameError:
+    xrange = range
+
+class Symbol(object):
+    """Any grammar symbol."""
+
+    def __init__(self, s):
+        self.s = s
+
+    def __repr__(self):
+        return '%s(%s)' % (type(self).__name__, str(self))
+
+    def __str__(self):
+        return str(self.s)
+
+    def __eq__(self, other):
+        return self.s == str(other)
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
+    def __hash__(self):
+        return hash((type(self), str(self.s)))
+
+
+class T(Symbol):
+    """Terminal."""
+
+    def match(self, s):
+        return self.s == s.type
+
+
+class NT(Symbol):
+    """Non-terminal."""
+    pass
+
+
+class Rule(object):
+    """Context-free grammar rule."""
+
+    def __init__(self, lhs, rhs, weight, alias):
+        super(Rule, self).__init__()
+        assert isinstance(lhs, NT), lhs
+        assert all(isinstance(x, NT) or isinstance(x, T) for x in rhs), rhs
+        self.lhs = lhs
+        self.rhs = rhs
+        self.weight = weight
+        self.alias = alias
+
+    def __str__(self):
+        return '%s -> %s' % (str(self.lhs), ' '.join(str(x) for x in self.rhs))
+
+    def __repr__(self):
+        return str(self)
+
+    def __hash__(self):
+        return hash((self.lhs, tuple(self.rhs)))
+
+    def __eq__(self, other):
+        return self.lhs == other.lhs and self.rhs == other.rhs
+
+    def __ne__(self, other):
+        return not (self == other)
+
+
+class Grammar(object):
+    """Context-free grammar."""
+
+    def __init__(self, rules):
+        self.rules = frozenset(rules)
+
+    def __eq__(self, other):
+        return self.rules == other.rules
+
+    def __str__(self):
+        return '\n' + '\n'.join(sorted(repr(x) for x in self.rules)) + '\n'
+
+    def __repr__(self):
+        return str(self)
+
+
+# Parse tree data structures
+class RuleNode(object):
+    """A node in the parse tree, which also contains the full rhs rule."""
+
+    def __init__(self, rule, children, weight=0):
+        self.rule = rule
+        self.children = children
+        self.weight = weight
+
+    def __repr__(self):
+        return 'RuleNode(%s, [%s])' % (repr(self.rule.lhs), ', '.join(str(x) for x in self.children))
+
+
+
+class Parser(object):
+    """Parser wrapper."""
+
+    def __init__(self, rules, start):
+        super(Parser, self).__init__()
+        self.orig_rules = {rule.alias: rule for rule in rules}
+        rules = [self._to_rule(rule) for rule in rules]
+        self.grammar = to_cnf(Grammar(rules))
+        self.start = NT(start)
+
+    def _to_rule(self, lark_rule):
+        """Converts a lark rule, (lhs, rhs, callback, options), to a Rule."""
+        return Rule(
+            NT(lark_rule.origin), [
+                T(x) if is_terminal(x) else NT(x) for x in lark_rule.expansion
+            ], weight=lark_rule.options.priority if lark_rule.options and lark_rule.options.priority else 0, alias=lark_rule.alias)
+
+    def parse(self, tokenized):  # pylint: disable=invalid-name
+        """Parses input, which is a list of tokens."""
+        table, trees = _parse(tokenized, self.grammar)
+        # Check if the parse succeeded.
+        if all(r.lhs != self.start for r in table[(0, len(tokenized) - 1)]):
+            raise ParseError('Parsing failed.')
+        parse = trees[(0, len(tokenized) - 1)][NT(self.start)]
+        return self._to_tree(revert_cnf(parse))
+
+    def _to_tree(self, rule_node):
+        """Converts a RuleNode parse tree to a lark Tree."""
+        orig_rule = self.orig_rules[rule_node.rule.alias]
+        children = []
+        for i, child in enumerate(rule_node.children):
+            if isinstance(child, RuleNode):
+                children.append(self._to_tree(child))
+            else:
+                assert isinstance(child.s, Token)
+                children.append(child.s)
+        return Tree(orig_rule.origin, children, rule=orig_rule)
+
+
+def print_parse(node, indent=0):
+    if isinstance(node, RuleNode):
+        print(' ' * (indent * 2) + str(node.rule.lhs))
+        for child in node.children:
+            print_parse(child, indent + 1)
+    else:
+        print(' ' * (indent * 2) + str(node.s))
+
+
+def _parse(s, g):
+    """Parses sentence 's' using CNF grammar 'g'."""
