using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Threading.Tasks;

namespace Elwig.Helpers {
    public static class AppDbUpdater {

        // Don't forget to update value in Tests/fetch-resources.bat!
        public static readonly int RequiredSchemaVersion = 40;

        private static int VersionOffset = 0;

        public static async Task<Version> CheckDb() {
            long? applId, schemaVers;
            using (var cnx = await AppDbContext.ConnectAsync()) {
                applId = (long?)await cnx.ExecuteScalar("PRAGMA application_id") ?? 0;
                if (applId != 0x454C5747) throw new Exception($"Invalid application_id in database (0x{applId:X08})");

                schemaVers = (long?)await cnx.ExecuteScalar("PRAGMA schema_version") ?? 0;
                VersionOffset = (int)(schemaVers % 100);
                if (VersionOffset != 0) {
                    // schema was modified manually/externally
                    // TODO issue warning
                }
            }

            await UpdateDbSchema((int)(schemaVers / 100), RequiredSchemaVersion);

            Version v;
            using (var cnx = await AppDbContext.ConnectAsync()) {
                var userVers = (long?)await cnx.ExecuteScalar("PRAGMA user_version") ?? 0;
                v = new Version((int)(userVers >> 24), (int)((userVers >> 16) & 0xFF), (int)((userVers >> 8) & 0xFF), (int)(userVers & 0xFF));

                if (App.Version > v) {
                    long vers = (App.Version.Major << 24) | (App.Version.Minor << 16) | (App.Version.Build << 8) | App.Version.Revision;
                    await cnx.ExecuteBatch($"PRAGMA user_version = {vers}");
                }
            }

            return v;
        }

        private static async Task UpdateDbSchema(int fromVersion, int toVersion) {
            if (fromVersion == toVersion) {
                return;
            } else if (fromVersion > toVersion) {
                throw new Exception("schema_version of database is too new");
            } else if (fromVersion <= 0) {
                throw new Exception("schema_version of database is invalid");
            }

            var asm = Assembly.GetExecutingAssembly();
            (int From, int To, string Name)[] scripts = [.. asm.GetManifestResourceNames()
                .Where(n => n.StartsWith("Elwig.Resources.Sql."))
                .Select(n => {
                    var p = n.Split(".")[^2].Split("-");
                    return (int.Parse(p[0]), int.Parse(p[1]), n);
                })
                .OrderBy(s => s.Item1).ThenBy(s => s.Item2)];

            List<string> toExecute = [];
            var vers = fromVersion;
            while (vers < toVersion) {
                var (_, to, name) = scripts.Last(s => s.From == vers);
                toExecute.Add(name);
                vers = to;
            }
            if (toExecute.Count == 0)
                return;

            var backup = Path.ChangeExtension(App.Config.DatabaseFile, $".v{fromVersion}.sqlite3");
            File.Copy(App.Config.DatabaseFile, backup, true);
            try {
                using var cnx = await AppDbContext.ConnectAsync();
                await cnx.ExecuteBatch("PRAGMA locking_mode = EXCLUSIVE");
                foreach (var script in toExecute) {
                    await cnx.ExecuteEmbeddedScript(asm, script);
                }

                var violations = await cnx.ForeignKeyCheck();
                if (violations.Length > 0) {
                    throw new Exception($"Foreign key violations ({violations.Length}):\n" + string.Join("\n", violations
                        .Take(50)
                        .Select(v => $"{v.Table} - {v.RowId} - {v.Parent} - {v.FkId}")));
                }

                await cnx.ExecuteBatch("VACUUM");
                await cnx.ExecuteBatch($"PRAGMA schema_version = {toVersion * 100 + VersionOffset}");
            } catch (Exception) {
                File.Move(backup, App.Config.DatabaseFile, true);
                throw;
            } finally {
                File.Delete(backup);
            }
        }
    }
}