from django.db.migrations.utils import field_references
from django.db.models import NOT_PROVIDED
from django.utils.functional import cached_property

from .base import Operation


class FieldOperation(Operation):
    def __init__(self, model_name, name, field=None):
        self.model_name = model_name
        self.name = name
        self.field = field

    @cached_property
    def model_name_lower(self):
        return self.model_name.lower()

    @cached_property
    def name_lower(self):
        return self.name.lower()

    def is_same_model_operation(self, operation):
        return self.model_name_lower == operation.model_name_lower

    def is_same_field_operation(self, operation):
        return self.is_same_model_operation(operation) and self.name_lower == operation.name_lower

    def references_model(self, name, app_label):
        name_lower = name.lower()
        if name_lower == self.model_name_lower:
            return True
        if self.field:
            return bool(field_references(
                (app_label, self.model_name_lower), self.field, (app_label, name_lower)
            ))
        return False

    def references_field(self, model_name, name, app_label):
        model_name_lower = model_name.lower()
        # Check if this operation locally references the field.
        if model_name_lower == self.model_name_lower:
            if name == self.name:
                return True
            elif self.field and hasattr(self.field, 'from_fields') and name in self.field.from_fields:
                return True
        # Check if this operation remotely references the field.
        if self.field is None:
            return False
        return bool(field_references(
            (app_label, self.model_name_lower),
            self.field,
            (app_label, model_name_lower),
            name,
        ))

    def reduce(self, operation, app_label):
        return (
            super().reduce(operation, app_label) or
            not operation.references_field(self.model_name, self.name, app_label)
        )


class AddField(FieldOperation):
    """Add a field to a model."""

    def __init__(self, model_name, name, field, preserve_default=True):
        self.preserve_default = preserve_default
        super().__init__(model_name, name, field)

    def deconstruct(self):
        kwargs = {
            'model_name': self.model_name,
            'name': self.name,
            'field': self.field,
        }
        if self.preserve_default is not True:
            kwargs['preserve_default'] = self.preserve_default
        return (
            self.__class__.__name__,
            [],
            kwargs
        )

    def state_forwards(self, app_label, state):
        state.add_field(
            app_label,
            self.model_name_lower,
            self.name,
            self.field,
            self.preserve_default,
        )

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        to_model = to_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            field = to_model._meta.get_field(self.name)
            if not self.preserve_default:
                field.default = self.field.default
            schema_editor.add_field(
                from_model,
                field,
            )
            if not self.preserve_default:
                field.default = NOT_PROVIDED

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        from_model = from_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, from_model):
            schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))

    def describe(self):
        return "Add field %s to %s" % (self.name, self.model_name)

    @property
    def migration_name_fragment(self):
        return '%s_%s' % (self.model_name_lower, self.name_lower)

    def reduce(self, operation, app_label):
        if isinstance(operation, FieldOperation) and self.is_same_field_operation(operation):
            if isinstance(operation, AlterField):
                return [
                    AddField(
                        model_name=self.model_name,
                        name=operation.name,
                        field=operation.field,
                    ),
                ]
            elif isinstance(operation, RemoveField):
                return []
            elif isinstance(operation, RenameField):
                return [
                    AddField(
                        model_name=self.model_name,
                        name=operation.new_name,
                        field=self.field,
                    ),
                ]
        return super().reduce(operation, app_label)


class RemoveField(FieldOperation):
    """Remove a field from a model."""

    def deconstruct(self):
        kwargs = {
            'model_name': self.model_name,
            'name': self.name,
        }
        return (
            self.__class__.__name__,
            [],
            kwargs
        )

    def state_forwards(self, app_label, state):
        state.remove_field(app_label, self.model_name_lower, self.name)

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        from_model = from_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, from_model):
            schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        to_model = to_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            schema_editor.add_field(from_model, to_model._meta.get_field(self.name))

    def describe(self):
        return "Remove field %s from %s" % (self.name, self.model_name)

