// Copyright (C) 2021-present sam (starshines.gay)
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published
// by the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

using System.Data.Common;
using Dapper;
using NodaTime;

namespace Catalogger.Backend.Database;

/// <summary>
/// Manages database migrations.
/// </summary>
public class DatabaseMigrator(ILogger logger, IClock clock, DatabaseConnection conn)
    : IDisposable,
        IAsyncDisposable
{
    private const string RootPath = "Catalogger.Backend.Database";
    private static readonly int MigrationsPathLength = $"{RootPath}.Migrations.".Length;

    /// <summary>
    /// Migrates the database to the latest version.
    /// </summary>
    public async Task MigrateUp()
    {
        var migrations = GetMigrationNames().ToArray();
        logger.Debug("Getting current database migration");
        var currentMigration = await GetCurrentMigration();
        if (currentMigration != null)
            migrations = migrations
                .Where(s => string.CompareOrdinal(s, currentMigration.MigrationName) > 0)
                .ToArray();

        logger.Information(
            "Current migration: {Migration}. Applying {Count} migrations",
            currentMigration?.MigrationName,
            migrations.Length
        );
        if (migrations.Length == 0)
        {
            return;
        }

        // Wrap all migrations in a transaction
        await using var tx = await conn.BeginTransactionAsync();
        var totalStartTime = clock.GetCurrentInstant();
        foreach (var migration in migrations)
        {
            logger.Debug("Executing migration {Migration}", migration);
            var startTime = clock.GetCurrentInstant();
            await ExecuteMigration(tx, migration);
            var took = clock.GetCurrentInstant() - startTime;
            logger.Debug("Executed migration {Migration} in {Took}", migration, took);
        }

        var totalTook = clock.GetCurrentInstant() - totalStartTime;
        logger.Information("Executed {Count} migrations in {Took}", migrations.Length, totalTook);

        // Finally, commit the transaction
        await tx.CommitAsync();
    }

    /// <summary>
    /// Migrates the database to a previous version.
    /// </summary>
    /// <param name="count">The number of migrations to revert. If higher than the number of applied migrations,
    /// reverts the database to a clean slate.</param>
    public async Task MigrateDown(int count = 1)
    {
        await using var tx = await conn.BeginTransactionAsync();

        var migrationCount = 0;
        var totalStartTime = clock.GetCurrentInstant();
        for (var i = count; i > 0; i--)
        {
            var migration = await GetCurrentMigration();
            if (migration == null)
            {
                logger.Information(
                    "More down migrations requested than were in the database, finishing early"
                );
                break;
            }

            logger.Debug("Reverting migration {Migration}", migration);
            var startTime = clock.GetCurrentInstant();
            await ExecuteMigration(tx, migration.MigrationName, up: false);
            var took = clock.GetCurrentInstant() - startTime;
            logger.Debug("Reverted migration {Migration} in {Took}", migration, took);
            migrationCount++;
        }

        var totalTook = clock.GetCurrentInstant() - totalStartTime;
        logger.Information("Reverted {Count} migrations in {Took}", migrationCount, totalTook);

        // Finally, commit the transaction
        await tx.CommitAsync();
    }

    private async Task ExecuteMigration(DbTransaction tx, string migrationName, bool up = true)
    {
        var query = await GetResource(
            $"{RootPath}.Migrations.{migrationName}.{(up ? "up" : "down")}.sql"
        );

        // Run the migration
        await conn.ExecuteAsync(query, transaction: tx);
        // Store that we ran the migration (or reverted it)
        if (up)
            await conn.ExecuteAsync(
                "INSERT INTO migrations (migration_name, applied_at) VALUES (@MigrationName, now())",
                new { MigrationName = migrationName }
            );
        else
            await conn.ExecuteAsync(
                "DELETE FROM migrations WHERE migration_name = @MigrationName",
                new { MigrationName = migrationName }
            );
    }

    /// Returns the current migration. If no migrations have been applied, returns null
    private async Task<MigrationEntry?> GetCurrentMigration()
    {
        // Check if the migrations table exists
        var hasMigrationTable =
            await conn.QuerySingleOrDefaultAsync<int>(
                "SELECT COUNT(*) FROM information_schema.tables WHERE table_name = 'migrations'"
            ) == 1;
        // If so, return the current migration
        if (hasMigrationTable)
        {
            return await conn.QuerySingleOrDefaultAsync<MigrationEntry>(
                "SELECT * FROM migrations ORDER BY applied_at DESC, migration_name DESC LIMIT 1"
            );
        }

        logger.Debug("Migrations table does not exist, assuming this is a new database");

        // Else, create the migrations table then return null
        var migrationTableQuery = await GetResource($"{RootPath}.setup_migrations.sql");
        await conn.ExecuteAsync(migrationTableQuery);
        return null;
    }

    /// Returns a resource by name as a string.
    private static async Task<string> GetResource(string name)
    {
        await using var stream =
            typeof(DatabasePool).Assembly.GetManifestResourceStream(name)
            ?? throw new ArgumentException($"Invalid resource '{name}'");
        using var reader = new StreamReader(stream);
        return await reader.ReadToEndAsync();
    }

    private static IEnumerable<string> GetMigrationNames() =>
        typeof(DatabasePool)
            .Assembly.GetManifestResourceNames()
            .Where(s => s.StartsWith($"{RootPath}.Migrations"))
            .Where(s => s.EndsWith(".up.sql"))
            .Select(s =>
                s.Substring(
                    MigrationsPathLength,
                    s.Length - MigrationsPathLength - ".up.sql".Length
                )
            )
            .OrderBy(s => s);

    private record MigrationEntry
    {
        public string MigrationName { get; init; } = null!;
        public Instant AppliedAt { get; init; }
    }

    public void Dispose()
    {
        conn.Dispose();
        GC.SuppressFinalize(this);
    }

    public async ValueTask DisposeAsync()
    {
        await conn.DisposeAsync();
        GC.SuppressFinalize(this);
    }
}