// Go parity: golang/nats-server/server/jetstream_cluster.go:2290-2439
// Covers: ProcessAddPeer, ProcessRemovePeer, RemovePeerFromStream, RemapStreamAssignment —
// peer-driven stream reassignment in the JetStreamMetaGroup.
using NATS.Server.JetStream.Cluster;
using NATS.Server.JetStream.Models;

namespace NATS.Server.JetStream.Tests.JetStream.Cluster;

/// <summary>
/// Tests for JetStreamMetaGroup peer management and stream reassignment.
/// Go reference: jetstream_cluster.go:2290-2439 (processAddPeer, processRemovePeer,
/// removePeerFromStreamLocked, remapStreamAssignment).
/// </summary>
public class PeerManagementTests
{
    // ---------------------------------------------------------------
    // ProcessAddPeer — peer registration
    // Go reference: jetstream_cluster.go:2290 processAddPeer
    // ---------------------------------------------------------------

    [Fact]
    public void ProcessAddPeer_registers_new_peer()
    {
        // Go reference: jetstream_cluster.go:2290 processAddPeer — peer is tracked
        var meta = new JetStreamMetaGroup(3);

        meta.ProcessAddPeer("peer-1");

        meta.GetKnownPeers().ShouldContain("peer-1");
    }

    [Fact]
    public void ProcessAddPeer_registers_multiple_peers_independently()
    {
        // Go reference: jetstream_cluster.go:2290 — each peer is independently tracked
        var meta = new JetStreamMetaGroup(3);

        meta.ProcessAddPeer("peer-1");
        meta.ProcessAddPeer("peer-2");
        meta.ProcessAddPeer("peer-3");

        var known = meta.GetKnownPeers();
        known.Count.ShouldBe(3);
        known.ShouldContain("peer-1");
        known.ShouldContain("peer-2");
        known.ShouldContain("peer-3");
    }

    [Fact]
    public void ProcessAddPeer_duplicate_add_is_idempotent()
    {
        // AddKnownPeer uses a HashSet so duplicates do not inflate the count.
        var meta = new JetStreamMetaGroup(3);

        meta.ProcessAddPeer("peer-1");
        meta.ProcessAddPeer("peer-1");

        meta.GetKnownPeers().Count.ShouldBe(1);
    }

    // ---------------------------------------------------------------
    // ProcessAddPeer — under-replication detection
    // Go reference: jetstream_cluster.go:2311-2339 missingPeers + peer append
    // ---------------------------------------------------------------

    [Fact]
    public void ProcessAddPeer_triggers_rereplication_of_underreplicated_stream()
    {
        // Go reference: jetstream_cluster.go:2315 sa.missingPeers() — adds new peer to group
        var meta = new JetStreamMetaGroup(3); // leader by default (selfIndex == leaderIndex == 1)

        // Stream assigned with 2 peers but DesiredReplicas == 3 → under-replicated
        var group = new RaftGroup
        {
            Name = "orders-rg",
            Peers = ["peer-1", "peer-2"],
            DesiredReplicas = 3,
        };
        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessAddPeer("peer-3");

        var updated = meta.GetStreamAssignment("ORDERS")!;
        updated.Group.Peers.ShouldContain("peer-3");
        updated.Group.Peers.Count.ShouldBe(3);
    }

    [Fact]
    public void ProcessAddPeer_does_not_add_peer_to_fully_replicated_stream()
    {
        // Go reference: jetstream_cluster.go:2315 missingPeers() returns false when at desired count
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup
        {
            Name = "events-rg",
            Peers = ["peer-1", "peer-2", "peer-3"],
            DesiredReplicas = 3,
        };
        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessAddPeer("peer-4");

        var updated = meta.GetStreamAssignment("EVENTS")!;
        updated.Group.Peers.Count.ShouldBe(3);
        updated.Group.Peers.ShouldNotContain("peer-4");
    }

    [Fact]
    public void ProcessAddPeer_does_not_add_peer_already_in_group()
    {
        // Peer already a member — should not be added twice.
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup
        {
            Name = "logs-rg",
            Peers = ["peer-1"],
            DesiredReplicas = 2,
        };
        var sa = new StreamAssignment { StreamName = "LOGS", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessAddPeer("peer-1");

        var updated = meta.GetStreamAssignment("LOGS")!;
        updated.Group.Peers.Count.ShouldBe(1);
    }

    [Fact]
    public void ProcessAddPeer_non_leader_does_not_modify_assignments()
    {
        // Go reference: jetstream_cluster.go:2301 — only leader triggers re-assignment
        var meta = new JetStreamMetaGroup(3, selfIndex: 2); // not leader

        var group = new RaftGroup
        {
            Name = "rg",
            Peers = ["peer-1"],
            DesiredReplicas = 3,
        };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessAddPeer("peer-2");

