using Microsoft.Extensions.Configuration;

namespace ZB.MOM.WW.OtOpcUa.Runtime;

/// <summary>
///     The definitions of "this node's LocalDb is replicated to a peer" (#603) — one place, two deliberately
///     different predicates for the two consumers: the Host's table-registration step
///     (<c>LocalDbSetup.ReplicationIsConfigured</c> → <see cref="CapturesChanges"/>: which nodes capture
///     changes) and the alarm-historian drain gate (<c>AddAlarmHistorian</c> → <see cref="SharesRowsWithPeer"/>:
///     which nodes stand down in favour of a peer). Lives in Runtime because Host references Runtime and not
///     the reverse.
/// </summary>
/// <remarks>
///     <para>
///         <b>Why two predicates (review R1 m3).</b> The two answers cost differently when wrong. Registering a
///         node that has no working sync path only costs capture triggers. Gating the drain of a node that
///         shares its queue with nobody parks its alarm events behind a Primary that does not hold them —
///         undelivered until the node is promoted, and drop-oldest at <c>Capacity</c> meanwhile. So
///         registration stays permissive and the gate is strict. Every node the gate treats as sharing also
///         captures, so the two never disagree in the harmful direction.
///     </para>
///     <para>
///         <b>Registration: <c>LocalDb:Replication:PeerAddress</c> OR <c>LocalDb:Replication:ApiKey</c>.</b>
///         The OR is required: only the dialling half of a pair sets <c>PeerAddress</c>, while the passive
///         half carries the shared <c>ApiKey</c> (plus a listen port) and nothing else. Keying on
///         <c>PeerAddress</c> alone would strip capture from every passive node.
///     </para>
///     <para>
///         <b>Drain gate: <c>ApiKey</c> AND (<c>PeerAddress</c> OR <c>LocalDb:SyncListenPort</c> &gt; 0).</b> A
///         peer holds this node's rows only over a working sync path: a dial (<c>PeerAddress</c>) or a
///         listener (<c>SyncListenPort</c>), each authenticated by the key. Both auth directions fail closed,
///         so a path without the key shares nothing, and a key with neither path (e.g. a fleet-wide
///         secret/env <c>ApiKey</c> on a node with no peer) shares nothing either. Every docker-dev shape
///         gives the same answer under both predicates: site-a-1 (PeerAddress + ApiKey + port) and site-a-2
///         (ApiKey + port) are gated; central and site-b (neither) are not.
///     </para>
/// </remarks>
public static class LocalDbReplicationPolicy
{
    /// <summary>Configuration section holding the LocalDb replication options.</summary>
    public const string ReplicationSectionPath = "LocalDb:Replication";

    /// <summary>The dedicated passive-sync listener port key (same key as <c>LocalDbRegistration.SyncListenPortKey</c>).</summary>
    public const string SyncListenPortKey = "LocalDb:SyncListenPort";

    /// <summary>Whether this node registers its replicated tables for change capture (table registration).</summary>
    /// <param name="configuration">The application configuration.</param>
    /// <returns><c>true</c> when <c>PeerAddress</c> or <c>ApiKey</c> is set under the replication section.</returns>
    public static bool CapturesChanges(IConfiguration configuration)
    {
        ArgumentNullException.ThrowIfNull(configuration);
        var section = configuration.GetSection(ReplicationSectionPath);
        return !string.IsNullOrWhiteSpace(section["PeerAddress"])
            || !string.IsNullOrWhiteSpace(section["ApiKey"]);
    }

    /// <summary>Whether a pair peer actually holds this node's replicated rows (the alarm drain gate).</summary>
    /// <param name="configuration">The application configuration.</param>
    /// <returns><c>true</c> when <c>ApiKey</c> is set AND either <c>PeerAddress</c> is set or
    ///     <c>LocalDb:SyncListenPort</c> is a positive port.</returns>
    public static bool SharesRowsWithPeer(IConfiguration configuration)
    {
        ArgumentNullException.ThrowIfNull(configuration);
        var section = configuration.GetSection(ReplicationSectionPath);
        if (string.IsNullOrWhiteSpace(section["ApiKey"])) return false;
        var dials = !string.IsNullOrWhiteSpace(section["PeerAddress"]);
        var listens = int.TryParse(configuration[SyncListenPortKey], out var port) && port > 0;
        return dials || listens;
    }
}