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# parser.py - simple top-down operator precedence parser for mercurial
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#
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# Copyright 2010 Matt Mackall <mpm@selenic.com>
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#
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# This software may be used and distributed according to the terms of the
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# GNU General Public License version 2 or any later version.
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# see http://effbot.org/zone/simple-top-down-parsing.txt and
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# http://eli.thegreenplace.net/2010/01/02/top-down-operator-precedence-parsing/
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# for background
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# takes a tokenizer and elements
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# tokenizer is an iterator that returns type, value pairs
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# elements is a mapping of types to binding strength, prefix and infix actions
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# an action is a tree node name, a tree label, and an optional match
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# __call__(program) parses program into a labelled tree
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import error
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class parser(object):
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def __init__(self, tokenizer, elements, methods=None):
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self._tokenizer = tokenizer
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self._elements = elements
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self._methods = methods
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def _advance(self):
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'advance the tokenizer'
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t = self.current
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try:
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self.current = self._iter.next()
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except StopIteration:
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pass
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return t
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def _match(self, m):
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'make sure the tokenizer matches an end condition'
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if self.current[0] != m:
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raise error.ParseError("unexpected token: %s" % self.current[2],
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pos)
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self._advance()
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def _parse(self, bind=0):
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token, value, pos = self._advance()
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# handle prefix rules on current token
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prefix = self._elements[token][1]
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if not prefix:
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raise error.ParseError("not a prefix: %s" % token, pos)
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if len(prefix) == 1:
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expr = (prefix[0], value)
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else:
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if len(prefix) > 2 and prefix[2] == self.current[0]:
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self._match(prefix[2])
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expr = (prefix[0], None)
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else:
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expr = (prefix[0], self._parse(prefix[1]))
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if len(prefix) > 2:
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self._match(prefix[2])
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# gather tokens until we meet a lower binding strength
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while bind < self._elements[self.current[0]][0]:
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token, value, pos = self._advance()
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e = self._elements[token]
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# check for suffix - next token isn't a valid prefix
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if len(e) == 4 and not self._elements[self.current[0]][1]:
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suffix = e[3]
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expr = (suffix[0], expr)
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else:
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# handle infix rules
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infix = self._elements[token][2]
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if len(infix) == 3 and infix[2] == self.current[0]:
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self._match(infix[2])
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expr = (infix[0], expr, (None))
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else:
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if not infix[0]:
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raise error.ParseError("not an infix: %s" % token, pos)
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expr = (infix[0], expr, self._parse(infix[1]))
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if len(infix) == 3:
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self._match(infix[2])
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return expr
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def parse(self, message):
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'generate a parse tree from a message'
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self._iter = self._tokenizer(message)
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self.current = self._iter.next()
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return self._parse()
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def eval(self, tree):
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'recursively evaluate a parse tree using node methods'
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if not isinstance(tree, tuple):
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return tree
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return self._methods[tree[0]](*[self.eval(t) for t in tree[1:]])
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def __call__(self, message):
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'parse a message into a parse tree and evaluate if methods given'
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t = self.parse(message)
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if self._methods:
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return self.eval(t)
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return t
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