import os
from django.utils.importlib import import_module
from django.utils.functional import cached_property
from django.db.models.loading import cache
from django.db.migrations.recorder import MigrationRecorder
from django.db.migrations.graph import MigrationGraph


class MigrationLoader(object):
    """
    Loads migration files from disk, and their status from the database.

    Migration files are expected to live in the "migrations" directory of
    an app. Their names are entirely unimportant from a code perspective,
    but will probably follow the 1234_name.py convention.

    On initialisation, this class will scan those directories, and open and
    read the python files, looking for a class called Migration, which should
    inherit from django.db.migrations.Migration. See
    django.db.migrations.migration for what that looks like.

    Some migrations will be marked as "replacing" another set of migrations.
    These are loaded into a separate set of migrations away from the main ones.
    If all the migrations they replace are either unapplied or missing from
    disk, then they are injected into the main set, replacing the named migrations.
    Any dependency pointers to the replaced migrations are re-pointed to the
    new migration.

    This does mean that this class MUST also talk to the database as well as
    to disk, but this is probably fine. We're already not just operating
    in memory.
    """

    def __init__(self, connection):
        self.connection = connection
        self.disk_migrations = None
        self.applied_migrations = None

    def load_disk(self):
        """
        Loads the migrations from all INSTALLED_APPS from disk.
        """
        self.disk_migrations = {}
        for app in cache.get_apps():
            # Get the migrations module directory
            module_name = ".".join(app.__name__.split(".")[:-1] + ["migrations"])
            app_label = module_name.split(".")[-2]
            try:
                module = import_module(module_name)
            except ImportError as e:
                # I hate doing this, but I don't want to squash other import errors.
                # Might be better to try a directory check directly.
                if "No module named migrations" in str(e):
                    continue
            directory = os.path.dirname(module.__file__)
            # Scan for .py[c|o] files
            migration_names = set()
            for name in os.listdir(directory):
                if name.endswith(".py") or name.endswith(".pyc") or name.endswith(".pyo"):
                    import_name = name.rsplit(".", 1)[0]
                    if import_name[0] not in "_.~":
                        migration_names.add(import_name)
            # Load them
            for migration_name in migration_names:
                migration_module = import_module("%s.%s" % (module_name, migration_name))
                if not hasattr(migration_module, "Migration"):
                    raise BadMigrationError("Migration %s in app %s has no Migration class" % (migration_name, app_label))
                self.disk_migrations[app_label, migration_name] = migration_module.Migration(migration_name, app_label)

    @cached_property
    def graph(self):
        """
        Builds a migration dependency graph using both the disk and database.
        """
        # Make sure we have the disk data
        if self.disk_migrations is None:
            self.load_disk()
        # And the database data
        if self.applied_migrations is None:
            recorder = MigrationRecorder(self.connection)
            self.applied_migrations = recorder.applied_migrations()
        # Do a first pass to separate out replacing and non-replacing migrations
        normal = {}
        replacing = {}
        for key, migration in self.disk_migrations.items():
            if migration.replaces:
                replacing[key] = migration
            else:
                normal[key] = migration
        # Calculate reverse dependencies - i.e., for each migration, what depends on it?
        # This is just for dependency re-pointing when applying replacements,
        # so we ignore run_before here.
        reverse_dependencies = {}
        for key, migration in normal.items():
            for parent in migration.dependencies:
                reverse_dependencies.setdefault(parent, set()).add(key)
        # Carry out replacements if we can - that is, if all replaced migrations
        # are either unapplied or missing.
        for key, migration in replacing.items():
            # Do the check
            can_replace = True
            for target in migration.replaces:
                if target in self.applied_migrations:
                    can_replace = False
                    break
            if not can_replace:
                continue
            # Alright, time to replace. Step through the replaced migrations
            # and remove, repointing dependencies if needs be.
            for replaced in migration.replaces:
                if replaced in normal:
                    del normal[replaced]
                for child_key in reverse_dependencies.get(replaced, set()):
                    normal[child_key].dependencies.remove(replaced)
                    normal[child_key].dependencies.append(key)
            normal[key] = migration
        # Finally, make a graph and load everything into it
        graph = MigrationGraph()
        for key, migration in normal.items():
            graph.add_node(key, migration)
        for key, migration in normal.items():
            for parent in migration.dependencies:
                graph.add_dependency(key, parent)
        return graph


class BadMigrationError(Exception):
    """
    Raised when there's a bad migration (unreadable/bad format/etc.)
    """
    pass