        // Peer is registered but stream is not modified since not leader.
        meta.GetKnownPeers().ShouldContain("peer-2");
        meta.GetStreamAssignment("S")!.Group.Peers.Count.ShouldBe(1);
    }

    // ---------------------------------------------------------------
    // ProcessRemovePeer — stream reassignment
    // Go reference: jetstream_cluster.go:2342 processRemovePeer
    // ---------------------------------------------------------------

    [Fact]
    public void ProcessRemovePeer_reassigns_streams_away_from_peer()
    {
        // Go reference: jetstream_cluster.go:2385-2392 — streams with removed peer get remapped
        var meta = new JetStreamMetaGroup(3);

        // Register three peers
        meta.AddKnownPeer("peer-1");
        meta.AddKnownPeer("peer-2");
        meta.AddKnownPeer("peer-3");

        var group = new RaftGroup
        {
            Name = "rg",
            Peers = ["peer-1", "peer-2"],
        };
        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessRemovePeer("peer-1");

        var updated = meta.GetStreamAssignment("ORDERS")!;
        updated.Group.Peers.ShouldNotContain("peer-1");
    }

    [Fact]
    public void ProcessRemovePeer_removes_peer_from_known_peers()
    {
        // Go reference: jetstream_cluster.go:2342 — peer is de-registered
        var meta = new JetStreamMetaGroup(3);
        meta.AddKnownPeer("peer-1");

        meta.ProcessRemovePeer("peer-1");

        meta.GetKnownPeers().ShouldNotContain("peer-1");
    }

    [Fact]
    public void ProcessRemovePeer_unknown_peer_is_noop()
    {
        // Go reference: jetstream_cluster.go:2342 — no crash when peer not known
        var meta = new JetStreamMetaGroup(3);
        var group = new RaftGroup { Name = "rg", Peers = ["peer-2", "peer-3"] };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        // Should not throw
        meta.ProcessRemovePeer("peer-99");

        // Stream unaffected
        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-2");
        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-3");
    }

    [Fact]
    public void ProcessRemovePeer_non_leader_only_deregisters_peer()
    {
        // Go reference: jetstream_cluster.go:2378 — non-leader skips re-assignment
        var meta = new JetStreamMetaGroup(3, selfIndex: 2);
        meta.AddKnownPeer("peer-1");
        meta.AddKnownPeer("peer-2");

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        meta.ProcessRemovePeer("peer-1");

        // Peer removed from known set
        meta.GetKnownPeers().ShouldNotContain("peer-1");

        // Stream assignments are NOT modified by a non-leader
        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-1");
    }

    // ---------------------------------------------------------------
    // RemovePeerFromStream
    // Go reference: jetstream_cluster.go:2403 removePeerFromStreamLocked
    // ---------------------------------------------------------------

    [Fact]
    public void RemovePeerFromStream_removes_peer_from_group()
    {
        // Go reference: jetstream_cluster.go:2404 — peer is removed from stream group
        var meta = new JetStreamMetaGroup(3);
        meta.AddKnownPeer("peer-1");
        meta.AddKnownPeer("peer-2");
        meta.AddKnownPeer("peer-3");

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
        meta.AddStreamAssignment(sa);

        meta.RemovePeerFromStream("EVENTS", "peer-2");

        var updated = meta.GetStreamAssignment("EVENTS")!;
        updated.Group.Peers.ShouldNotContain("peer-2");
    }

    [Fact]
    public void RemovePeerFromStream_returns_false_for_nonexistent_stream()
    {
        // RemovePeerFromStream silently returns false when stream not found.
        var meta = new JetStreamMetaGroup(3);

        var result = meta.RemovePeerFromStream("GHOST", "peer-1");

        result.ShouldBeFalse();
    }

    [Fact]
    public void RemovePeerFromStream_returns_false_when_peer_not_in_group()
    {
        // Peer not a member of the stream's group.
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        var result = meta.RemovePeerFromStream("S", "peer-99");

        result.ShouldBeFalse();
    }

    [Fact]
    public void RemovePeerFromStream_replaces_peer_when_replacement_available()
    {
        // Go reference: jetstream_cluster.go:7088-7094 — replacement peer picked from available pool
        var meta = new JetStreamMetaGroup(3);
        meta.AddKnownPeer("peer-1");
        meta.AddKnownPeer("peer-2");
        meta.AddKnownPeer("peer-3"); // replacement candidate

