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+# A Boneless Datastructure Language
+# Copyright (C) 2019  Soni L.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+"""A Boneless Datastructure Language, version 2.0.1.
+
+ABDL expressions are regex-like constructs for matching and validating object structures. They can be used
+with JSON and similar formats, and even self-referential data structures.
+
+Language Reference:
+
+    ABDL expressions have the ability to iterate, index, validate and filter data structures. This is
+    done with the use of the syntax elements listed below.
+
+    Syntax Elements:
+
+        An ABDL expression is a sequence of zero or more sequences starting with arrows followed by zero or
+        more subvalues.
+
+        An arrow is ``->`` and indicates indexing/iteration (Mappings, Sequences, Sets). It must be followed
+        by a variable, literal, parameter, regex or key match.
+
+        A variable is a string of alphanumeric characters, not starting with a digit. It may be followed by a
+        literal, parameter, regex, key match, or one or more type tests. A ``(key, value)`` tuple containing
+        the corresponding matched element will be identified by this name in the results dict.
+
+        A literal is a string delimited by single quotes (use ``%'`` to escape ``'`` and ``%%`` to escape ``%``).
+        A literal can be made "non-validating" by appending an ``?`` after it. It may be followed by one or more
+        type tests. It is exactly equivalent to indexing an object with a string key.
+
+        A parameter is the symbol ``$`` followed by a string of alphanumeric characters, not starting with
+        a digit. A parameter can be made "non-validating" by appending an ``?`` after it. It may be followed by
+        one or more type tests. It is exactly equivalent to indexing an object with an arbitrary object key.
+
+        A regex is an RE, as defined by the ``re`` module, delimited by forward slashes (use ``%/`` to escape
+        ``/`` and ``%%`` to escape ``%``). A regex can be made "non-validating" by appending an ``?`` after it.
+        It may be followed by one or more type tests. It attempts to match each key in the object.
+
+        A type test is ``:`` followed by a parameter. A type test can be made "non-validating" by appending
+        an ``?`` after the ``:``. It attempts to match the type of each matched value in the object.
+
+        A key match is an ABDL expression enclosed in ``[`` and ``]``, optionally prefixed with one or more type
+        tests. This matches keys (including the type tests).
+
+        A subvalue is an ABDL expression enclosed in ``(`` and ``)``. This allows matching multiple values on
+        the same object.
+
+        Some syntax elements can be validating or non-validating. Validating syntax elements will raise a
+        :py:exc:`abdl.ValidationError` whenever a non-matching element is encountered, whereas non-validating
+        ones will skip them. Note that it is possible for a validating syntax element to still yield results
+        before raising a :py:exc:`abdl.ValidationError`, so one needs to be careful when writing code where such
+        behaviour could result in a security vulnerability.
+
+    Examples:
+
+        >>> import abdl
+        >>> for m in abdl.match("->X:?$dict->Y", {"foo": 1, "bar": {"baz": 2}}, {'dict': dict}):
+        ...     print(m['X'][0], m['Y'][0], m['Y'][1])
+        bar baz 2
+
+        >>> pat = abdl.compile('''-> 'projects'?
+        ...                          -> commit /[0-9a-fA-F]{40}|[0-9a-fA-F]{64}/? :?$dict
+        ...                             -> url :?$dict
+        ...                                -> branch :?$dict''', {'dict': dict})
+        >>> data = {"projects": {
+        ...     "385e734a52e13949a7a5c71827f6de920dbfea43": {
+        ...         "https://soniex2.autistic.space/git-repos/ganarchy.git": {"HEAD": {"active": True}}
+        ...     }
+        ... }}
+        >>> for m in pat.match(data):
+        ...     print(m['commit'][0], m['url'][0], m['branch'][0], m['branch'][1])
+        385e734a52e13949a7a5c71827f6de920dbfea43 https://soniex2.autistic.space/git-repos/ganarchy.git HEAD {'active': True}
+
+    (If ``:?$dict`` wasn't present, a TypeError would be raised when trying to iterate the ``1`` from ``"foo": 1``.)
