import os
import sys
from itertools import takewhile

from django.apps import apps
from django.conf import settings
from django.core.management.base import (
    BaseCommand, CommandError, no_translations,
)
from django.db import DEFAULT_DB_ALIAS, connections, router
from django.db.migrations import Migration
from django.db.migrations.autodetector import MigrationAutodetector
from django.db.migrations.loader import MigrationLoader
from django.db.migrations.questioner import (
    InteractiveMigrationQuestioner, MigrationQuestioner,
    NonInteractiveMigrationQuestioner,
)
from django.db.migrations.state import ProjectState
from django.db.migrations.utils import get_migration_name_timestamp
from django.db.migrations.writer import MigrationWriter


class Command(BaseCommand):
    help = "Creates new migration(s) for apps."

    def add_arguments(self, parser):
        parser.add_argument(
            'args', metavar='app_label', nargs='*',
            help='Specify the app label(s) to create migrations for.',
        )
        parser.add_argument(
            '--dry-run', action='store_true',
            help="Just show what migrations would be made; don't actually write them.",
        )
        parser.add_argument(
            '--merge', action='store_true',
            help="Enable fixing of migration conflicts.",
        )
        parser.add_argument(
            '--empty', action='store_true',
            help="Create an empty migration.",
        )
        parser.add_argument(
            '--noinput', '--no-input', action='store_false', dest='interactive',
            help='Tells Django to NOT prompt the user for input of any kind.',
        )
        parser.add_argument(
            '-n', '--name',
            help="Use this name for migration file(s).",
        )
        parser.add_argument(
            '--no-header', action='store_false', dest='include_header',
            help='Do not add header comments to new migration file(s).',
        )
        parser.add_argument(
            '--check', action='store_true', dest='check_changes',
            help='Exit with a non-zero status if model changes are missing migrations.',
        )

    @no_translations
    def handle(self, *app_labels, **options):
        self.verbosity = options['verbosity']
        self.interactive = options['interactive']
        self.dry_run = options['dry_run']
        self.merge = options['merge']
        self.empty = options['empty']
        self.migration_name = options['name']
        if self.migration_name and not self.migration_name.isidentifier():
            raise CommandError('The migration name must be a valid Python identifier.')
        self.include_header = options['include_header']
        check_changes = options['check_changes']

        # Make sure the app they asked for exists
        app_labels = set(app_labels)
        has_bad_labels = False
        for app_label in app_labels:
            try:
                apps.get_app_config(app_label)
            except LookupError as err:
                self.stderr.write(str(err))
                has_bad_labels = True
        if has_bad_labels:
            sys.exit(2)

        # Load the current graph state. Pass in None for the connection so
        # the loader doesn't try to resolve replaced migrations from DB.
        loader = MigrationLoader(None, ignore_no_migrations=True)

        # Raise an error if any migrations are applied before their dependencies.
        consistency_check_labels = {config.label for config in apps.get_app_configs()}
        # Non-default databases are only checked if database routers used.
        aliases_to_check = connections if settings.DATABASE_ROUTERS else [DEFAULT_DB_ALIAS]
        for alias in sorted(aliases_to_check):
            connection = connections[alias]
            if (connection.settings_dict['ENGINE'] != 'django.db.backends.dummy' and any(
                    # At least one model must be migrated to the database.
                    router.allow_migrate(connection.alias, app_label, model_name=model._meta.object_name)
                    for app_label in consistency_check_labels
                    for model in apps.get_app_config(app_label).get_models()
            )):
                loader.check_consistent_history(connection)

        # Before anything else, see if there's conflicting apps and drop out
        # hard if there are any and they don't want to merge
        conflicts = loader.detect_conflicts()

        # If app_labels is specified, filter out conflicting migrations for unspecified apps
        if app_labels:
            conflicts = {
                app_label: conflict for app_label, conflict in conflicts.items()
                if app_label in app_labels
            }

        if conflicts and not self.merge:
            name_str = "; ".join(
                "%s in %s" % (", ".join(names), app)
                for app, names in conflicts.items()
            )
            raise CommandError(
                "Conflicting migrations detected; multiple leaf nodes in the "
                "migration graph: (%s).\nTo fix them run "
                "'python manage.py makemigrations --merge'" % name_str
            )

        # If they want to merge and there's nothing to merge, then politely exit
        if self.merge and not conflicts:
            self.stdout.write("No conflicts detected to merge.")
            return

        # If they want to merge and there is something to merge, then
        # divert into the merge code
        if self.merge and conflicts:
            return self.handle_merge(loader, conflicts)

        if self.interactive:
            questioner = InteractiveMigrationQuestioner(specified_apps=app_labels, dry_run=self.dry_run)
        else:
            questioner = NonInteractiveMigrationQuestioner(specified_apps=app_labels, dry_run=self.dry_run)
        # Set up autodetector
        autodetector = MigrationAutodetector(
            loader.project_state(),
            ProjectState.from_apps(apps),
            questioner,
        )

        # If they want to make an empty migration, make one for each app
        if self.empty:
            if not app_labels:
                raise CommandError("You must supply at least one app label when using --empty.")
            # Make a fake changes() result we can pass to arrange_for_graph
            changes = {
                app: [Migration("custom", app)]
                for app in app_labels
            }
            changes = autodetector.arrange_for_graph(
                changes=changes,
                graph=loader.graph,
                migration_name=self.migration_name,
            )
            self.write_migration_files(changes)
            return

