# Copyright 2004-2005 Elemental Security, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.

"""Parser engine for the grammar tables generated by pgen.

The grammar table must be loaded first.

See Parser/parser.c in the Python distribution for additional info on
how this parsing engine works.

"""

# Local imports
from . import token

class ParseError(Exception):
 """Exception to signal the parser is stuck."""

 def __init__(self, msg, type, value, context):
 Exception.__init__(self, "%s: type=%r, value=%r, context=%r" %
 (msg, type, value, context))
 self.msg = msg
 self.type = type
 self.value = value
 self.context = context

class Parser(object):
 """Parser engine.

 The proper usage sequence is:

 p = Parser(grammar, [converter]) # create instance
 p.setup([start]) # prepare for parsing
 <for each input token>:
 if p.addtoken(...): # parse a token; may raise ParseError
 break
 root = p.rootnode # root of abstract syntax tree

 A Parser instance may be reused by calling setup() repeatedly.

 A Parser instance contains state pertaining to the current token
 sequence, and should not be used concurrently by different threads
 to parse separate token sequences.

 See driver.py for how to get input tokens by tokenizing a file or
 string.

 Parsing is complete when addtoken() returns True; the root of the
 abstract syntax tree can then be retrieved from the rootnode
 instance variable. When a syntax error occurs, addtoken() raises
 the ParseError exception. There is no error recovery; the parser
 cannot be used after a syntax error was reported (but it can be
 reinitialized by calling setup()).

 """

 def __init__(self, grammar, convert=None):
 """Constructor.

 The grammar argument is a grammar.Grammar instance; see the
 grammar module for more information.

 The parser is not ready yet for parsing; you must call the
 setup() method to get it started.

 The optional convert argument is a function mapping concrete
 syntax tree nodes to abstract syntax tree nodes. If not
 given, no conversion is done and the syntax tree produced is
 the concrete syntax tree. If given, it must be a function of
 two arguments, the first being the grammar (a grammar.Grammar
 instance), and the second being the concrete syntax tree node
 to be converted. The syntax tree is converted from the bottom
 up.

 A concrete syntax tree node is a (type, value, context, nodes)
 tuple, where type is the node type (a token or symbol number),
 value is None for symbols and a string for tokens, context is
 None or an opaque value used for error reporting (typically a
 (lineno, offset) pair), and nodes is a list of children for
 symbols, and None for tokens.

 An abstract syntax tree node may be anything; this is entirely
 up to the converter function.

 """
 self.grammar = grammar
 self.convert = convert or (lambda grammar, node: node)

 def setup(self, start=None):
 """Prepare for parsing.

 This *must* be called before starting to parse.

 The optional argument is an alternative start symbol; it
 defaults to the grammar's start symbol.

 You can use a Parser instance to parse any number of programs;
 each time you call setup() the parser is reset to an initial
 state determined by the (implicit or explicit) start symbol.

 """
 if start is None:
 start = self.grammar.start
 # Each stack entry is a tuple: (dfa, state, node).
 # A node is a tuple: (type, value, context, children),
 # where children is a list of nodes or None, and context may be None.
 newnode = (start, None, None, [])
 stackentry = (self.grammar.dfas[start], 0, newnode)
 self.stack = [stackentry]
 self.rootnode = None
 self.used_names = set() # Aliased to self.rootnode.used_names in pop()

 def addtoken(self, type, value, context):
 """Add a token; return True iff this is the end of the program."""
 # Map from token to label
 ilabel = self.classify(type, value, context)
 # Loop until the token is shifted; may raise exceptions
 while True:
 dfa, state, node = self.stack[-1]
 states, first = dfa
 arcs = states[state]
 # Look for a state with this label
 for i, newstate in arcs:
 t, v = self.grammar.labels[i]
 if ilabel == i:
 # Look it up in the list of labels
 assert t < 256
 # Shift a token; we're done with it
 self.shift(type, value, newstate, context)
 # Pop while we are in an accept-only state
 state = newstate
 while states[state] == [(0, state)]:
 self.pop()
 if not self.stack:
 # Done parsing!
 return True
 dfa, state, node = self.stack[-1]
 states, first = dfa
 # Done with this token
 return False
 elif t >= 256:
 # See if it's a symbol and if we're in its first set
 itsdfa = self.grammar.dfas[t]
 itsstates, itsfirst = itsdfa
 if ilabel in itsfirst:
 # Push a symbol
 self.push(t, self.grammar.dfas[t], newstate, context)
 break # To continue the outer while loop
 else:
 if (0, state) in arcs:
 # An accepting state, pop it and try something else
 self.pop()
 if not self.stack:
 # Done parsing, but another token is input
 raise ParseError("too much input",
 type, value, context)
 else:
 # No success finding a transition
 raise ParseError("bad input", type, value, context)

 def classify(self, type, value, context):
 """Turn a token into a label. (Internal)"""
 if type == token.NAME:
 # Keep a listing of all used names
 self.used_names.add(value)
 # Check for reserved words
 ilabel = self.grammar.keywords.get(value)
 if ilabel is not None:
 return ilabel
 ilabel = self.grammar.tokens.get(type)
 if ilabel is None:
 raise ParseError("bad token", type, value, context)
 return ilabel

 def shift(self, type, value, newstate, context):
 """Shift a token. (Internal)"""
 dfa, state, node = self.stack[-1]
 newnode = (type, value, context, None)
 newnode = self.convert(self.grammar, newnode)
 if newnode is not None:
 node[-1].append(newnode)
 self.stack[-1] = (dfa, newstate, node)

 def push(self, type, newdfa, newstate, context):
 """Push a nonterminal. (Internal)"""
 dfa, state, node = self.stack[-1]
 newnode = (type, None, context, [])
 self.stack[-1] = (dfa, newstate, node)
 self.stack.append((newdfa, 0, newnode))

 def pop(self):
 """Pop a nonterminal. (Internal)"""
 popdfa, popstate, popnode = self.stack.pop()
 newnode = self.convert(self.grammar, popnode)
 if newnode is not None:
 if self.stack:
 dfa, state, node = self.stack[-1]
 node[-1].append(newnode)
 else:
 self.rootnode = newnode
 self.rootnode.used_names = self.used_names