+"""
+
+import re
+
+from collections.abc import Mapping, Sequence, Iterator, Set
+
+class DeprecationError(Exception):
+    """Raised for deprecated features, if they are disabled.
+
+    This class controls warning/error behaviour of deprecated features."""
+    #enable_key_match_compat = False
+    #warn_key_match_compat = False
+
+    @classmethod
+    def warn_all(cls):
+        """Enables all deprecation warnings."""
+        pass
+
+class PatternError(Exception):
+    """Raised for invalid input or output expressions."""
+    # TODO implement formatting
+
+    def __init__(self, msg, pattern, defs, pos, toks):
+        self.msg = msg
+        self.pattern = pattern
+        self.defs = defs
+        self.pos = pos
+        self._toks = toks # internal use
+
+    def _normalize(self, pattern, defs):
+        if pattern is not None:
+            if self.pattern is not None:
+                raise ValueError("Attempt to normalize normalized pattern")
+            else:
+                self.pattern = pattern
+        if defs is not None:
+            if self.defs is not None:
+                raise ValueError("Attempt to normalize normalized defs")
+            else:
+                self.defs = defs
+
+    @classmethod
+    def _str_escape(cls, s, pos, toks):
+        raise cls("Error in string escape", None, None, pos, toks)
+
+    @classmethod
+    def _str_end(cls, s, pos, toks):
+        raise cls("Unfinished string", None, None, pos, toks)
+
+    @classmethod
+    def _re_escape(cls, s, pos, toks):
+        raise cls("Error in regex escape", None, None, pos, toks)
+
+    @classmethod
+    def _re_end(cls, s, pos, toks):
+        raise cls("Unfinished regex", None, None, pos, toks)
+
+    @classmethod
+    def _unexpected_tok(cls, s, pos, toks):
+        raise cls("Unexpected token", None, None, pos, toks)
+
+class ValidationError(Exception):
+    """Raised when the object tree doesn't validate against the given pattern."""
+    # FIXME TODO?
+
+class _PatternElement:
+    def on_not_in_key(self, frame, path, defs):
+        raise NotImplementedError
+
+    def on_in_key(self, frame, path, defs):
+        raise NotImplementedError
+
+    def collect_params(self, res: list):
+        pass
+
+class _Arrow(_PatternElement):
+    def on_not_in_key(self, frame, path, defs):
+        assert not path[-1].empty
+        path.append(_Holder(key=None, value=None, name=None, parent=path[-1].value, empty=True))
+        return False
+
+class _StringKey(_PatternElement):
+    def __init__(self, toks):
+        self.key = toks[0]
+        self.skippable = toks[1] == '?'
+
+    def on_in_key(self, frame, path, defs):
+        return self.on_not_in_key(frame, path, defs)
+
+    def on_not_in_key(self, frame, path, defs):
+        path[-1].iterator = self.extract(path[-1].parent)
+        path[-1].empty = False
+        return True
+
+    def extract(self, obj):
+        try:
+            yield (self.key, obj[self.key])
+        except (TypeError, IndexError, KeyError):
+            if not self.skippable:
+                raise ValidationError
+
+class _RegexKey(_PatternElement):
+    def __init__(self, toks):
+        self.key = toks[0]
+        self.compiled = re.compile(self.key)
+        self.skippable = toks[1] == '?'
+
+    def on_in_key(self, frame, path, defs):
+        return self.on_not_in_key(frame, path, defs)
+
+    def on_not_in_key(self, frame, path, defs):
+        filtered_iterator = self.filter(path[-1].iterator)
+        del path[-1].iterator
+        path[-1].iterator = filtered_iterator
+        del filtered_iterator
+        path[-1].empty = False
+        return True
+
+    def filter(self, it):
+        for el in it:
+            try:
+                if self.compiled.search(el[0]):
+                    yield el
+                elif not self.skippable:
+                    raise ValidationError
+            except TypeError:
+                if not self.skippable:
+                    raise ValidationError
+
+class _KeySubtree(_PatternElement):
+    def __init__(self, toks):
+        self.key = toks[0]
+        self.skippable = toks[1] == '?'