        # Detect changes
        changes = autodetector.changes(
            graph=loader.graph,
            trim_to_apps=app_labels or None,
            convert_apps=app_labels or None,
            migration_name=self.migration_name,
        )

        if not changes:
            # No changes? Tell them.
            if self.verbosity >= 1:
                if app_labels:
                    if len(app_labels) == 1:
                        self.stdout.write("No changes detected in app '%s'" % app_labels.pop())
                    else:
                        self.stdout.write("No changes detected in apps '%s'" % ("', '".join(app_labels)))
                else:
                    self.stdout.write("No changes detected")
        else:
            self.write_migration_files(changes)
            if check_changes:
                sys.exit(1)

    def write_migration_files(self, changes):
        """
        Take a changes dict and write them out as migration files.
        """
        directory_created = {}
        for app_label, app_migrations in changes.items():
            if self.verbosity >= 1:
                self.stdout.write(self.style.MIGRATE_HEADING("Migrations for '%s':" % app_label))
            for migration in app_migrations:
                # Describe the migration
                writer = MigrationWriter(migration, self.include_header)
                if self.verbosity >= 1:
                    # Display a relative path if it's below the current working
                    # directory, or an absolute path otherwise.
                    try:
                        migration_string = os.path.relpath(writer.path)
                    except ValueError:
                        migration_string = writer.path
                    if migration_string.startswith('..'):
                        migration_string = writer.path
                    self.stdout.write('  %s\n' % self.style.MIGRATE_LABEL(migration_string))
                    for operation in migration.operations:
                        self.stdout.write('    - %s' % operation.describe())
                if not self.dry_run:
                    # Write the migrations file to the disk.
                    migrations_directory = os.path.dirname(writer.path)
                    if not directory_created.get(app_label):
                        os.makedirs(migrations_directory, exist_ok=True)
                        init_path = os.path.join(migrations_directory, "__init__.py")
                        if not os.path.isfile(init_path):
                            open(init_path, "w").close()
                        # We just do this once per app
                        directory_created[app_label] = True
                    migration_string = writer.as_string()
                    with open(writer.path, "w", encoding='utf-8') as fh:
                        fh.write(migration_string)
                elif self.verbosity == 3:
                    # Alternatively, makemigrations --dry-run --verbosity 3
                    # will output the migrations to stdout rather than saving
                    # the file to the disk.
                    self.stdout.write(self.style.MIGRATE_HEADING(
                        "Full migrations file '%s':" % writer.filename
                    ))
                    self.stdout.write(writer.as_string())

    def handle_merge(self, loader, conflicts):
        """
        Handles merging together conflicted migrations interactively,
        if it's safe; otherwise, advises on how to fix it.
        """
        if self.interactive:
            questioner = InteractiveMigrationQuestioner()
        else:
            questioner = MigrationQuestioner(defaults={'ask_merge': True})

        for app_label, migration_names in conflicts.items():
            # Grab out the migrations in question, and work out their
            # common ancestor.
            merge_migrations = []
            for migration_name in migration_names:
                migration = loader.get_migration(app_label, migration_name)
                migration.ancestry = [
                    mig for mig in loader.graph.forwards_plan((app_label, migration_name))
                    if mig[0] == migration.app_label
                ]
                merge_migrations.append(migration)

            def all_items_equal(seq):
                return all(item == seq[0] for item in seq[1:])

            merge_migrations_generations = zip(*(m.ancestry for m in merge_migrations))
            common_ancestor_count = sum(1 for common_ancestor_generation
                                        in takewhile(all_items_equal, merge_migrations_generations))
            if not common_ancestor_count:
                raise ValueError("Could not find common ancestor of %s" % migration_names)
            # Now work out the operations along each divergent branch
            for migration in merge_migrations:
                migration.branch = migration.ancestry[common_ancestor_count:]
                migrations_ops = (loader.get_migration(node_app, node_name).operations
                                  for node_app, node_name in migration.branch)
                migration.merged_operations = sum(migrations_ops, [])
            # In future, this could use some of the Optimizer code
            # (can_optimize_through) to automatically see if they're
            # mergeable. For now, we always just prompt the user.
            if self.verbosity > 0:
                self.stdout.write(self.style.MIGRATE_HEADING("Merging %s" % app_label))
                for migration in merge_migrations:
                    self.stdout.write(self.style.MIGRATE_LABEL("  Branch %s" % migration.name))
                    for operation in migration.merged_operations:
                        self.stdout.write('    - %s' % operation.describe())
            if questioner.ask_merge(app_label):
                # If they still want to merge it, then write out an empty
                # file depending on the migrations needing merging.
                numbers = [
                    MigrationAutodetector.parse_number(migration.name)
                    for migration in merge_migrations
                ]
                try:
                    biggest_number = max(x for x in numbers if x is not None)
                except ValueError:
                    biggest_number = 1
                subclass = type("Migration", (Migration,), {
                    "dependencies": [(app_label, migration.name) for migration in merge_migrations],
                })
                migration_name = "%04i_%s" % (
                    biggest_number + 1,
                    self.migration_name or ("merge_%s" % get_migration_name_timestamp())
                )
                new_migration = subclass(migration_name, app_label)
                writer = MigrationWriter(new_migration, self.include_header)

                if not self.dry_run:
                    # Write the merge migrations file to the disk
                    with open(writer.path, "w", encoding='utf-8') as fh:
                        fh.write(writer.as_string())
                    if self.verbosity > 0:
                        self.stdout.write("\nCreated new merge migration %s" % writer.path)
                elif self.verbosity == 3:
                    # Alternatively, makemigrations --merge --dry-run --verbosity 3
                    # will output the merge migrations to stdout rather than saving
                    # the file to the disk.
                    self.stdout.write(self.style.MIGRATE_HEADING(
                        "Full merge migrations file '%s':" % writer.filename
                    ))
                    self.stdout.write(writer.as_string())
