using Npgsql;
using StellaOps.Doctor.Models;
using StellaOps.Doctor.Plugins;
using StellaOps.Doctor.Plugins.Builders;
using System.Globalization;

namespace StellaOps.Doctor.Plugins.Database.Checks;

/// <summary>
/// Checks the health of database connection pool.
/// </summary>
public sealed class ConnectionPoolHealthCheck : DatabaseCheckBase
{
    /// <inheritdoc />
    public override string CheckId => "check.db.pool.health";

    /// <inheritdoc />
    public override string Name => "Connection Pool Health";

    /// <inheritdoc />
    public override string Description => "Verifies the database connection pool is healthy";

    /// <inheritdoc />
    public override IReadOnlyList<string> Tags => ["database", "pool", "connectivity"];

    /// <inheritdoc />
    protected override async Task<DoctorCheckResult> ExecuteCheckAsync(
        DoctorPluginContext context,
        string connectionString,
        CheckResultBuilder result,
        CancellationToken ct)
    {
        await using var connection = await CreateConnectionAsync(connectionString, ct);

        // Get connection statistics from pg_stat_activity
        await using var cmd = new NpgsqlCommand(@"
            SELECT
                COUNT(*) AS total_connections,
                COUNT(*) FILTER (WHERE state = 'active') AS active_connections,
                COUNT(*) FILTER (WHERE state = 'idle') AS idle_connections,
                COUNT(*) FILTER (WHERE state = 'idle in transaction') AS idle_in_transaction,
                COUNT(*) FILTER (WHERE wait_event IS NOT NULL) AS waiting_connections,
                MAX(EXTRACT(EPOCH FROM (now() - backend_start))) AS oldest_connection_seconds
            FROM pg_stat_activity
            WHERE datname = current_database()
              AND pid <> pg_backend_pid()",
            connection);

        await using var reader = await cmd.ExecuteReaderAsync(ct);

        if (await reader.ReadAsync(ct))
        {
            var totalConnections = reader.GetInt64(0);
            var activeConnections = reader.GetInt64(1);
            var idleConnections = reader.GetInt64(2);
            var idleInTransaction = reader.GetInt64(3);
            var waitingConnections = reader.GetInt64(4);
            var oldestConnectionSeconds = reader.IsDBNull(5) ? 0 : reader.GetDouble(5);

            await reader.CloseAsync();

            // Get max connections setting
            await using var maxCmd = new NpgsqlCommand("SHOW max_connections", connection);
            var maxConnectionsStr = await maxCmd.ExecuteScalarAsync(ct) as string ?? "100";
            var maxConnections = int.Parse(maxConnectionsStr, CultureInfo.InvariantCulture);

            var usagePercent = (double)totalConnections / maxConnections * 100;

            // Check for issues
            if (idleInTransaction > 5)
            {
                return result
                    .Warn($"{idleInTransaction} connections idle in transaction")
                    .WithEvidence("Connection pool status", e => e
                        .Add("TotalConnections", totalConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("ActiveConnections", activeConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("IdleConnections", idleConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("IdleInTransaction", idleInTransaction.ToString(CultureInfo.InvariantCulture))
                        .Add("WaitingConnections", waitingConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("MaxConnections", maxConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("UsagePercent", $"{usagePercent:F1}%"))
                    .WithCauses(
                        "Long-running transactions not committed",
                        "Application not properly closing transactions",
                        "Deadlock or lock contention")
                    .WithRemediation(r => r
                        .AddShellStep(1, "Find idle transactions", "psql -c \"SELECT pid, query FROM pg_stat_activity WHERE state = 'idle in transaction'\"")
                        .AddManualStep(2, "Review application code", "Ensure transactions are properly committed or rolled back")
                        .WithRunbookUrl("docs/doctor/articles/postgres/db-pool-health.md"))
                    .WithVerification("stella doctor --check check.db.pool.health")
                    .Build();
            }

            if (usagePercent > 80)
            {
                return result
                    .Warn($"Connection pool usage at {usagePercent:F1}%")
                    .WithEvidence("Connection pool status", e => e
                        .Add("TotalConnections", totalConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("ActiveConnections", activeConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("IdleConnections", idleConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("MaxConnections", maxConnections.ToString(CultureInfo.InvariantCulture))
                        .Add("UsagePercent", $"{usagePercent:F1}%"))
                    .WithCauses(
                        "Connection leak in application",
                        "Too many concurrent requests",
                        "max_connections too low for workload")
                    .WithRemediation(r => r
                        .AddManualStep(1, "Review connection pool settings", "Check Npgsql connection string pool size")
                        .AddManualStep(2, "Consider increasing max_connections", "Edit postgresql.conf if appropriate")
                        .WithRunbookUrl("docs/doctor/articles/postgres/db-pool-health.md"))
                    .WithVerification("stella doctor --check check.db.pool.health")
                    .Build();
            }

            return result
                .Pass($"Connection pool healthy: {totalConnections}/{maxConnections} connections ({usagePercent:F1}%)")
                .WithEvidence("Connection pool status", e => e
                    .Add("TotalConnections", totalConnections.ToString(CultureInfo.InvariantCulture))
                    .Add("ActiveConnections", activeConnections.ToString(CultureInfo.InvariantCulture))
                    .Add("IdleConnections", idleConnections.ToString(CultureInfo.InvariantCulture))
                    .Add("IdleInTransaction", idleInTransaction.ToString(CultureInfo.InvariantCulture))
                    .Add("WaitingConnections", waitingConnections.ToString(CultureInfo.InvariantCulture))
                    .Add("MaxConnections", maxConnections.ToString(CultureInfo.InvariantCulture))
                    .Add("UsagePercent", $"{usagePercent:F1}%")
                    .Add("OldestConnectionAge", $"{oldestConnectionSeconds:F0}s"))
                .Build();
        }

        return result
            .Fail("Unable to retrieve connection pool statistics")
            .Build();
    }
}