+    # The CYK table. Indexed with a 2-tuple: (start pos, end pos)
+    table = defaultdict(set)
+    # Top-level structure is similar to the CYK table. Each cell is a dict from
+    # rule name to the best (lightest) tree for that rule.
+    trees = defaultdict(dict)
+    # Populate base case with existing terminal production rules
+    for i, w in enumerate(s):
+        for terminal, rules in g.terminal_rules.items():
+            if terminal.match(w):
+                for rule in rules:
+                    table[(i, i)].add(rule)
+                    if (rule.lhs not in trees[(i, i)] or
+                        rule.weight < trees[(i, i)][rule.lhs].weight):
+                        trees[(i, i)][rule.lhs] = RuleNode(rule, [T(w)], weight=rule.weight)
+
+    # Iterate over lengths of sub-sentences
+    for l in xrange(2, len(s) + 1):
+        # Iterate over sub-sentences with the given length
+        for i in xrange(len(s) - l + 1):
+            # Choose partition of the sub-sentence in [1, l)
+            for p in xrange(i + 1, i + l):
+                span1 = (i, p - 1)
+                span2 = (p, i + l - 1)
+                for r1, r2 in itertools.product(table[span1], table[span2]):
+                    for rule in g.nonterminal_rules.get((r1.lhs, r2.lhs), []):
+                        table[(i, i + l - 1)].add(rule)
+                        r1_tree = trees[span1][r1.lhs]
+                        r2_tree = trees[span2][r2.lhs]
+                        rule_total_weight = rule.weight + r1_tree.weight + r2_tree.weight
+                        if (rule.lhs not in trees[(i, i + l - 1)]
+                            or rule_total_weight < trees[(i, i + l - 1)][rule.lhs].weight):
+                            trees[(i, i + l - 1)][rule.lhs] = RuleNode(rule, [r1_tree, r2_tree], weight=rule_total_weight)
+    return table, trees
+
+
+# This section implements context-free grammar converter to Chomsky normal form.
+# It also implements a conversion of parse trees from its CNF to the original
+# grammar.
+# Overview:
+# Applies the following operations in this order:
+# * TERM: Eliminates non-solitary terminals from all rules
+# * BIN: Eliminates rules with more than 2 symbols on their right-hand-side.
+# * UNIT: Eliminates non-terminal unit rules
+#
+# The following grammar characteristics aren't featured:
+# * Start symbol appears on RHS
+# * Empty rules (epsilon rules)
+
+
+class CnfWrapper(object):
+    """CNF wrapper for grammar.
+
+  Validates that the input grammar is CNF and provides helper data structures.
+  """
+
+    def __init__(self, grammar):
+        super(CnfWrapper, self).__init__()
+        self.grammar = grammar
+        self.rules = grammar.rules
+        self.terminal_rules = defaultdict(list)
+        self.nonterminal_rules = defaultdict(list)
+        for r in self.rules:
+            # Validate that the grammar is CNF and populate auxiliary data structures.
+            assert isinstance(r.lhs, NT), r
+            assert len(r.rhs) in [1, 2], r
+            if len(r.rhs) == 1 and isinstance(r.rhs[0], T):
+                self.terminal_rules[r.rhs[0]].append(r)
+            elif len(r.rhs) == 2 and all(isinstance(x, NT) for x in r.rhs):
+                self.nonterminal_rules[tuple(r.rhs)].append(r)
+            else:
+                assert False, r
+
+    def __eq__(self, other):
+        return self.grammar == other.grammar
+
+    def __repr__(self):
+        return repr(self.grammar)
+
+
+class UnitSkipRule(Rule):
+    """A rule that records NTs that were skipped during transformation."""
+
+    def __init__(self, lhs, rhs, skipped_rules, weight, alias):
+        super(UnitSkipRule, self).__init__(lhs, rhs, weight, alias)
+        self.skipped_rules = skipped_rules
+
+    def __eq__(self, other):
+        return isinstance(other, type(self)) and self.skipped_rules == other.skipped_rules
+
+    __hash__ = Rule.__hash__
+
+
+def build_unit_skiprule(unit_rule, target_rule):
+    skipped_rules = []
+    if isinstance(unit_rule, UnitSkipRule):
+        skipped_rules += unit_rule.skipped_rules
+    skipped_rules.append(target_rule)
+    if isinstance(target_rule, UnitSkipRule):
+        skipped_rules += target_rule.skipped_rules
