gitmirror/lark/parsers/earley_forest.py

""""This module implements an SPPF implementation

This is used as the primary output mechanism for the Earley parser
in order to store complex ambiguities.

Full reference and more details is here:
http://www.bramvandersanden.com/post/2014/06/shared-packed-parse-forest/
"""

from ..tree import Tree
from ..exceptions import ParseError
from ..lexer import Token
from ..utils import Str
from ..grammar import NonTerminal, Terminal
from .earley_common import Column, Derivation

from collections import deque

class SymbolNode(object):
    """
    A Symbol Node represents a symbol (or Intermediate LR0).

    Symbol nodes are keyed by the symbol (s). For intermediate nodes
    s will be an LR0, stored as a tuple of (rule, ptr). For completed symbol
    nodes, s will be a string representing the non-terminal origin (i.e.
    the left hand side of the rule).

    The children of a Symbol or Intermediate Node will always be Packed Nodes;
    with each Packed Node child representing a single derivation of a production.

    Hence a Symbol Node with a single child is unambiguous.
    """
    __slots__ = ('s', 'start', 'end', 'children', 'priority', 'is_intermediate')
    def __init__(self, s, start, end):
        self.s = s
        self.start = start
        self.end = end
        self.children = set()
        self.priority = None
        self.is_intermediate = isinstance(s, tuple)

    def add_family(self, lr0, rule, start, left, right):
        self.children.add(PackedNode(self, lr0, rule, start, left, right))

    @property
    def is_ambiguous(self):
        return len(self.children) > 1

    def __iter__(self):
        return iter(self.children)

    def __eq__(self, other):
        if not isinstance(other, SymbolNode):
            return False
        return self is other or (self.s == other.s and self.start == other.start and self.end is other.end)

    def __hash__(self):
        return hash((self.s, self.start.i, self.end.i))

    def __repr__(self):
        symbol = self.s.name if isinstance(self.s, (NonTerminal, Terminal)) else self.s[0].origin.name
        return "(%s, %d, %d, %d)" % (symbol, self.start.i, self.end.i, self.priority if self.priority is not None else 0)

class PackedNode(object):
    """
    A Packed Node represents a single derivation in a symbol node.
    """
    __slots__ = ('parent', 's', 'rule', 'start', 'left', 'right', 'priority', '_hash')
    def __init__(self, parent, s, rule, start, left, right):
        self.parent = parent
        self.s = s
        self.start = start
        self.rule = rule
        self.left = left
        self.right = right
        self.priority = None
        self._hash = hash((self.s, self.start.i, self.left, self.right))

    @property
    def is_empty(self):
        return self.left is None and self.right is None

    def __iter__(self):
        return iter([self.left, self.right])

    def __lt__(self, other):
        if self.is_empty and not other.is_empty: return True
        if self.priority < other.priority:       return True
        return False

    def __gt__(self, other):
        if self.is_empty and not other.is_empty: return True
        if self.priority > other.priority:       return True
        return False

    def __eq__(self, other):
        if not isinstance(other, PackedNode):
            return False
        return self is other or (self.s == other.s and self.start == other.start and self.left == other.left and self.right == other.right)

    def __hash__(self):
        return self._hash

    def __repr__(self):
        symbol = self.s.name if isinstance(self.s, (NonTerminal, Terminal)) else self.s[0].origin.name
        return "{%s, %d, %s, %s, %s}" % (symbol, self.start.i, self.left, self.right, self.priority if self.priority is not None else 0)

class ForestVisitor(object):
    """
    An abstract base class for building forest visitors.

