Merge CYK parser.

LICENSE

@@ -1,4 +1,5 @@
 Copyright © 2017 Erez Shinan
+Copyright (c) 2018 Google LLC
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of
 this software and associated documentation files (the "Software"), to deal in

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 +1,12 @@
 import re
 import sre_parse
 
+from parsers.grammar_analysis import GrammarAnalyzer
 from .lexer import Lexer, ContextualLexer, Token
 
-from .common import is_terminal, GrammarError, ParserConf, Terminal_Regexp, Terminal_Token
-from .parsers import lalr_parser, earley, xearley, resolve_ambig
+from .common import is_terminal, GrammarError, Terminal_Regexp, Terminal_Token
+from .parsers import lalr_parser, earley, xearley, resolve_ambig, cyk
+from .tree import Tree
 
 class WithLexer:
     def __init__(self, lexer_conf):
@@ -142,6 +144,50 @@ class XEarley:
     def parse(self, text):
         return self.parser.parse(text)
 
+
+class CYK(WithLexer):
+
+  def __init__(self, lexer_conf, parser_conf, options=None):
+    WithLexer.__init__(self, lexer_conf)
+    # TokenDef from synthetic rule to terminal value
+    self._token_by_name = {t.name: t for t in lexer_conf.tokens}
+    rules = [(lhs, self._prepare_expansion(rhs), cb, opt) for lhs, rhs, cb, opt in parser_conf.rules]
+    self._analysis = GrammarAnalyzer(rules, parser_conf.start)
+    self._parser = cyk.Parser(self._analysis.rules, parser_conf.start)
+
+    self._postprocess = {}
+    for rule in self._analysis.rules:
+        if rule.origin != '$root':  # XXX kinda ugly
+            a = rule.alias
+            self._postprocess[a] = a if callable(a) else (a and getattr(parser_conf.callback, a))
+
+  def _prepare_expansion(self, expansion):
+    return [
+        Terminal_Regexp(sym, self._token_by_name[sym].pattern.to_regexp())
+        if is_terminal(sym) else sym for sym in expansion
+    ]
+
+  def parse(self, text):
+    tokenized = [token.value for token in self.lex(text)]
+    parse = self._parser.parse(tokenized)
+    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:
@@ -163,6 +209,11 @@ 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 +1,401 @@
+"""This module implements a CYK parser."""
+from collections import defaultdict
+import itertools
+import re
+
+from ..common import ParseError, Terminal, Terminal_Regexp
+from ..lexer import Token
+from ..tree import Tree
+
+
+def TypeName(x):
+  return type(x).__name__
+
+
+class Symbol(object):
+  """Any grammar symbol."""
+
+  def __init__(self, s):
+    self.s = s
+
+  def __repr__(self):
+    return '%s(%s)' % (TypeName(self), str(self))
+
+  def __str__(self):
+    return str(self.s)
+
+  def __eq__(self, other):
+    return str(self) == str(other)
+
+  def __ne__(self, other):
+    return not self.__eq__(other)
+
+  def __hash__(self):
+    return hash(TypeName(self) + '&' + self.__str__())
+
+
+class T(Symbol):
+  """Terminal."""
+
+  def __init__(self, s):
+    super(T, self).__init__(s)
+    self.regexp = re.compile(s)
+
+  def match(self, s):
+    m = self.regexp.match(s)
+    return bool(m) and len(m.group(0)) == len(s)
+
+  def __eq__(self, other):
+    return super(T, self).__eq__(other) and isinstance(other, T)
+
+
+class NT(Symbol):
+  """Non-terminal."""
+
+  def __eq__(self, other):
+    return super(NT, self).__eq__(other) and isinstance(other, NT)
+
+
+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.__repr__())
+
+  def __eq__(self, other):
+    return self.lhs == other.lhs and self.rhs == other.rhs
+
+  def __ne__(self, other):
+    return not self.__eq__(other)
+
+
+class Grammar(object):
+  """Context-free grammar."""
+
+  def __init__(self, rules):
+    super(Grammar, self).__init__()
+    self.rules = sorted(rules, key=lambda x: str(x))
+
+  def __eq__(self, other):
+    return set(self.rules) == set(other.rules)
+
+  def __str__(self):
+    return '\n' + '\n'.join(sorted(x.__repr__() 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):
+    super(RuleNode, self).__init__()
+    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))
+
+  def __hash__(self):
+    return hash(self.__repr__())
+
+
+class Node(object):