+    return UnitSkipRule(unit_rule.lhs, target_rule.rhs, skipped_rules,
+                      weight=unit_rule.weight + target_rule.weight, alias=unit_rule.alias)
+
+
+def get_any_nt_unit_rule(g):
+    """Returns a non-terminal unit rule from 'g', or None if there is none."""
+    for rule in g.rules:
+        if len(rule.rhs) == 1 and isinstance(rule.rhs[0], NT):
+            return rule
+    return None
+
+
+def _remove_unit_rule(g, rule):
+    """Removes 'rule' from 'g' without changing the langugage produced by 'g'."""
+    new_rules = [x for x in g.rules if x != rule]
+    refs = [x for x in g.rules if x.lhs == rule.rhs[0]]
+    new_rules += [build_unit_skiprule(rule, ref) for ref in refs]
+    return Grammar(new_rules)
+
+
+def _split(rule):
+    """Splits a rule whose len(rhs) > 2 into shorter rules."""
+    rule_str = str(rule.lhs) + '__' + '_'.join(str(x) for x in rule.rhs)
+    rule_name = '__SP_%s' % (rule_str) + '_%d'
+    yield Rule(rule.lhs, [rule.rhs[0], NT(rule_name % 1)], weight=rule.weight, alias=rule.alias)
+    for i in xrange(1, len(rule.rhs) - 2):
+        yield Rule(NT(rule_name % i), [rule.rhs[i], NT(rule_name % (i + 1))], weight=0, alias='Split')
+    yield Rule(NT(rule_name % (len(rule.rhs) - 2)), rule.rhs[-2:], weight=0, alias='Split')
+
+
+def _term(g):
+    """Applies the TERM rule on 'g' (see top comment)."""
+    all_t = {x for rule in g.rules for x in rule.rhs if isinstance(x, T)}
+    t_rules = {t: Rule(NT('__T_%s' % str(t)), [t], weight=0, alias='Term') for t in all_t}
+    new_rules = []
+    for rule in g.rules:
+        if len(rule.rhs) > 1 and any(isinstance(x, T) for x in rule.rhs):
+            new_rhs = [t_rules[x].lhs if isinstance(x, T) else x for x in rule.rhs]
+            new_rules.append(Rule(rule.lhs, new_rhs, weight=rule.weight, alias=rule.alias))
+            new_rules.extend(v for k, v in t_rules.items() if k in rule.rhs)
+        else:
+            new_rules.append(rule)
+    return Grammar(new_rules)
+
+
+def _bin(g):
+    """Applies the BIN rule to 'g' (see top comment)."""
+    new_rules = []
+    for rule in g.rules:
+        if len(rule.rhs) > 2:
+            new_rules += _split(rule)
+        else:
+            new_rules.append(rule)
+    return Grammar(new_rules)
+
+
+def _unit(g):
+    """Applies the UNIT rule to 'g' (see top comment)."""
+    nt_unit_rule = get_any_nt_unit_rule(g)
+    while nt_unit_rule:
+        g = _remove_unit_rule(g, nt_unit_rule)
+        nt_unit_rule = get_any_nt_unit_rule(g)
+    return g
+
+
+def to_cnf(g):
+    """Creates a CNF grammar from a general context-free grammar 'g'."""
+    g = _unit(_bin(_term(g)))
+    return CnfWrapper(g)
+
+
+def unroll_unit_skiprule(lhs, orig_rhs, skipped_rules, children, weight, alias):
+    if not skipped_rules:
+        return RuleNode(Rule(lhs, orig_rhs, weight=weight, alias=alias), children, weight=weight)
+    else:
+        weight = weight - skipped_rules[0].weight
+        return RuleNode(
+            Rule(lhs, [skipped_rules[0].lhs], weight=weight, alias=alias), [
+                unroll_unit_skiprule(skipped_rules[0].lhs, orig_rhs,
+                                skipped_rules[1:], children,
+                                skipped_rules[0].weight, skipped_rules[0].alias)
+            ], weight=weight)
+
+
+def revert_cnf(node):
+    """Reverts a parse tree (RuleNode) to its original non-CNF form (Node)."""
+    if isinstance(node, T):
+        return node
+    # Reverts TERM rule.
+    if node.rule.lhs.s.startswith('__T_'):
+        return node.children[0]
+    else:
+        children = []
+        for child in map(revert_cnf, node.children):
+            # Reverts BIN rule.
+            if isinstance(child, RuleNode) and child.rule.lhs.s.startswith('__SP_'):
+                children += child.children
+            else:
+                children.append(child)
+        # Reverts UNIT rule.
+        if isinstance(node.rule, UnitSkipRule):
+            return unroll_unit_skiprule(node.rule.lhs, node.rule.rhs,
+                                    node.rule.skipped_rules, children,
+                                    node.rule.weight, node.rule.alias)
+        else:
+            return RuleNode(node.rule, children)

