import collections
import re

from sqlalchemy import util as sqlautil

from .. import util
from ..util import compat

_relative_destination = re.compile(r"(?:(.+?)@)?(\w+)?((?:\+|-)\d+)")
_revision_illegal_chars = ["@", "-", "+"]


class RevisionError(Exception):
    pass


class RangeNotAncestorError(RevisionError):
    def __init__(self, lower, upper):
        self.lower = lower
        self.upper = upper
        super(RangeNotAncestorError, self).__init__(
            "Revision %s is not an ancestor of revision %s"
            % (lower or "base", upper or "base")
        )


class MultipleHeads(RevisionError):
    def __init__(self, heads, argument):
        self.heads = heads
        self.argument = argument
        super(MultipleHeads, self).__init__(
            "Multiple heads are present for given argument '%s'; "
            "%s" % (argument, ", ".join(heads))
        )


class ResolutionError(RevisionError):
    def __init__(self, message, argument):
        super(ResolutionError, self).__init__(message)
        self.argument = argument


class RevisionMap(object):
    """Maintains a map of :class:`.Revision` objects.

    :class:`.RevisionMap` is used by :class:`.ScriptDirectory` to maintain
    and traverse the collection of :class:`.Script` objects, which are
    themselves instances of :class:`.Revision`.

    """

    def __init__(self, generator):
        """Construct a new :class:`.RevisionMap`.

        :param generator: a zero-arg callable that will generate an iterable
         of :class:`.Revision` instances to be used.   These are typically
         :class:`.Script` subclasses within regular Alembic use.

        """
        self._generator = generator

    @util.memoized_property
    def heads(self):
        """All "head" revisions as strings.

        This is normally a tuple of length one,
        unless unmerged branches are present.

        :return: a tuple of string revision numbers.

        """
        self._revision_map
        return self.heads

    @util.memoized_property
    def bases(self):
        """All "base" revisions as strings.

        These are revisions that have a ``down_revision`` of None,
        or empty tuple.

        :return: a tuple of string revision numbers.

        """
        self._revision_map
        return self.bases

    @util.memoized_property
    def _real_heads(self):
        """All "real" head revisions as strings.

        :return: a tuple of string revision numbers.

        """
        self._revision_map
        return self._real_heads

    @util.memoized_property
    def _real_bases(self):
        """All "real" base revisions as strings.

        :return: a tuple of string revision numbers.

        """
        self._revision_map
        return self._real_bases

    @util.memoized_property
    def _revision_map(self):
        """memoized attribute, initializes the revision map from the
        initial collection.

        """
        map_ = {}

        heads = sqlautil.OrderedSet()
        _real_heads = sqlautil.OrderedSet()
        self.bases = ()
        self._real_bases = ()

        has_branch_labels = set()
        has_depends_on = set()
        for revision in self._generator():

            if revision.revision in map_:
                util.warn(
                    "Revision %s is present more than once" % revision.revision
                )
            map_[revision.revision] = revision
            if revision.branch_labels:
                has_branch_labels.add(revision)
            if revision.dependencies:
                has_depends_on.add(revision)
            heads.add(revision.revision)
            _real_heads.add(revision.revision)
            if revision.is_base:
                self.bases += (revision.revision,)
            if revision._is_real_base:
                self._real_bases += (revision.revision,)

        # add the branch_labels to the map_.  We'll need these
        # to resolve the dependencies.
        for revision in has_branch_labels:
            self._map_branch_labels(revision, map_)

        for revision in has_depends_on:
            self._add_depends_on(revision, map_)

        for rev in map_.values():
            for downrev in rev._all_down_revisions:
                if downrev not in map_:
                    util.warn(
                        "Revision %s referenced from %s is not present"
                        % (downrev, rev)
                    )
                down_revision = map_[downrev]
                down_revision.add_nextrev(rev)
                if downrev in rev._versioned_down_revisions:
                    heads.discard(downrev)
                _real_heads.discard(downrev)

