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# Version: MPL 1.1/GPL 2.0/LGPL 2.1
# The contents of this file are subject to the Mozilla Public License Version
# 1.1 (the "License"); you may not use this file except in compliance with
# the License. You may obtain a copy of the License at
# http://www.mozilla.org/MPL/
# Software distributed under the License is distributed on an "AS IS" basis,
# WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
# for the specific language governing rights and limitations under the
# License.
# The Original Code is Mozilla build system.
# The Initial Developer of the Original Code is
# Mozilla Foundation.
# Portions created by the Initial Developer are Copyright (C) 2007
# the Initial Developer. All Rights Reserved.
# Contributor(s):
#  Axel Hecht <axel@pike.org>
# Alternatively, the contents of this file may be used under the terms of
# either the GNU General Public License Version 2 or later (the "GPL"), or
# the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
# in which case the provisions of the GPL or the LGPL are applicable instead
# of those above. If you wish to allow use of your version of this file only
# under the terms of either the GPL or the LGPL, and not to allow others to
# use your version of this file under the terms of the MPL, indicate your
# decision by deleting the provisions above and replace them with the notice
# and other provisions required by the GPL or the LGPL. If you do not delete
# the provisions above, a recipient may use your version of this file under
# the terms of any one of the MPL, the GPL or the LGPL.
# ***** END LICENSE BLOCK *****

Parses and evaluates simple statements for Preprocessor:

Expression currently supports the following grammar, whitespace is ignored:

expression :
  unary ( ( '==' | '!=' ) unary ) ? ;
unary :
  '!'? value ;
value :
  [0-9]+ # integer
  | \w+  # string identifier or value;

import re

class Expression:
  def __init__(self, expression_string):
    Create a new expression with this string.
    The expression will already be parsed into an Abstract Syntax Tree.
    self.content = expression_string
    self.offset = 0
    self.e = self.__get_equality()
    if self.content:
      raise Expression.ParseError, self

  def __get_equality(self):
    Production: unary ( ( '==' | '!=' ) unary ) ?
    if not len(self.content):
      return None
    rv = Expression.__AST("equality")
    # unary 
    if not re.match('[=!]=', self.content):
      # no equality needed, short cut to our prime unary
      return rv[0]
    # append operator
    rv.append(Expression.__ASTLeaf('op', self.content[:2]))
    return rv

  def __get_unary(self):
    Production: '!'? value
    # eat whitespace right away, too
    not_ws = re.match('!\s*', self.content)
    if not not_ws:
      return self.__get_value()
    rv = Expression.__AST('not')
    return rv

  def __get_value(self):
    Production: ( [0-9]+ | \w+)
    Note that the order is important, and the expression is kind-of
    ambiguous as \w includes 0-9. One could make it unambiguous by
    removing 0-9 from the first char of a string literal.
    rv = None
    word_len = re.match('[0-9]*', self.content).end()
    if word_len:
      rv = Expression.__ASTLeaf('int', int(self.content[:word_len]))
      word_len = re.match('\w*', self.content).end()
      if word_len:
        rv = Expression.__ASTLeaf('string', self.content[:word_len])
        raise Expression.ParseError, self
    return rv

  def __ignore_whitespace(self):
    ws_len = re.match('\s*', self.content).end()

  def __strip(self, length):
    Remove a given amount of chars from the input and update
    the offset.
    self.content = self.content[length:]
    self.offset += length
  def evaluate(self, context):
    Evaluate the expression with the given context
    # Helper function to evaluate __get_equality results
    def eval_equality(tok):
      left = opmap[tok[0].type](tok[0])
      right = opmap[tok[2].type](tok[2])
      rv = left == right
      if tok[1].value == '!=':
        rv = not rv
      return rv
    # Mapping from token types to evaluator functions
    # Apart from (non-)equality, all these can be simple lambda forms.
    opmap = {
      'equality': eval_equality,
      'not': lambda tok: not opmap[tok[0].type](tok[0]),
      'string': lambda tok: context[tok.value],
      'int': lambda tok: tok.value}

    return opmap[self.e.type](self.e);
00159   class __AST(list):
    Internal class implementing Abstract Syntax Tree nodes
    def __init__(self, type):
      self.type = type
      super(self.__class__, self).__init__(self)
00167   class __ASTLeaf:
    Internal class implementing Abstract Syntax Tree leafs
    def __init__(self, type, value):
      self.value = value
      self.type = type
    def __str__(self):
      return self.value.__str__()
    def __repr__(self):
      return self.value.__repr__()
00179   class ParseError(StandardError):
    Error raised when parsing fails.
    It has two members, offset and content, which give the offset of the
    error and the offending content.
    def __init__(self, expression):
      self.offset = expression.offset
      self.content = expression.content[:3]
    def __str__(self):
      return 'Unexpected content at offset %i, "%s"'%(self.offset, self.content)

00191 class Context(dict):
  This class holds variable values by subclassing dict, and while it
  truthfully reports True and False on
  name in context
  it returns the variable name itself on

  to reflect the ambiguity between string literals and preprocessor
  def __getitem__(self, key):
    if key in self:
      return super(self.__class__, self).__getitem__(key)
    return key

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