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+"This module implements an Earley Parser"
+
+# The parser uses a parse-forest to keep track of derivations and ambiguations.
+# When the parse ends successfully, a disambiguation stage resolves all ambiguity
+# (right now ambiguity resolution is not developed beyond the needs of lark)
+# Afterwards the parse tree is reduced (transformed) according to user callbacks.
+# I use the no-recursion version of Transformer, because the tree might be
+# deeper than Python's recursion limit (a bit absurd, but that's life)
+#
+# The algorithm keeps track of each state set, using a corresponding Column instance.
+# Column keeps track of new items using NewsList instances.
+#
+# Author: Erez Shinan (2017)
+# Email : erezshin@gmail.com
+
+from ..grammar import NonTerminal, Terminal
+
+class Item(object):
+ "An Earley Item, the atom of the algorithm."
+
+ __slots__ = ('s', 'rule', 'ptr', 'start', 'is_complete', 'expect', 'previous', 'node', '_hash')
+ def __init__(self, rule, ptr, start):
+ self.is_complete = len(rule.expansion) == ptr
+ self.rule = rule # rule
+ self.ptr = ptr # ptr
+ self.start = start # j
+ self.node = None # w
+ if self.is_complete:
+ self.s = rule.origin
+ self.expect = None
+ self.previous = rule.expansion[ptr - 1] if ptr > 0 and len(rule.expansion) else None
+ else:
+ self.s = (rule, ptr)
+ self.expect = rule.expansion[ptr]
+ self.previous = rule.expansion[ptr - 1] if ptr > 0 and len(rule.expansion) else None
+ self._hash = hash((self.s, self.start))
+
+ def advance(self):
+ return Item(self.rule, self.ptr + 1, self.start)
+
+ def __eq__(self, other):
+ return self is other or (self.s == other.s and self.start == other.start)
+
+ def __hash__(self):
+ return self._hash
+
+ def __repr__(self):
+ before = ( expansion.name for expansion in self.rule.expansion[:self.ptr] )
+ after = ( expansion.name for expansion in self.rule.expansion[self.ptr:] )
+ symbol = "{} ::= {}* {}".format(self.rule.origin.name, ' '.join(before), ' '.join(after))
+ return '%s (%d)' % (symbol, self.start)
+
+
+class TransitiveItem(Item):
+ __slots__ = ('recognized', 'reduction', 'column', 'next_titem')
+ def __init__(self, recognized, trule, originator, start):
+ super(TransitiveItem, self).__init__(trule.rule, trule.ptr, trule.start)
+ self.recognized = recognized
+ self.reduction = originator
+ self.column = start
+ self.next_titem = None
+ self._hash = hash((self.s, self.start, self.recognized))
+
+ def __eq__(self, other):
+ if not isinstance(other, TransitiveItem):
+ return False
+ return self is other or (type(self.s) == type(other.s) and self.s == other.s and self.start == other.start and self.recognized == other.recognized)
+
+ def __hash__(self):
+ return self._hash
+
+ def __repr__(self):
+ before = ( expansion.name for expansion in self.rule.expansion[:self.ptr] )
+ after = ( expansion.name for expansion in self.rule.expansion[self.ptr:] )
+ return '{} : {} -> {}* {} ({}, {})'.format(self.recognized.name, self.rule.origin.name, ' '.join(before), ' '.join(after), self.column, self.start)