        map_[None] = map_[()] = None
        self.heads = tuple(heads)
        self._real_heads = tuple(_real_heads)

        for revision in has_branch_labels:
            self._add_branches(revision, map_, map_branch_labels=False)
        return map_

    def _map_branch_labels(self, revision, map_):
        if revision.branch_labels:
            for branch_label in revision._orig_branch_labels:
                if branch_label in map_:
                    raise RevisionError(
                        "Branch name '%s' in revision %s already "
                        "used by revision %s"
                        % (
                            branch_label,
                            revision.revision,
                            map_[branch_label].revision,
                        )
                    )
                map_[branch_label] = revision

    def _add_branches(self, revision, map_, map_branch_labels=True):
        if map_branch_labels:
            self._map_branch_labels(revision, map_)

        if revision.branch_labels:
            revision.branch_labels.update(revision.branch_labels)
            for node in self._get_descendant_nodes(
                [revision], map_, include_dependencies=False
            ):
                node.branch_labels.update(revision.branch_labels)

            parent = node
            while (
                parent
                and not parent._is_real_branch_point
                and not parent.is_merge_point
            ):

                parent.branch_labels.update(revision.branch_labels)
                if parent.down_revision:
                    parent = map_[parent.down_revision]
                else:
                    break

    def _add_depends_on(self, revision, map_):
        if revision.dependencies:
            deps = [map_[dep] for dep in util.to_tuple(revision.dependencies)]
            revision._resolved_dependencies = tuple([d.revision for d in deps])

    def add_revision(self, revision, _replace=False):
        """add a single revision to an existing map.

        This method is for single-revision use cases, it's not
        appropriate for fully populating an entire revision map.

        """
        map_ = self._revision_map
        if not _replace and revision.revision in map_:
            util.warn(
                "Revision %s is present more than once" % revision.revision
            )
        elif _replace and revision.revision not in map_:
            raise Exception("revision %s not in map" % revision.revision)

        map_[revision.revision] = revision
        self._add_branches(revision, map_)
        self._add_depends_on(revision, map_)

        if revision.is_base:
            self.bases += (revision.revision,)
        if revision._is_real_base:
            self._real_bases += (revision.revision,)
        for downrev in revision._all_down_revisions:
            if downrev not in map_:
                util.warn(
                    "Revision %s referenced from %s is not present"
                    % (downrev, revision)
                )
            map_[downrev].add_nextrev(revision)
        if revision._is_real_head:
            self._real_heads = tuple(
                head
                for head in self._real_heads
                if head
                not in set(revision._all_down_revisions).union(
                    [revision.revision]
                )
            ) + (revision.revision,)
        if revision.is_head:
            self.heads = tuple(
                head
                for head in self.heads
                if head
                not in set(revision._versioned_down_revisions).union(
                    [revision.revision]
                )
            ) + (revision.revision,)

    def get_current_head(self, branch_label=None):
        """Return the current head revision.

        If the script directory has multiple heads
        due to branching, an error is raised;
        :meth:`.ScriptDirectory.get_heads` should be
        preferred.

        :param branch_label: optional branch name which will limit the
         heads considered to those which include that branch_label.

        :return: a string revision number.

        .. seealso::

            :meth:`.ScriptDirectory.get_heads`

        """
        current_heads = self.heads
        if branch_label:
            current_heads = self.filter_for_lineage(
                current_heads, branch_label
            )
        if len(current_heads) > 1:
            raise MultipleHeads(
                current_heads,
                "%s@head" % branch_label if branch_label else "head",
            )

        if current_heads:
            return current_heads[0]
        else:
            return None

    def _get_base_revisions(self, identifier):
        return self.filter_for_lineage(self.bases, identifier)

    def get_revisions(self, id_):
        """Return the :class:`.Revision` instances with the given rev id
        or identifiers.