lark/parsers/earley.py

@@ -22,8 +22,7 @@ class Derivation(Tree):
     _hash = None
 
     def __init__(self, rule, items=None):
-        Tree.__init__(self, 'drv', items or [])
-        self.rule = rule
+        Tree.__init__(self, 'drv', items or [], rule=rule)
 
     def _pretty_label(self):    # Nicer pretty for debugging the parser
         return self.rule.origin if self.rule else self.data

lark/tree.py

@@ -9,9 +9,10 @@ from .utils import inline_args
 
 ###{standalone
 class Tree(object):
-    def __init__(self, data, children):
+    def __init__(self, data, children, rule=None):
         self.data = data
         self.children = list(children)
+        self.rule = rule
 
     def __repr__(self):
         return 'Tree(%s, %s)' % (self.data, self.children)

tests/__main__.py

@@ -16,6 +16,7 @@ except ImportError:
 from .test_parser import (
         TestLalrStandard,
         TestEarleyStandard,
+        TestCykStandard,
         TestLalrContextual,
         TestEarleyScanless,
         TestEarleyDynamic,

tests/test_parser.py

@@ -382,6 +382,7 @@ def _make_parser_test(LEXER, PARSER):
             g.parse(u'\xa3\u0101\u00a3\u0203\n')
 
 
+        @unittest.skipIf(PARSER == 'cyk', "Takes forever")
         def test_stack_for_ebnf(self):
             """Verify that stack depth isn't an issue for EBNF grammars"""
             g = _Lark(r"""start: a+
@@ -455,6 +456,7 @@ def _make_parser_test(LEXER, PARSER):
 
 
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_empty_expand1_list(self):
             g = _Lark(r"""start: list
                             ?list: item*
@@ -473,6 +475,7 @@ def _make_parser_test(LEXER, PARSER):
             [list] = r.children
             self.assertSequenceEqual([item.data for item in list.children], ())
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_empty_expand1_list_2(self):
             g = _Lark(r"""start: list
                             ?list: item* "!"?
@@ -492,6 +495,7 @@ def _make_parser_test(LEXER, PARSER):
             self.assertSequenceEqual([item.data for item in list.children], ())
 
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_empty_flatten_list(self):
             g = _Lark(r"""start: list
                             list: | item "," list
@@ -645,6 +649,7 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(len(x.children), 1, '/a/ should not be considered anonymous')
             self.assertEqual(x.children[0].type, "A")
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_maybe(self):
             g = _Lark("""start: ["a"] """)
             x = g.parse('a')
@@ -702,6 +707,7 @@ def _make_parser_test(LEXER, PARSER):
         #                  B: A
         #               """)
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_empty(self):
             # Fails an Earley implementation without special handling for empty rules,
             # or re-processing of already completed rules.
@@ -732,6 +738,8 @@ def _make_parser_test(LEXER, PARSER):
 
         def test_float_without_lexer(self):
             expected_error = UnexpectedInput if LEXER == 'dynamic' else UnexpectedToken
+            if PARSER == 'cyk':
+                expected_error = ParseError
 
             g = _Lark("""start: ["+"|"-"] float
                          float: digit* "." digit+ exp?
@@ -796,6 +804,7 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(tree.children, ['a', 'A'])
 
 
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_twice_empty(self):
             g = """!start: [["A"]]
                 """
@@ -1001,6 +1010,7 @@ def _make_parser_test(LEXER, PARSER):
 
 
         @unittest.skipIf(LEXER==None, "Scanless doesn't support regular expressions")
+        @unittest.skipIf(PARSER == 'cyk', "No empty rules")
         def test_ignore(self):
             grammar = r"""
             COMMENT: /(!|(\/\/))[^\n]*/
@@ -1026,7 +1036,6 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(tree.children, [])
 
 
-
         @unittest.skipIf(LEXER==None, "Scanless doesn't support regular expressions")
         def test_regex_escaping(self):
             g = _Lark("start: /[ab]/")
@@ -1075,6 +1084,7 @@ def _make_parser_test(LEXER, PARSER):
 # Note: You still have to import them in __main__ for the tests to run
 _TO_TEST = [
         ('standard', 'earley'),
+        ('standard', 'cyk'),
         ('dynamic', 'earley'),
         ('standard', 'lalr'),
         ('contextual', 'lalr'),