    Use this as a base when you need to walk the forest.
    """
    def __init__(self, root):
        self.root = root
        self.result = None

    def visit_token_node(self, node): pass
    def visit_symbol_node_in(self, node): pass
    def visit_symbol_node_out(self, node): pass
    def visit_packed_node_in(self, node): pass
    def visit_packed_node_out(self, node): pass

    def go(self):
        # Visiting is a list of IDs of all symbol/intermediate nodes currently in
        # the stack. It serves two purposes: to detect when we 'recurse' in and out
        # of a symbol/intermediate so that we can process both up and down. Also,
        # since the SPPF can have cycles it allows us to detect if we're trying
        # to recurse into a node that's already on the stack (infinite recursion).
        visiting = set()

        # We do not use recursion here to walk the Forest due to the limited
        # stack size in python. Therefore input_stack is essentially our stack.
        input_stack = deque([self.root])

        # It is much faster to cache these as locals since they are called
        # many times in large parses.
        vpno = getattr(self, 'visit_packed_node_out')
        vpni = getattr(self, 'visit_packed_node_in')
        vsno = getattr(self, 'visit_symbol_node_out')
        vsni = getattr(self, 'visit_symbol_node_in')
        vtn = getattr(self, 'visit_token_node')
        while input_stack:
            current = next(reversed(input_stack))
            try:
                next_node = next(current)
            except StopIteration:
                input_stack.pop()
                continue
            except TypeError:
                ### If the current object is not an iterator, pass through to Token/SymbolNode
                pass
            else:
                if next_node is None:
                    continue

                if id(next_node) in visiting:
                    raise ParseError("Infinite recursion in grammar!")

                input_stack.append(next_node)
                continue

            if isinstance(current, Str):
                vtn(current)
                input_stack.pop()
                continue

            current_id = id(current)
            if current_id in visiting:
                if isinstance(current, PackedNode): vpno(current)
                else:                               vsno(current)
                input_stack.pop()
                visiting.remove(current_id)
                continue
            else:
                visiting.add(current_id)
                if isinstance(current, PackedNode): next_node = vpni(current)
                else:                               next_node = vsni(current)
                if next_node is None:
                    continue

                if id(next_node) in visiting:
                    raise ParseError("Infinite recursion in grammar!")

                input_stack.append(next_node)
                continue

        return self.result

class ForestSumVisitor(ForestVisitor):
    """
    A visitor for prioritizing ambiguous parts of the Forest.

    This visitor is the default when resolving ambiguity. It pushes the priorities
    from the rules into the SPPF nodes; and then sorts the packed node children
    of ambiguous symbol or intermediate node according to the priorities.
    This relies on the custom sort function provided in PackedNode.__lt__; which
    uses these properties (and other factors) to sort the ambiguous packed nodes.
    """
    def visit_packed_node_in(self, node):
        return iter([node.left, node.right])

    def visit_symbol_node_in(self, node):
        return iter(node.children)

    def visit_packed_node_out(self, node):
        node.priority = 0
        if node.rule.options and node.rule.options.priority:           node.priority += node.rule.options.priority
        if node.right is not None and hasattr(node.right, 'priority'): node.priority += node.right.priority
        if node.left is not None and hasattr(node.left, 'priority'):   node.priority += node.left.priority

    def visit_symbol_node_out(self, node):
        node.priority = max(child.priority for child in node.children)
        node.children = sorted(node.children, reverse = True)

class ForestAntiscoreSumVisitor(ForestSumVisitor):
    """
    A visitor for prioritizing ambiguous parts of the Forest.

    This visitor is used when resolve_ambiguity == 'resolve__antiscore_sum'.
    It pushes the priorities from the rules into the SPPF nodes, and implements
    a 'least cost' mechanism for resolving ambiguity (reverse of the default
    priority mechanism). It uses a custom __lt__ comparator key for sorting
    the packed node children.
    """
    def visit_symbol_node_out(self, node):
        node.priority = min(child.priority for child in node.children)
        node.children = sorted(node.children, key=AntiscoreSumComparator, reverse = True)

class AntiscoreSumComparator(object):
    """
    An antiscore-sum comparator for PackedNode objects.