        May be given a single identifier, a sequence of identifiers, or the
        special symbols "head" or "base".  The result is a tuple of one
        or more identifiers, or an empty tuple in the case of "base".

        In the cases where 'head', 'heads' is requested and the
        revision map is empty, returns an empty tuple.

        Supports partial identifiers, where the given identifier
        is matched against all identifiers that start with the given
        characters; if there is exactly one match, that determines the
        full revision.

        """

        if isinstance(id_, (list, tuple, set, frozenset)):
            return sum([self.get_revisions(id_elem) for id_elem in id_], ())
        else:
            resolved_id, branch_label = self._resolve_revision_number(id_)
            return tuple(
                self._revision_for_ident(rev_id, branch_label)
                for rev_id in resolved_id
            )

    def get_revision(self, id_):
        """Return the :class:`.Revision` instance with the given rev id.

        If a symbolic name such as "head" or "base" is given, resolves
        the identifier into the current head or base revision.  If the symbolic
        name refers to multiples, :class:`.MultipleHeads` is raised.

        Supports partial identifiers, where the given identifier
        is matched against all identifiers that start with the given
        characters; if there is exactly one match, that determines the
        full revision.

        """

        resolved_id, branch_label = self._resolve_revision_number(id_)
        if len(resolved_id) > 1:
            raise MultipleHeads(resolved_id, id_)
        elif resolved_id:
            resolved_id = resolved_id[0]

        return self._revision_for_ident(resolved_id, branch_label)

    def _resolve_branch(self, branch_label):
        try:
            branch_rev = self._revision_map[branch_label]
        except KeyError:
            try:
                nonbranch_rev = self._revision_for_ident(branch_label)
            except ResolutionError:
                raise ResolutionError(
                    "No such branch: '%s'" % branch_label, branch_label
                )
            else:
                return nonbranch_rev
        else:
            return branch_rev

    def _revision_for_ident(self, resolved_id, check_branch=None):
        if check_branch:
            branch_rev = self._resolve_branch(check_branch)
        else:
            branch_rev = None

        try:
            revision = self._revision_map[resolved_id]
        except KeyError:
            # break out to avoid misleading py3k stack traces
            revision = False
        if revision is False:
            # do a partial lookup
            revs = [
                x
                for x in self._revision_map
                if x and len(x) > 3 and x.startswith(resolved_id)
            ]

            if branch_rev:
                revs = self.filter_for_lineage(revs, check_branch)
            if not revs:
                raise ResolutionError(
                    "No such revision or branch '%s'%s"
                    % (
                        resolved_id,
                        (
                            "; please ensure at least four characters are "
                            "present for partial revision identifier matches"
                            if len(resolved_id) < 4
                            else ""
                        ),
                    ),
                    resolved_id,
                )
            elif len(revs) > 1:
                raise ResolutionError(
                    "Multiple revisions start "
                    "with '%s': %s..."
                    % (resolved_id, ", ".join("'%s'" % r for r in revs[0:3])),
                    resolved_id,
                )
            else:
                revision = self._revision_map[revs[0]]

        if check_branch and revision is not None:
            if not self._shares_lineage(
                revision.revision, branch_rev.revision
            ):
                raise ResolutionError(
                    "Revision %s is not a member of branch '%s'"
                    % (revision.revision, check_branch),
                    resolved_id,
                )
        return revision

    def _filter_into_branch_heads(self, targets):
        targets = set(targets)

        for rev in list(targets):
            if targets.intersection(
                self._get_descendant_nodes([rev], include_dependencies=False)
            ).difference([rev]):
                targets.discard(rev)
        return targets

    def filter_for_lineage(
        self, targets, check_against, include_dependencies=False
    ):
        id_, branch_label = self._resolve_revision_number(check_against)

        shares = []
        if branch_label:
            shares.append(branch_label)
        if id_:
            shares.extend(id_)