    @property
    def migration_name_fragment(self):
        return 'remove_%s_%s' % (self.model_name_lower, self.name_lower)

    def reduce(self, operation, app_label):
        from .models import DeleteModel
        if isinstance(operation, DeleteModel) and operation.name_lower == self.model_name_lower:
            return [operation]
        return super().reduce(operation, app_label)


class AlterField(FieldOperation):
    """
    Alter a field's database column (e.g. null, max_length) to the provided
    new field.
    """

    def __init__(self, model_name, name, field, preserve_default=True):
        self.preserve_default = preserve_default
        super().__init__(model_name, name, field)

    def deconstruct(self):
        kwargs = {
            'model_name': self.model_name,
            'name': self.name,
            'field': self.field,
        }
        if self.preserve_default is not True:
            kwargs['preserve_default'] = self.preserve_default
        return (
            self.__class__.__name__,
            [],
            kwargs
        )

    def state_forwards(self, app_label, state):
        state.alter_field(
            app_label,
            self.model_name_lower,
            self.name,
            self.field,
            self.preserve_default,
        )

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        to_model = to_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            from_field = from_model._meta.get_field(self.name)
            to_field = to_model._meta.get_field(self.name)
            if not self.preserve_default:
                to_field.default = self.field.default
            schema_editor.alter_field(from_model, from_field, to_field)
            if not self.preserve_default:
                to_field.default = NOT_PROVIDED

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        self.database_forwards(app_label, schema_editor, from_state, to_state)

    def describe(self):
        return "Alter field %s on %s" % (self.name, self.model_name)

    @property
    def migration_name_fragment(self):
        return 'alter_%s_%s' % (self.model_name_lower, self.name_lower)

    def reduce(self, operation, app_label):
        if isinstance(operation, RemoveField) and self.is_same_field_operation(operation):
            return [operation]
        elif isinstance(operation, RenameField) and self.is_same_field_operation(operation):
            return [
                operation,
                AlterField(
                    model_name=self.model_name,
                    name=operation.new_name,
                    field=self.field,
                ),
            ]
        return super().reduce(operation, app_label)


class RenameField(FieldOperation):
    """Rename a field on the model. Might affect db_column too."""

    def __init__(self, model_name, old_name, new_name):
        self.old_name = old_name
        self.new_name = new_name
        super().__init__(model_name, old_name)

    @cached_property
    def old_name_lower(self):
        return self.old_name.lower()

    @cached_property
    def new_name_lower(self):
        return self.new_name.lower()

    def deconstruct(self):
        kwargs = {
            'model_name': self.model_name,
            'old_name': self.old_name,
            'new_name': self.new_name,
        }
        return (
            self.__class__.__name__,
            [],
            kwargs
        )

    def state_forwards(self, app_label, state):
        state.rename_field(app_label, self.model_name_lower, self.old_name, self.new_name)

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        to_model = to_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            schema_editor.alter_field(
                from_model,
                from_model._meta.get_field(self.old_name),
                to_model._meta.get_field(self.new_name),
            )

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        to_model = to_state.apps.get_model(app_label, self.model_name)
        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            schema_editor.alter_field(
                from_model,
                from_model._meta.get_field(self.new_name),
                to_model._meta.get_field(self.old_name),
            )

    def describe(self):
        return "Rename field %s on %s to %s" % (self.old_name, self.model_name, self.new_name)

    @property
    def migration_name_fragment(self):
        return 'rename_%s_%s_%s' % (
            self.old_name_lower,
            self.model_name_lower,
            self.new_name_lower,
        )

    def references_field(self, model_name, name, app_label):
        return self.references_model(model_name, app_label) and (
            name.lower() == self.old_name_lower or
            name.lower() == self.new_name_lower
        )

    def reduce(self, operation, app_label):
        if (isinstance(operation, RenameField) and
                self.is_same_model_operation(operation) and
                self.new_name_lower == operation.old_name_lower):
            return [
                RenameField(
                    self.model_name,
                    self.old_name,
                    operation.new_name,
                ),
            ]
        # Skip `FieldOperation.reduce` as we want to run `references_field`
        # against self.old_name and self.new_name.
        return (
            super(FieldOperation, self).reduce(operation, app_label) or
            not (
                operation.references_field(self.model_name, self.old_name, app_label) or
                operation.references_field(self.model_name, self.new_name, app_label)
            )
        )
