// Copyright 2026 The NATS Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// This file contains mock raft transport implementations and helper functions
// for testing. The `raftTransportHub` manages multiple `mockTransport` instances,
// allowing for the simulation of network partitions (`partition`, `heal`)
// and message interception (`setAfterMsgHook`). This setup is used to test
// raft interactions without actual network communication, providing control
// over message delivery and network conditions.

package server

import (
	"sync"
)

type msgHook func(subject, reply string, msg []byte)

type raftTransportHub struct {
	mu         sync.Mutex
	transports map[string]*mockTransport
	partitions map[string]int
	afterMsg   msgHook
}

func newRaftTransportHub() *raftTransportHub {
	return &raftTransportHub{
		partitions: make(map[string]int),
		transports: make(map[string]*mockTransport),
	}
}

func (h *raftTransportHub) newTransport(s *Server, n RaftNode) raftTransport {
	return &mockTransport{
		server: s,
		node:   n,
		hub:    h,
	}
}

func (h *raftTransportHub) register(t *mockTransport) {
	h.mu.Lock()
	defer h.mu.Unlock()
	h.transports[t.Node().ID()] = t
}

func (h *raftTransportHub) unregister(t *mockTransport) {
	h.mu.Lock()
	defer h.mu.Unlock()
	delete(h.transports, t.Node().ID())
}

// Simulate a network partition. Nodes assigned to different `partitionID`s
// will not be able to exchange messages, effectively isolating them.
// By default, all nodes are considered to be in partition ID 0.
// See `heal(nodeID)` and `healPartitions()` to heal a partition.
func (h *raftTransportHub) partition(nodeID string, partitionID int) {
	h.mu.Lock()
	defer h.mu.Unlock()
	h.partitions[nodeID] = partitionID
}

// Reassign the node with the given ID to the default partition 0.
func (h *raftTransportHub) heal(nodeID string) {
	h.mu.Lock()
	defer h.mu.Unlock()
	delete(h.partitions, nodeID)
}

// Reassign all nodes to the default partition 0.
func (h *raftTransportHub) healPartitions() {
	h.mu.Lock()
	defer h.mu.Unlock()
	clear(h.partitions)
}

// Set a hook that is called back after a message is published. The after
// message hook can expect the raftTransportHub to be unlocked. It is OK
// to interact with the raftTransportHub inside the message hook. On the
// other hand, the underlying RaftNode that has sent the message remains
// locked throughout the execution of the hook. The hook can only call
// methods that do not lock the RaftNode.
func (h *raftTransportHub) setAfterMsgHook(hook msgHook) {
	h.mu.Lock()
	defer h.mu.Unlock()
	h.afterMsg = hook
}

func (h *raftTransportHub) publish(t *mockTransport, subject, reply string, msg []byte) {
	h.mu.Lock()
	afterMsgHook := h.afterMsg
	sender := t.Node().ID()
	partition := h.partitions[sender]
	var subscriptions []*subscription

	for id, transport := range h.transports {
		if sender == id {
			continue
		}

		if partition != h.partitions[id] {
			continue
		}

		result := transport.sub.Match(subject)
		subscriptions = append(subscriptions, result.psubs...)
	}
	h.mu.Unlock()

	// Deliver without holding the hub lock, to avoid a lock inversion between
	// Raft and hub locks.
	for _, sub := range subscriptions {
		sub.icb(sub, nil, nil, subject, reply, msg)
	}

	if afterMsgHook != nil {
		afterMsgHook(subject, reply, msg)
	}
}

type mockTransport struct {
	server *Server
	node   RaftNode
	hub    *raftTransportHub
	sub    *Sublist
	acc    *Account
}

func (t *mockTransport) Reset(acc *Account) {
	t.Close()
	t.acc = acc
	t.sub = NewSublist(false)
	t.hub.register(t)
}

func (t *mockTransport) Close() {
	t.hub.unregister(t)
	t.sub = nil
}

func (t *mockTransport) Node() RaftNode {
	return t.node
}

func (t *mockTransport) Account() *Account {
	return t.acc
}

func (t *mockTransport) Publish(subject string, reply string, msg []byte) {
	t.hub.publish(t, subject, reply, msg)
}

func (t *mockTransport) Subscribe(subject string, cb msgHandler) (*subscription, error) {
	if t.sub == nil {
		return nil, errNoInternalClient
	}
	sub := &subscription{subject: []byte(subject), sid: nil, icb: cb}
	if err := t.sub.Insert(sub); err != nil {
		return nil, err
	}
	return sub, nil
}

func (t *mockTransport) Unsubscribe(sub *subscription) {
	if t.sub != nil {
		t.sub.Remove(sub)
	}
}