    This allows 'sorting' an iterable of PackedNode objects so that they
    are arranged lowest priority first.
    """
    __slots__ = ['obj']
    def __init__(self, obj, *args):
        self.obj = obj

    def __lt__(self, other):
        if self.obj.is_empty and not other.obj.is_empty: return True
        if self.obj.priority > other.obj.priority:       return True
        return False

    def __gt__(self, other):
        if self.obj.is_empty and not other.obj.is_empty: return True
        if self.obj.priority < other.obj.priority:       return True
        return False

class ForestToTreeVisitor(ForestVisitor):
    """
    A Forest visitor which converts an SPPF forest to an unambiguous AST.

    The implementation in this visitor walks only the first ambiguous child
    of each symbol node. When it finds an ambiguous symbol node it first
    calls the forest_sum_visitor implementation to sort the children
    into preference order using the algorithms defined there; so the first
    child should always be the highest preference. The forest_sum_visitor
    implementation should be another ForestVisitor which sorts the children
    according to some priority mechanism.
    """
    def __init__(self, root, forest_sum_visitor = ForestSumVisitor, callbacks = None):
        super(ForestToTreeVisitor, self).__init__(root)
        self.forest_sum_visitor = forest_sum_visitor
        self.output_stack = deque()
        self.callbacks = callbacks
        self.result = None

    def visit_token_node(self, node):
        self.output_stack[-1].append(node)

    def visit_symbol_node_in(self, node):
        if node.is_ambiguous and node.priority is None:
            self.forest_sum_visitor(node).go()
        return next(iter(node.children))

    def visit_packed_node_in(self, node):
        if not node.parent.is_intermediate:
            self.output_stack.append([])
        return iter([node.left, node.right])

    def visit_packed_node_out(self, node):
        if not node.parent.is_intermediate:
            result = self.callbacks[node.rule](self.output_stack.pop())
            if self.output_stack:
                self.output_stack[-1].append(result)
            else:
                self.result = result

class ForestToAmbiguousTreeVisitor(ForestVisitor):
    """
    A Forest visitor which converts an SPPF forest to an ambiguous AST.

    Because of the fundamental disparity between what can be stored in
    an SPPF and what can be stored in a Tree; this implementation is not
    complete. It correctly deals with ambiguities that occur on symbol nodes only,
    and cannot deal with ambiguities that occur on intermediate nodes.

    Usually, most parsers can be rewritten to avoid intermediate node
    ambiguities. Also, this implementation could be fixed, however
    the code to handle intermediate node ambiguities is messy and
    would not be performant. It is much better not to use this and
    instead to correctly disambiguate the forest and only store unambiguous
    parses in Trees. It is here just to provide some parity with the
    old ambiguity='explicit'.

    This is mainly used by the test framework, to make it simpler to write
    tests ensuring the SPPF contains the right results.
    """
    def __init__(self, root, callbacks):
        super(ForestToAmbiguousTreeVisitor, self).__init__(root)
        self.output_stack = deque()
        self.callbacks = callbacks
        self.result = None

    def visit_token_node(self, node):
        self.output_stack[-1].children.append(node)

    def visit_symbol_node_in(self, node):
        if not node.is_intermediate and node.is_ambiguous:
            self.output_stack.append(Tree('_ambig', []))
        return iter(node.children)

    def visit_symbol_node_out(self, node):
        if node.is_ambiguous:
            result = self.output_stack.pop()
            if self.output_stack:
                self.output_stack[-1].children.append(result)
            else:
                self.result = result

    def visit_packed_node_in(self, node):
        #### NOTE:
        ## When an intermediate node (node.parent.s == tuple) has ambiguous children this
        ## forest visitor will break.
        if not node.parent.is_intermediate:
            self.output_stack.append(Tree('drv', []))
        return iter([node.left, node.right])

    def visit_packed_node_out(self, node):
        if not node.parent.is_intermediate:
            result = self.callbacks[node.rule](self.output_stack.pop().children)
            if self.output_stack:
                self.output_stack[-1].children.append(result)
            else:
                self.result = result