+
+    def on_not_in_key(self, frame, path, defs):
+        path[-1].subtree = True
+        filtered_iterator = self.filter(path[-1].iterator, defs)
+        del path[-1].iterator
+        path[-1].iterator = filtered_iterator
+        del filtered_iterator
+        path[-1].empty = False
+        return True
+
+    def filter(self, it, defs):
+        for x in it:
+            for y in _match_helper(self.key, defs, x[0]):
+                yield (y, x[1])
+
+    def collect_params(self, res: list):
+        for sub in self.key:
+            sub.collect_params(res)
+
+class _ValueSubtree(_PatternElement):
+    def __init__(self, toks):
+        self.key = toks[0]
+        self.skippable = toks[1] == '?'
+
+    def on_not_in_key(self, frame, path, defs):
+        assert not path[-1].empty
+        path.append(_Holder(key=None, value=None, name=None, parent=path[-1].value, empty=False, subtree=True))
+        path[-1].iterator = self.filter(path[-1].parent, defs)
+        return True
+
+    def filter(self, parent, defs):
+        for x in _match_helper(self.key, defs, parent):
+            yield (x, parent)
+
+    def collect_params(self, res: list):
+        for sub in self.key:
+            sub.collect_params(res)
+
+class _Ident(_PatternElement):
+    def __init__(self, toks):
+        self.key = toks[0]
+
+    def on_not_in_key(self, frame, path, defs):
+        path[-1].name = self.key
+        path[-1].empty = False
+        return True
+
+class _Param(_PatternElement):
+    def __init__(self, toks):
+        assert isinstance(toks[1], _Ident)
+        self.skippable = toks[0] == '?'
+        self.key = toks[1].key
+
+    def on_in_key(self, frame, path, defs):
+        return self.on_not_in_key(frame, path, defs)
+
+    def on_not_in_key(self, frame, path, defs):
+        path[-1].iterator = self.extract(path[-1].parent, defs[self.key])
+        path[-1].empty = False
+        return True
+
+    def extract(self, obj, key):
+        try:
+            yield (key, obj[key])
+        except (TypeError, IndexError, KeyError):
+            if not self.skippable:
+                raise ValidationError
+
+    def collect_params(self, res: list):
+        res.append(self.key)
+    
+    def get_value(self, defs):
+        return defs[self.key]
+
+class _Ty(_PatternElement):
+    def __init__(self, toks):
+        assert isinstance(toks[1], _Ident)
+        self.skippable = toks[0] == '?'
+        self.key = toks[1].key
+
+    def on_in_key(self, frame, path, defs):
+        filtered_iterator = self.filter(path[-1].iterator, defs[self.key])
+        del path[-1].iterator
+        path[-1].iterator = filtered_iterator
+        del filtered_iterator
+        path[-1].empty = False
+        return True
+
+    def on_not_in_key(self, frame, path, defs):
+        assert len(path) == 1
+        if isinstance(path[-1].value, defs[self.key]):
+            return False
+        elif not self.skippable:
+            raise ValidationError
+        path.clear()
+        return False
+
+    def filter(self, it, ty):
+        for el in it:
+            # this may TypeError if ty is not a type nor a tuple of types
+            # but that's actually the programmer's error
+            if isinstance(el[1], ty):
+                yield el
+            elif not self.skippable:
+                # and this one is for actual validation
+                raise ValidationError
+
+    def collect_params(self, res: list):
+        res.append(self.key)
+
+class _End(_PatternElement):
+    def on_in_key(self, frame, path, defs):
+        try:
+            path[-1].next()
+            return False
+        except StopIteration:
+            path.pop()
+            while frame.prev() and not isinstance(frame.current_op, _End):
+                pass
+            if not frame.prev():
+                # FIXME?