        return [
            tg
            for tg in targets
            if self._shares_lineage(
                tg, shares, include_dependencies=include_dependencies
            )
        ]

    def _shares_lineage(
        self, target, test_against_revs, include_dependencies=False
    ):
        if not test_against_revs:
            return True
        if not isinstance(target, Revision):
            target = self._revision_for_ident(target)

        test_against_revs = [
            self._revision_for_ident(test_against_rev)
            if not isinstance(test_against_rev, Revision)
            else test_against_rev
            for test_against_rev in util.to_tuple(
                test_against_revs, default=()
            )
        ]

        return bool(
            set(
                self._get_descendant_nodes(
                    [target], include_dependencies=include_dependencies
                )
            )
            .union(
                self._get_ancestor_nodes(
                    [target], include_dependencies=include_dependencies
                )
            )
            .intersection(test_against_revs)
        )

    def _resolve_revision_number(self, id_):
        if isinstance(id_, compat.string_types) and "@" in id_:
            branch_label, id_ = id_.split("@", 1)

        elif id_ is not None and (
            (
                isinstance(id_, tuple)
                and id_
                and not isinstance(id_[0], compat.string_types)
            )
            or not isinstance(id_, compat.string_types + (tuple,))
        ):
            raise RevisionError(
                "revision identifier %r is not a string; ensure database "
                "driver settings are correct" % (id_,)
            )

        else:
            branch_label = None

        # ensure map is loaded
        self._revision_map
        if id_ == "heads":
            if branch_label:
                return (
                    self.filter_for_lineage(self.heads, branch_label),
                    branch_label,
                )
            else:
                return self._real_heads, branch_label
        elif id_ == "head":
            current_head = self.get_current_head(branch_label)
            if current_head:
                return (current_head,), branch_label
            else:
                return (), branch_label
        elif id_ == "base" or id_ is None:
            return (), branch_label
        else:
            return util.to_tuple(id_, default=None), branch_label

    def _relative_iterate(
        self,
        destination,
        source,
        is_upwards,
        implicit_base,
        inclusive,
        assert_relative_length,
    ):
        if isinstance(destination, compat.string_types):
            match = _relative_destination.match(destination)
            if not match:
                return None
        else:
            return None

        relative = int(match.group(3))
        symbol = match.group(2)
        branch_label = match.group(1)

        reldelta = 1 if inclusive and not symbol else 0

        if is_upwards:
            if branch_label:
                from_ = "%s@head" % branch_label
            elif symbol:
                if symbol.startswith("head"):
                    from_ = symbol
                else:
                    from_ = "%s@head" % symbol
            else:
                from_ = "head"
            to_ = source
        else:
            if branch_label:
                to_ = "%s@base" % branch_label
            elif symbol:
                to_ = "%s@base" % symbol
            else:
                to_ = "base"
            from_ = source

        revs = list(
            self._iterate_revisions(
                from_, to_, inclusive=inclusive, implicit_base=implicit_base
            )
        )

        if symbol:
            if branch_label:
                symbol_rev = self.get_revision(
                    "%s@%s" % (branch_label, symbol)
                )
            else:
                symbol_rev = self.get_revision(symbol)
            if symbol.startswith("head"):
                index = 0
            elif symbol == "base":
                index = len(revs) - 1
            else:
                range_ = compat.range(len(revs) - 1, 0, -1)
                for index in range_:
                    if symbol_rev.revision == revs[index].revision:
                        break
                else:
                    index = 0
        else:
            index = 0
        if is_upwards:
            revs = revs[index - relative - reldelta :]
            if (
                not index
                and assert_relative_length
                and len(revs) < abs(relative - reldelta)
            ):
                raise RevisionError(
                    "Relative revision %s didn't "
                    "produce %d migrations" % (destination, abs(relative))
                )
        else:
            revs = revs[0 : index - relative + reldelta]
            if (
                not index
                and assert_relative_length
                and len(revs) != abs(relative) + reldelta
            ):
                raise RevisionError(
                    "Relative revision %s didn't "
                    "produce %d migrations" % (destination, abs(relative))
                )

        return iter(revs)

    def iterate_revisions(
        self,
        upper,
        lower,
        implicit_base=False,
        inclusive=False,
        assert_relative_length=True,
        select_for_downgrade=False,
    ):
        """Iterate through script revisions, starting at the given
        upper revision identifier and ending at the lower.