+  """A node in the parse tree."""
+
+  def __init__(self, lhs, children):
+    super(Node, self).__init__()
+    self.lhs = lhs
+    self.children = children
+
+  def __repr__(self):
+    return 'Node(%s, [%s])' % (repr(self.lhs), ', '.join(
+        str(x) for x in self.children))
+
+  def __hash__(self):
+    return hash(self.__repr__())
+
+
+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._ToRule(rule) for rule in rules]
+    self.grammar = ToCnf(Grammar(rules))
+    self.start = NT(start)
+
+  def _ToRule(self, lark_rule):
+    """Converts a lark rule, (lhs, rhs, callback, options), to a Rule."""
+    return Rule(
+        NT(lark_rule.origin), [
+            T(x.data) if (isinstance(x, Terminal_Regexp) or
+                          isinstance(x, Terminal)) 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._ToTree(RevertCnf(parse))
+
+  def _ToTree(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._ToTree(child))
+      elif isinstance(child, Terminal_Regexp):
+        children.append(Token(orig_rule.expansion[i].name, child.s))
+      else:
+        children.append(Token(orig_rule.expansion[i], child.s))
+    return Tree(orig_rule.origin, children, rule=orig_rule)
+
+
+def PrintParse(node, indent=0):
+  if isinstance(node, RuleNode):
+    print(' ' * (indent * 2) + str(node.rule.lhs))
+    for child in node.children:
+      PrintParse(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.iteritems():
+      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 self.grammar.__repr__()
+
+
+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 (super(UnitSkipRule, self).__eq__(other) and
+            isinstance(other, type(self)) and
+            self.skipped_rules == other.skipped_rules)
+
+
+def BuildUnitSkipRule(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 GetAnyNtUnitRule(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 RemoveUnitRule(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]]
+  for ref in refs:
+    new_rules.append(BuildUnitSkipRule(rule, ref))
+  return Grammar(new_rules)
+
+
+def Split(rule):
+  """Splits a rule whose len(rhs) > 2 into shorter rules."""
+  # if len(rule.rhs) <= 2:
+  #   return [rule]
+  rule_str = str(rule.lhs) + '__' + '_'.join(str(x) for x in rule.rhs)
+  rule_name = '__SP_%s' % (rule_str) + '_%d'
+  new_rules = [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):
+    new_rules.append(
+        Rule(NT(rule_name % i),
+             [rule.rhs[i], NT(rule_name % (i + 1))], weight=0, alias='Split'))
+  new_rules.append(Rule(NT(rule_name % (len(rule.rhs) - 2)), rule.rhs[-2:], weight=0, alias='Split'))
+  return new_rules
+
+
+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.iteritems() 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.extend(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 = GetAnyNtUnitRule(g)
+  while nt_unit_rule:
+    g = RemoveUnitRule(g, nt_unit_rule)
+    nt_unit_rule = GetAnyNtUnitRule(g)
+  return g
+
+
+def ToCnf(g):
+  """Creates a CNF grammar from a general context-free grammar 'g'."""
+  g = Unit(Bin(Term(g)))
+  return CnfWrapper(g)
+
+
+def UnrollUnitSkipRule(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), [
+            UnrollUnitSkipRule(skipped_rules[0].lhs, orig_rhs,
+                               skipped_rules[1:], children,
+                               skipped_rules[0].weight, skipped_rules[0].alias)
+        ], weight=weight)
+
+
+def RevertCnf(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 = []
+    reverted_children = [RevertCnf(x) for x in node.children]
+    for child in reverted_children:
+      # Reverts BIN rule.
+      if isinstance(child, RuleNode) and child.rule.lhs.s.startswith('__SP_'):
+        children.extend(child.children)
+      else:
+        children.append(child)
+    # Reverts UNIT rule.
+    if isinstance(node.rule, UnitSkipRule):
+      return UnrollUnitSkipRule(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

@@ -25,8 +25,7 @@ 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

@@ -8,9 +8,10 @@ from copy import deepcopy
 from .utils import inline_args
 
 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)