// Copyright 2018 New Vector Ltd
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package postgres

import (
	"context"
	"database/sql"

	"github.com/matrix-org/dendrite/common"
)

// The purpose of this table is to keep track of backwards extremities for a room.
// Backwards extremities are the earliest (DAG-wise) known events which we have
// the entire event JSON. These event IDs are used in federation requests to fetch
// even earlier events.
//
// We persist the previous event IDs as well, one per row, so when we do fetch even
// earlier events we can simply delete rows which referenced it. Consider the graph:
//        A
//        |   Event C has 1 prev_event ID: A.
//    B   C
//    |___|   Event D has 2 prev_event IDs: B and C.
//      |
//      D
// The earliest known event we have is D, so this table has 2 rows.
// A backfill request gives us C but not B. We delete rows where prev_event=C. This
// still means that D is a backwards extremity as we do not have event B. However, event
// C is *also* a backwards extremity at this point as we do not have event A. Later,
// when we fetch event B, we delete rows where prev_event=B. This then removes D as
// a backwards extremity because there are no more rows with event_id=B.
const backwardExtremitiesSchema = `
-- Stores output room events received from the roomserver.
CREATE TABLE IF NOT EXISTS syncapi_backward_extremities (
	-- The 'room_id' key for the event.
	room_id TEXT NOT NULL,
	-- The event ID for the last known event. This is the backwards extremity.
	event_id TEXT NOT NULL,
	-- The prev_events for the last known event. This is used to update extremities.
	prev_event_id TEXT NOT NULL,

	PRIMARY KEY(room_id, event_id, prev_event_id)
);
`

const insertBackwardExtremitySQL = "" +
	"INSERT INTO syncapi_backward_extremities (room_id, event_id, prev_event_id)" +
	" VALUES ($1, $2, $3)" +
	" ON CONFLICT DO NOTHING"

const selectBackwardExtremitiesForRoomSQL = "" +
	"SELECT event_id FROM syncapi_backward_extremities WHERE room_id = $1"

const deleteBackwardExtremitySQL = "" +
	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"

type backwardExtremitiesStatements struct {
	insertBackwardExtremityStmt          *sql.Stmt
	selectBackwardExtremitiesForRoomStmt *sql.Stmt
	deleteBackwardExtremityStmt          *sql.Stmt
}

func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
	_, err = db.Exec(backwardExtremitiesSchema)
	if err != nil {
		return
	}
	if s.insertBackwardExtremityStmt, err = db.Prepare(insertBackwardExtremitySQL); err != nil {
		return
	}
	if s.selectBackwardExtremitiesForRoomStmt, err = db.Prepare(selectBackwardExtremitiesForRoomSQL); err != nil {
		return
	}
	if s.deleteBackwardExtremityStmt, err = db.Prepare(deleteBackwardExtremitySQL); err != nil {
		return
	}
	return
}

func (s *backwardExtremitiesStatements) insertsBackwardExtremity(
	ctx context.Context, txn *sql.Tx, roomID, eventID string, prevEventID string,
) (err error) {
	_, err = txn.Stmt(s.insertBackwardExtremityStmt).ExecContext(ctx, roomID, eventID, prevEventID)
	return
}

func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
	ctx context.Context, roomID string,
) (eventIDs []string, err error) {
	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
	if err != nil {
		return
	}
	defer common.CloseAndLogIfError(ctx, rows, "selectBackwardExtremitiesForRoom: rows.close() failed")

	for rows.Next() {
		var eID string
		if err = rows.Scan(&eID); err != nil {
			return
		}

		eventIDs = append(eventIDs, eID)
	}

	return eventIDs, rows.Err()
}

func (s *backwardExtremitiesStatements) deleteBackwardExtremity(
	ctx context.Context, txn *sql.Tx, roomID, knownEventID string,
) (err error) {
	_, err = txn.Stmt(s.deleteBackwardExtremityStmt).ExecContext(ctx, roomID, knownEventID)
	return
}