        The traversal uses strictly the `down_revision`
        marker inside each migration script, so
        it is a requirement that upper >= lower,
        else you'll get nothing back.

        The iterator yields :class:`.Revision` objects.

        """

        relative_upper = self._relative_iterate(
            upper,
            lower,
            True,
            implicit_base,
            inclusive,
            assert_relative_length,
        )
        if relative_upper:
            return relative_upper

        relative_lower = self._relative_iterate(
            lower,
            upper,
            False,
            implicit_base,
            inclusive,
            assert_relative_length,
        )
        if relative_lower:
            return relative_lower

        return self._iterate_revisions(
            upper,
            lower,
            inclusive=inclusive,
            implicit_base=implicit_base,
            select_for_downgrade=select_for_downgrade,
        )

    def _get_descendant_nodes(
        self,
        targets,
        map_=None,
        check=False,
        omit_immediate_dependencies=False,
        include_dependencies=True,
    ):

        if omit_immediate_dependencies:

            def fn(rev):
                if rev not in targets:
                    return rev._all_nextrev
                else:
                    return rev.nextrev

        elif include_dependencies:

            def fn(rev):
                return rev._all_nextrev

        else:

            def fn(rev):
                return rev.nextrev

        return self._iterate_related_revisions(
            fn, targets, map_=map_, check=check
        )

    def _get_ancestor_nodes(
        self, targets, map_=None, check=False, include_dependencies=True
    ):

        if include_dependencies:

            def fn(rev):
                return rev._all_down_revisions

        else:

            def fn(rev):
                return rev._versioned_down_revisions

        return self._iterate_related_revisions(
            fn, targets, map_=map_, check=check
        )

    def _iterate_related_revisions(self, fn, targets, map_, check=False):
        if map_ is None:
            map_ = self._revision_map

        seen = set()
        todo = collections.deque()
        for target in targets:

            todo.append(target)
            if check:
                per_target = set()

            while todo:
                rev = todo.pop()
                if check:
                    per_target.add(rev)

                if rev in seen:
                    continue
                seen.add(rev)
                todo.extend(map_[rev_id] for rev_id in fn(rev))
                yield rev
            if check:
                overlaps = per_target.intersection(targets).difference(
                    [target]
                )
                if overlaps:
                    raise RevisionError(
                        "Requested revision %s overlaps with "
                        "other requested revisions %s"
                        % (
                            target.revision,
                            ", ".join(r.revision for r in overlaps),
                        )
                    )

    def _iterate_revisions(
        self,
        upper,
        lower,
        inclusive=True,
        implicit_base=False,
        select_for_downgrade=False,
    ):
        """iterate revisions from upper to lower.

        The traversal is depth-first within branches, and breadth-first
        across branches as a whole.

        """

        requested_lowers = self.get_revisions(lower)

        # some complexity to accommodate an iteration where some
        # branches are starting from nothing, and others are starting
        # from a given point.  Additionally, if the bottom branch
        # is specified using a branch identifier, then we limit operations
        # to just that branch.