+                path.clear()
+        return True # FIXME?
+
+def _build_syntax():
+    from pyparsing import Suppress, Literal, Forward, CharsNotIn, StringEnd, Combine, Optional, Group, Word, srange, Empty
+    # original regex order: arrow, type/parameter/identifier, string, regex, failure
+    # better syntax: "arrow" changes from "value" to "key" and thus you need at least one key match before an arrow
+    subtree = Forward()
+    # where relevant, enforces match behaviour (skippable object tree branch vs required object tree branch)
+    skippable = Optional("?", default="")
+    #   r"|'(?:%'|%%|%(?P<EES>.|$)|[^%'])*?(?:'|(?P<ES>$))\??" # string literals
+    str_literal = (Combine(Suppress("'")
+            + (Suppress("%") + "'" | Suppress("%") + "%" | Literal("%") + (CharsNotIn("") | StringEnd()).setParseAction(PatternError._str_escape) | CharsNotIn("%'"))[...]
+            + (Suppress("'") | StringEnd().setParseAction(PatternError._str_end))) + skippable).setParseAction(lambda toks: [_StringKey(toks)])
+    #   r"|/(?:%/|%%|%(?P<EER>.|$)|[^%/])*?(?:/|(?P<ER>$))\??" # regex
+    re_literal = (Combine(Suppress("/")
+            + (Suppress("%") + "/" | Suppress("%") + "%" | Literal("%") + (CharsNotIn("") | StringEnd()).setParseAction(PatternError._re_escape) | CharsNotIn("%/"))[...]
+            + (Suppress("/") | StringEnd().setParseAction(PatternError._re_end))) + skippable).setParseAction(lambda toks: [_RegexKey(toks)])
+    arrow = Literal("->").setParseAction(lambda: [_Arrow()])
+    #   r"|(?::\??)?\$?[A-Za-z][A-Za-z0-9]*"                   # identifiers, parameters and type matches
+    identifier = Word(srange("[A-Za-z_]"), srange("[A-Za-z0-9_]")).setParseAction(lambda toks: [_Ident(toks)])
+    parameter = (Suppress("$") + skippable + identifier).setParseAction(lambda toks: [_Param(toks)])
+    ty = (Suppress(":") + skippable + Suppress("$") + identifier).setParseAction(lambda toks: [_Ty(toks)])
+    # support for objects-as-keys
+    keysubtree = (Suppress("[") + Group(ty[...] + subtree) + (Suppress("]") | CharsNotIn("").setParseAction(PatternError._unexpected_tok) | StringEnd().setParseAction(PatternError._unexpected_tok)) + Optional("?", default="")).setParseAction(lambda toks: [_KeySubtree(toks)])
+    # represents key matching - switches from "key" to "value"
+    tag = (identifier + Optional(parameter | str_literal | re_literal | keysubtree) | parameter | str_literal | re_literal | keysubtree) + ty[...] + Empty().setParseAction(lambda: [_End()])
+    # multiple value matching
+    valuesubtree = (Suppress("(") + Group(subtree) + (Suppress(")") | CharsNotIn("").setParseAction(PatternError._unexpected_tok) | StringEnd().setParseAction(PatternError._unexpected_tok)) + Optional("?", default="")).setParseAction(lambda toks: [_ValueSubtree(toks)])
+    # arrow and tag, value subtree
+    subtree <<= (arrow + tag)[...] + (valuesubtree + Empty().setParseAction(lambda: [_End()]))[...]
