using SQLInsight.Core.Data;
using SQLInsight.Core.Instances;

namespace SQLInsight.Collectors.Concurrency;

/// <summary>
/// Blocked sessions and their blockers right now. Head blockers are often sleeping sessions with an open
/// transaction, which have no row in <c>sys.dm_exec_requests</c>, so sessions are the starting point.
/// </summary>
public sealed class BlockingCollector : Collector<BlockingSnapshot>
{
    public override string Name => "blocking";
    public override TimeSpan DefaultInterval => TimeSpan.FromMinutes(1);

    protected override async Task<BlockingSnapshot> CollectAsync(CollectorContext context, BlockingSnapshot? previous)
    {
        var version = context.Instance.Version;
        var sessions = version.Major >= 9
            ? await Db.QueryAsync(context.Connection, BuildSql2005(version), Map2005, context.CancellationToken).ConfigureAwait(false)
            : await Db.QueryAsync(context.Connection, Sql2000, Map2000, context.CancellationToken).ConfigureAwait(false);

        return new BlockingSnapshot { CapturedAt = context.Now, Sessions = sessions };
    }

    internal static string BuildSql2005(SqlServerVersion version)
    {
        var openTran = version.Major >= 11
            ? "s.open_transaction_count"
            : "(SELECT COUNT(*) FROM sys.dm_tran_session_transactions st WHERE st.session_id = s.session_id)";
        var database = version.Major >= 11 ? "COALESCE(r.database_id, s.database_id)" : "r.database_id";

        // CONTEXT_INFO, where Dynamics AX 2009/2012 AOS can put the AX user (sys.dm_exec_sessions has it from 2008).
        var contextInfo = version.Major >= 10 ? "s.context_info" : "(SELECT TOP 1 p.context_info FROM sys.sysprocesses p WHERE p.spid = s.session_id AND p.ecid = 0)";

        // blocking_session_id < 0 are special cases (orphaned DTC, deferred recovery, latch) and not a session.
        return $"""
            SELECT s.session_id, NULLIF(r.blocking_session_id, 0), COALESCE(r.status, s.status), r.wait_type, r.wait_time, r.wait_resource,
                   DB_NAME({database}), s.login_name, s.host_name, s.program_name, {openTran},
                   LEFT(CASE WHEN r.sql_handle IS NOT NULL AND r.statement_start_offset IS NOT NULL
                             THEN SUBSTRING(t.text, r.statement_start_offset / 2 + 1,
                                  (CASE WHEN r.statement_end_offset = -1 THEN DATALENGTH(t.text) ELSE r.statement_end_offset END
                                   - r.statement_start_offset) / 2 + 1)
                             ELSE t.text END, 4000),
                   s.last_request_end_time, {contextInfo}
            FROM sys.dm_exec_sessions s
            LEFT JOIN sys.dm_exec_requests r ON r.session_id = s.session_id
            LEFT JOIN sys.dm_exec_connections c ON c.session_id = s.session_id
            OUTER APPLY sys.dm_exec_sql_text(COALESCE(r.sql_handle, c.most_recent_sql_handle)) t
            WHERE r.blocking_session_id > 0
               OR s.session_id IN (SELECT blocking_session_id FROM sys.dm_exec_requests WHERE blocking_session_id > 0)
            """;
    }

    // blocked = spid is a session waiting on its own latch, not blocking.
    internal const string Sql2000 = """
        SELECT p.spid, NULLIF(CASE WHEN p.blocked = p.spid THEN 0 ELSE p.blocked END, 0), RTRIM(p.status), RTRIM(p.lastwaittype),
               p.waittime, RTRIM(p.waitresource), DB_NAME(p.dbid), RTRIM(p.loginame), RTRIM(p.hostname), RTRIM(p.program_name),
               p.open_tran, CAST(NULL AS nvarchar(4000)), p.last_batch, p.context_info
        FROM master.dbo.sysprocesses p
        WHERE (p.blocked > 0 AND p.blocked <> p.spid)
           OR p.spid IN (SELECT blocked FROM master.dbo.sysprocesses WHERE blocked > 0 AND blocked <> spid)
        """;

    private static SessionInfo Map2005(Row r) => new()
    {
        SessionId = r.Int(0),
        BlockingSessionId = r.NullableInt(1),
        Status = r.NullableString(2),
        WaitType = r.NullableString(3),
        WaitTimeMs = r.NullableLong(4),
        WaitResource = r.NullableString(5),
        DatabaseName = r.NullableString(6),
        LoginName = r.NullableString(7),
        HostName = r.NullableString(8),
        ProgramName = r.NullableString(9),
        OpenTransactionCount = r.NullableInt(10) ?? 0,
        SqlText = r.NullableString(11),
        LastRequestEndTime = r.NullableDateTime(12),
        ContextInfo = r.NullableHex(13),
    };

    private static SessionInfo Map2000(Row r) => Map2005(r);
}