        limit_to_lower_branch = isinstance(
            lower, compat.string_types
        ) and lower.endswith("@base")

        uppers = util.dedupe_tuple(self.get_revisions(upper))

        if not uppers and not requested_lowers:
            return

        upper_ancestors = set(self._get_ancestor_nodes(uppers, check=True))

        if limit_to_lower_branch:
            lowers = self.get_revisions(self._get_base_revisions(lower))
        elif implicit_base and requested_lowers:
            lower_ancestors = set(self._get_ancestor_nodes(requested_lowers))
            lower_descendants = set(
                self._get_descendant_nodes(requested_lowers)
            )
            base_lowers = set()
            candidate_lowers = upper_ancestors.difference(
                lower_ancestors
            ).difference(lower_descendants)
            for rev in candidate_lowers:
                for downrev in rev._all_down_revisions:
                    if self._revision_map[downrev] in candidate_lowers:
                        break
                else:
                    base_lowers.add(rev)
            lowers = base_lowers.union(requested_lowers)
        elif implicit_base:
            base_lowers = set(self.get_revisions(self._real_bases))
            lowers = base_lowers.union(requested_lowers)
        elif not requested_lowers:
            lowers = set(self.get_revisions(self._real_bases))
        else:
            lowers = requested_lowers

        # represents all nodes we will produce
        total_space = set(
            rev.revision for rev in upper_ancestors
        ).intersection(
            rev.revision
            for rev in self._get_descendant_nodes(
                lowers,
                check=True,
                omit_immediate_dependencies=(
                    select_for_downgrade and requested_lowers
                ),
            )
        )

        if not total_space:
            # no nodes.  determine if this is an invalid range
            # or not.
            start_from = set(requested_lowers)
            start_from.update(
                self._get_ancestor_nodes(
                    list(start_from), include_dependencies=True
                )
            )

            # determine all the current branch points represented
            # by requested_lowers
            start_from = self._filter_into_branch_heads(start_from)

            # if the requested start is one of those branch points,
            # then just return empty set
            if start_from.intersection(upper_ancestors):
                return
            else:
                # otherwise, they requested nodes out of
                # order
                raise RangeNotAncestorError(lower, upper)

        # organize branch points to be consumed separately from
        # member nodes
        branch_todo = set(
            rev
            for rev in (self._revision_map[rev] for rev in total_space)
            if rev._is_real_branch_point
            and len(total_space.intersection(rev._all_nextrev)) > 1
        )

        # it's not possible for any "uppers" to be in branch_todo,
        # because the ._all_nextrev of those nodes is not in total_space
        # assert not branch_todo.intersection(uppers)

        todo = collections.deque(
            r for r in uppers if r.revision in total_space
        )

        # iterate for total_space being emptied out
        total_space_modified = True
        while total_space:

            if not total_space_modified:
                raise RevisionError(
                    "Dependency resolution failed; iteration can't proceed"
                )
            total_space_modified = False
            # when everything non-branch pending is consumed,
            # add to the todo any branch nodes that have no
            # descendants left in the queue
            if not todo:
                todo.extendleft(
                    sorted(
                        (
                            rev
                            for rev in branch_todo
                            if not rev._all_nextrev.intersection(total_space)
                        ),
                        # favor "revisioned" branch points before
                        # dependent ones
                        key=lambda rev: 0 if rev.is_branch_point else 1,
                    )
                )
                branch_todo.difference_update(todo)
            # iterate nodes that are in the immediate todo
            while todo:
                rev = todo.popleft()
                total_space.remove(rev.revision)
                total_space_modified = True

                # do depth first for elements within branches,
                # don't consume any actual branch nodes
                todo.extendleft(
                    [
                        self._revision_map[downrev]
                        for downrev in reversed(rev._all_down_revisions)
                        if self._revision_map[downrev] not in branch_todo
                        and downrev in total_space
                    ]
                )

                if not inclusive and rev in requested_lowers:
                    continue
                yield rev

        assert not branch_todo


class Revision(object):
    """Base class for revisioned objects.

    The :class:`.Revision` class is the base of the more public-facing
    :class:`.Script` object, which represents a migration script.
    The mechanics of revision management and traversal are encapsulated
    within :class:`.Revision`, while :class:`.Script` applies this logic
    to Python files in a version directory.

    """

    nextrev = frozenset()
    """following revisions, based on down_revision only."""