+    return ((subtree | CharsNotIn("").setParseAction(PatternError._unexpected_tok)) + StringEnd()).parseWithTabs()
+
+_built_syntax = _build_syntax()
+
+def _pairs(o):
+    if isinstance(o, Mapping):
+        return iter(o.items())
+    elif isinstance(o, Sequence):
+        return iter(enumerate(o, 0))
+    elif isinstance(o, Set):
+        return iter(((e, e) for e in o))
+    else:
+        # maybe there's more stuff I can implement later
+        raise TypeError
+
+class _Holder:
+    def __init__(self, key, value, name, parent=None, it=None, empty=False, subtree=False):
+        self.name = name
+        self.key = key
+        self.value = value
+        self.empty = empty
+        self._it = it
+        self.parent = parent
+        self.subtree = subtree
+
+    @property
+    def iterator(self):
+        if self._it is None:
+            self._it = _pairs(self.parent)
+        return self._it
+
+    @iterator.setter
+    def iterator(self, value):
+        assert self._it is None
+        self._it = value
+
+    @iterator.deleter
+    def iterator(self):
+        self._it = None
+
+    def next(self):
+        self.key, self.value = next(self.iterator)
+
+class _Frame:
+    def __init__(self, ops):
+        self.ops = ops
+        self.pc = -1
+
+    def next(self):
+        pc = self.pc + 1
+        if pc >= len(self.ops):
+            return False
+        self.pc = pc
+        return True
+
+    @property
+    def current_op(self):
+        return self.ops[self.pc]
+
+    def prev(self):
+        pc = self.pc - 1
+        if pc < 0:
+            return False
+        self.pc = pc
+        return True
+
+def _match_helper(ops, defs, tree):
+    frame = _Frame(ops)
+
+    path = [_Holder(key=None, value=tree, parent=None, it=iter(()), name=None)]
+    in_key = False
+    while path:
+        if not frame.next():
+            assert not path[-1].empty
+            res = {}
+            for h in path:
+                if h.subtree:
+                    for name, kv in h.key.items():
+                        res[name] = kv
+                elif h.name is not None:
+                    res[h.name] = (h.key, h.value)
+            yield res
+            assert len(path) == 1 or isinstance(frame.current_op, _End)
+            frame.prev()
+            in_key = True
+        else:
+            op = frame.current_op
+            if in_key:
+                in_key = op.on_in_key(frame, path, defs)
+            else:
+                in_key = op.on_not_in_key(frame, path, defs)
+
+class Pattern:
+    """A compiled pattern object.
+
+    Warning:
+        Do not create instances of this class manually. Use :py:func:`abdl.compile`.
+
+    """
+
+    def __init__(self, pattern, defs):
+        try:
+            self._ops = _built_syntax.parseString(pattern)
+        except PatternError as e:
+            e._normalize(pattern, defs)
+            raise
+        else:
+            self._params = []
+            for op in self._ops:
+                op.collect_params(self._params)
+            self._defs = {param: defs[param] for param in self._params}
+
+    def match(self, obj):
+        """Matches this compiled pattern against the given object.
+
+        Args:
+            obj: The object to match against.
+
+        Returns:
+            An iterator. This iterator yields ``(key, value)`` pairs
+            wrapped in a dict for each variable in the pattern.
+
+        """
+        return _match_helper(self._ops, self._defs, obj)
+
+def compile(pattern, defs={}):
+    """Compiles the pattern and returns a compiled :py:class:`abdl.Pattern` object.
+
+    Args:
+        pattern (str): The pattern. Refer to module-level documentation for
+            pattern syntax.
+        defs (dict): The parameter list. Used by parameters in the pattern.
+
+    Returns:
+        Pattern: A compiled pattern object.
+
+    """
+    # TODO caching
+    return Pattern(pattern, defs)
+
+def match(pattern, obj, defs={}):
+    """Matches the pattern against the given obj.
+
+    This method is equivalent to ``abdl.compile(pattern, defs).match(obj)``.
+
+    Args:
+        pattern (str): The pattern. Refer to module-level documentation for
+            pattern syntax.
+        obj: The object to match against.
+        defs (dict): The parameter list. Used by parameters in the pattern.
+
+    Returns:
+        An iterator. This iterator yields ``(key, value)`` pairs
+        wrapped in a dict for each variable in the pattern.
+
+    """
+    return compile(pattern, defs).match(obj)