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
        meta.AddStreamAssignment(sa);

        var result = meta.RemovePeerFromStream("ORDERS", "peer-1");

        result.ShouldBeTrue();
        var updated = meta.GetStreamAssignment("ORDERS")!;
        updated.Group.Peers.ShouldNotContain("peer-1");
        updated.Group.Peers.Count.ShouldBe(2);
        updated.Group.Peers.ShouldContain("peer-3");
    }

    [Fact]
    public void RemovePeerFromStream_shrinks_group_when_no_replacement_available()
    {
        // Go reference: jetstream_cluster.go:7102-7110 — R>1 bare removal fallback
        var meta = new JetStreamMetaGroup(3);
        // Only peer-1 and peer-2 are known; peer-1 is in the group; no replacement
        meta.AddKnownPeer("peer-1");
        meta.AddKnownPeer("peer-2");

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
        var sa = new StreamAssignment { StreamName = "LOGS", Group = group };
        meta.AddStreamAssignment(sa);

        var result = meta.RemovePeerFromStream("LOGS", "peer-1");

        // No replacement found → group shrinks
        result.ShouldBeFalse();
        var updated = meta.GetStreamAssignment("LOGS")!;
        updated.Group.Peers.ShouldNotContain("peer-1");
        updated.Group.Peers.Count.ShouldBe(1);
    }

    // ---------------------------------------------------------------
    // RemapStreamAssignment
    // Go reference: jetstream_cluster.go:7077 remapStreamAssignment
    // ---------------------------------------------------------------

    [Fact]
    public void RemapStreamAssignment_selects_new_peers()
    {
        // Go reference: jetstream_cluster.go:7077 — retain existing minus removed, add candidate
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
        meta.AddStreamAssignment(sa);

        var available = new List<string> { "peer-1", "peer-2", "peer-3", "peer-4" };
        var result = meta.RemapStreamAssignment(sa, available, removePeer: "peer-3");

        result.ShouldBeTrue();
        var updated = meta.GetStreamAssignment("EVENTS")!;
        updated.Group.Peers.ShouldNotContain("peer-3");
        updated.Group.Peers.Count.ShouldBe(3);
        updated.Group.Peers.ShouldContain("peer-4");
    }

    [Fact]
    public void RemapStreamAssignment_retains_existing_peers()
    {
        // Retained peers (not removed) remain in the new assignment.
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        var available = new List<string> { "peer-1", "peer-2", "peer-3", "peer-4" };
        meta.RemapStreamAssignment(sa, available, removePeer: "peer-1");

        var updated = meta.GetStreamAssignment("S")!;
        updated.Group.Peers.ShouldContain("peer-2");
        updated.Group.Peers.ShouldContain("peer-3");
    }

    [Fact]
    public void RemapStreamAssignment_returns_false_when_no_replacement()
    {
        // Go reference: jetstream_cluster.go:7098-7110 — no placement, R1 returns false
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1"] };
        var sa = new StreamAssignment { StreamName = "R1", Group = group };
        meta.AddStreamAssignment(sa);

        var available = new List<string> { "peer-1" };
        var result = meta.RemapStreamAssignment(sa, available, removePeer: "peer-1");

        // Only peer-1 available and it is the removed one → nothing to add
        result.ShouldBeFalse();
    }

    [Fact]
    public void RemapStreamAssignment_empty_available_shrinks_group()
    {
        // When the available-peer list is empty, the group simply loses the removed peer.
        var meta = new JetStreamMetaGroup(3);

        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
        var sa = new StreamAssignment { StreamName = "S", Group = group };
        meta.AddStreamAssignment(sa);

        var result = meta.RemapStreamAssignment(sa, [], removePeer: "peer-1");

        result.ShouldBeFalse();
        meta.GetStreamAssignment("S")!.Group.Peers.ShouldNotContain("peer-1");
    }

    // ---------------------------------------------------------------
    // AddKnownPeer / RemoveKnownPeer
    // ---------------------------------------------------------------

    [Fact]
    public void AddKnownPeer_and_RemoveKnownPeer_are_consistent()
    {
        var meta = new JetStreamMetaGroup(3);

        meta.AddKnownPeer("p1");
        meta.AddKnownPeer("p2");
        meta.RemoveKnownPeer("p1");

        var known = meta.GetKnownPeers();
        known.ShouldNotContain("p1");
        known.ShouldContain("p2");
    }

    [Fact]
    public void RemoveKnownPeer_unknown_peer_is_noop()
    {
        var meta = new JetStreamMetaGroup(3);
        meta.AddKnownPeer("p1");

        // Should not throw
        meta.RemoveKnownPeer("p99");

        meta.GetKnownPeers().Count.ShouldBe(1);
    }
}