    _all_nextrev = frozenset()

    revision = None
    """The string revision number."""

    down_revision = None
    """The ``down_revision`` identifier(s) within the migration script.

    Note that the total set of "down" revisions is
    down_revision + dependencies.

    """

    dependencies = None
    """Additional revisions which this revision is dependent on.

    From a migration standpoint, these dependencies are added to the
    down_revision to form the full iteration.  However, the separation
    of down_revision from "dependencies" is to assist in navigating
    a history that contains many branches, typically a multi-root scenario.

    """

    branch_labels = None
    """Optional string/tuple of symbolic names to apply to this
    revision's branch"""

    @classmethod
    def verify_rev_id(cls, revision):
        illegal_chars = set(revision).intersection(_revision_illegal_chars)
        if illegal_chars:
            raise RevisionError(
                "Character(s) '%s' not allowed in revision identifier '%s'"
                % (", ".join(sorted(illegal_chars)), revision)
            )

    def __init__(
        self, revision, down_revision, dependencies=None, branch_labels=None
    ):
        self.verify_rev_id(revision)
        self.revision = revision
        self.down_revision = tuple_rev_as_scalar(down_revision)
        self.dependencies = tuple_rev_as_scalar(dependencies)
        self._resolved_dependencies = ()
        self._orig_branch_labels = util.to_tuple(branch_labels, default=())
        self.branch_labels = set(self._orig_branch_labels)

    def __repr__(self):
        args = [repr(self.revision), repr(self.down_revision)]
        if self.dependencies:
            args.append("dependencies=%r" % (self.dependencies,))
        if self.branch_labels:
            args.append("branch_labels=%r" % (self.branch_labels,))
        return "%s(%s)" % (self.__class__.__name__, ", ".join(args))

    def add_nextrev(self, revision):
        self._all_nextrev = self._all_nextrev.union([revision.revision])
        if self.revision in revision._versioned_down_revisions:
            self.nextrev = self.nextrev.union([revision.revision])

    @property
    def _all_down_revisions(self):
        return (
            util.to_tuple(self.down_revision, default=())
            + self._resolved_dependencies
        )

    @property
    def _versioned_down_revisions(self):
        return util.to_tuple(self.down_revision, default=())

    @property
    def is_head(self):
        """Return True if this :class:`.Revision` is a 'head' revision.

        This is determined based on whether any other :class:`.Script`
        within the :class:`.ScriptDirectory` refers to this
        :class:`.Script`.   Multiple heads can be present.

        """
        return not bool(self.nextrev)

    @property
    def _is_real_head(self):
        return not bool(self._all_nextrev)

    @property
    def is_base(self):
        """Return True if this :class:`.Revision` is a 'base' revision."""

        return self.down_revision is None

    @property
    def _is_real_base(self):
        """Return True if this :class:`.Revision` is a "real" base revision,
        e.g. that it has no dependencies either."""

        # we use self.dependencies here because this is called up
        # in initialization where _real_dependencies isn't set up
        # yet
        return self.down_revision is None and self.dependencies is None

    @property
    def is_branch_point(self):
        """Return True if this :class:`.Script` is a branch point.

        A branchpoint is defined as a :class:`.Script` which is referred
        to by more than one succeeding :class:`.Script`, that is more
        than one :class:`.Script` has a `down_revision` identifier pointing
        here.

        """
        return len(self.nextrev) > 1

    @property
    def _is_real_branch_point(self):
        """Return True if this :class:`.Script` is a 'real' branch point,
        taking into account dependencies as well.

        """
        return len(self._all_nextrev) > 1

    @property
    def is_merge_point(self):
        """Return True if this :class:`.Script` is a merge point."""

        return len(self._versioned_down_revisions) > 1


def tuple_rev_as_scalar(rev):
    if not rev:
        return None
    elif len(rev) == 1:
        return rev[0]
    else:
        return rev
