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

namespace SQLInsight.Collectors.Performance;

/// <summary>
/// Active session sampling: every 15 seconds, what each waiting user request waits on, in which database and for
/// which statement (the same key as top SQL). Sums of these samples show which statements a wait type comes from.
/// The agent's own sessions and system sessions are left out. One light query on sys.dm_exec_requests; SQL Server 2005+.
/// </summary>
public sealed class WaitSamplingCollector : Collector<WaitSampleSnapshot>
{
    private const string From = """
        FROM sys.dm_exec_requests r
        JOIN sys.dm_exec_sessions s ON s.session_id = r.session_id
        OUTER APPLY sys.dm_exec_sql_text(r.sql_handle) st
        WHERE s.is_user_process = 1 AND r.session_id <> @@SPID AND r.wait_type IS NOT NULL
          AND (s.program_name IS NULL OR s.program_name NOT LIKE N'SQLInsight Pro%')
        """;

    private const string Columns = """
        r.session_id, r.wait_type, r.wait_time, DB_NAME(r.database_id), r.blocking_session_id,
        """;

    private const string Text = """
        CAST(SUBSTRING(st.text, r.statement_start_offset / 2 + 1,
             CASE WHEN r.statement_end_offset = -1 THEN 4000 ELSE (r.statement_end_offset - r.statement_start_offset) / 2 + 1 END) AS nvarchar(4000))
        """;

    /// <summary>The statement key as top SQL makes it on 2008+: query_hash.</summary>
    internal const string Sql2008 = "SELECT " + Columns + """
        CASE WHEN r.query_hash IS NULL OR r.query_hash = 0x0000000000000000 THEN NULL ELSE CONVERT(varchar(20), r.query_hash, 1) END,
        """ + "\n" + Text + "\n" + From;

    /// <summary>SQL Server 2005 has no query_hash: the batch (sql_handle) and the statement's offsets, as top SQL.</summary>
    internal const string Sql2005 = "SELECT " + Columns + """
        CASE WHEN r.sql_handle IS NULL THEN NULL
             ELSE sys.fn_varbintohexstr(r.sql_handle) + ':' + CAST(r.statement_start_offset AS varchar(12)) + ':' + CAST(r.statement_end_offset AS varchar(12)) END,
        """ + "\n" + Text + "\n" + From;

    public override string Name => "wait-sampling";
    public override TimeSpan DefaultInterval => WaitSampling.Interval;
    public override IReadOnlyCollection<Capability> RequiredCapabilities => [Capability.DynamicManagementViews];
    public override bool IsSampling => true;

    protected override async Task<WaitSampleSnapshot> CollectAsync(CollectorContext context, WaitSampleSnapshot? previous)
    {
        var sql = context.Instance.Version.Major >= 10 ? Sql2008 : Sql2005;
        var requests = await Db.QueryAsync(context.Connection, sql, r => new WaitingRequest(
            r.Int(0), r.String(1), r.Long(2), r.NullableString(3), r.NullableString(5), r.NullableString(6), r.NullableInt(4) ?? 0),
            context.CancellationToken).ConfigureAwait(false);
        return WaitSampling.Aggregate(requests, context.Now);
    }
}