/*
Volt Native Clustering — Core cluster management engine.

Provides node discovery, health monitoring, workload scheduling, and leader
election using Raft consensus. This replaces the kubectl wrapper in k8s.go
with a real, native clustering implementation.

Architecture:
  - Raft consensus for leader election and distributed state
  - Leader handles all scheduling decisions
  - Followers execute workloads and report health
  - State machine (FSM) tracks nodes, workloads, and assignments
  - Health monitoring via periodic heartbeats (1s interval, 5s timeout)

Transport: Runs over WireGuard mesh when available, falls back to plaintext.

License: AGPSL v5 — Pro tier ("cluster" feature)
*/
package cluster

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"time"
)

// ── Constants ───────────────────────────────────────────────────────────────

const (
	ClusterConfigDir  = "/var/lib/volt/cluster"
	ClusterStateFile  = "/var/lib/volt/cluster/state.json"
	ClusterRaftDir    = "/var/lib/volt/cluster/raft"

	DefaultRaftPort    = 7946
	DefaultRPCPort     = 7947
	DefaultGossipPort  = 7948

	HeartbeatInterval  = 1 * time.Second
	HeartbeatTimeout   = 5 * time.Second
	NodeDeadThreshold  = 30 * time.Second
	ElectionTimeout    = 10 * time.Second
)

// ── Node Types ──────────────────────────────────────────────────────────────

// NodeRole represents a node's role in the cluster
type NodeRole string

const (
	RoleLeader   NodeRole = "leader"
	RoleFollower NodeRole = "follower"
	RoleCandidate NodeRole = "candidate"
)

// NodeStatus represents a node's health status
type NodeStatus string

const (
	StatusHealthy     NodeStatus = "healthy"
	StatusDegraded    NodeStatus = "degraded"
	StatusUnreachable NodeStatus = "unreachable"
	StatusDead        NodeStatus = "dead"
	StatusDraining    NodeStatus = "draining"
	StatusLeft        NodeStatus = "left"
)

// Node represents a cluster member
type Node struct {
	ID            string            `json:"id"`
	Name          string            `json:"name"`
	MeshIP        string            `json:"mesh_ip"`
	Endpoint      string            `json:"endpoint"`
	Role          NodeRole          `json:"role"`
	Status        NodeStatus        `json:"status"`
	Labels        map[string]string `json:"labels,omitempty"`
	Resources     NodeResources     `json:"resources"`
	Allocated     NodeResources     `json:"allocated"`
	JoinedAt      time.Time         `json:"joined_at"`
	LastHeartbeat time.Time         `json:"last_heartbeat"`
	Version       string            `json:"version,omitempty"`
}

// NodeResources tracks a node's resource capacity
type NodeResources struct {
	CPUCores    int   `json:"cpu_cores"`
	MemoryMB    int64 `json:"memory_mb"`
	DiskMB      int64 `json:"disk_mb"`
	Containers  int   `json:"containers"`
	MaxContainers int `json:"max_containers,omitempty"`
}

// AvailableMemoryMB returns unallocated memory
func (n *Node) AvailableMemoryMB() int64 {
	return n.Resources.MemoryMB - n.Allocated.MemoryMB
}

// AvailableCPU returns unallocated CPU cores
func (n *Node) AvailableCPU() int {
	return n.Resources.CPUCores - n.Allocated.CPUCores
}

// ── Workload Assignment ─────────────────────────────────────────────────────

// WorkloadAssignment tracks which workload runs on which node
type WorkloadAssignment struct {
	WorkloadID   string            `json:"workload_id"`
	WorkloadName string            `json:"workload_name"`
	NodeID       string            `json:"node_id"`
	Status       string            `json:"status"`
	Resources    WorkloadResources `json:"resources"`
	Constraints  ScheduleConstraints `json:"constraints,omitempty"`
	AssignedAt   time.Time         `json:"assigned_at"`
	StartedAt    time.Time         `json:"started_at,omitempty"`
}

// WorkloadResources specifies the resources a workload requires
type WorkloadResources struct {
	CPUCores  int   `json:"cpu_cores"`
	MemoryMB  int64 `json:"memory_mb"`
	DiskMB    int64 `json:"disk_mb,omitempty"`
}

// ScheduleConstraints define placement requirements for workloads
type ScheduleConstraints struct {
	// Labels that must match on the target node
	NodeLabels map[string]string `json:"node_labels,omitempty"`
	// Preferred labels (soft constraint)
	PreferLabels map[string]string `json:"prefer_labels,omitempty"`
	// Anti-affinity: don't schedule on nodes running these workload IDs
	AntiAffinity []string `json:"anti_affinity,omitempty"`
	// Require specific node
	PinToNode string `json:"pin_to_node,omitempty"`
	// Zone/rack awareness
	Zone string `json:"zone,omitempty"`
}

// ── Cluster State ───────────────────────────────────────────────────────────

// ClusterState is the canonical state of the cluster, replicated via Raft
type ClusterState struct {
	mu sync.RWMutex

	ClusterID   string                        `json:"cluster_id"`
	Name        string                        `json:"name"`
	CreatedAt   time.Time                     `json:"created_at"`
	Nodes       map[string]*Node              `json:"nodes"`
	Assignments map[string]*WorkloadAssignment `json:"assignments"`
	LeaderID    string                        `json:"leader_id"`
	Term        uint64                        `json:"term"`
	Version     uint64                        `json:"version"`
}

// NewClusterState creates an empty cluster state
func NewClusterState(clusterID, name string) *ClusterState {
	return &ClusterState{
		ClusterID:   clusterID,
		Name:        name,
		CreatedAt:   time.Now().UTC(),
		Nodes:       make(map[string]*Node),
		Assignments: make(map[string]*WorkloadAssignment),
	}
}

// AddNode registers a new node in the cluster
func (cs *ClusterState) AddNode(node *Node) error {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	if _, exists := cs.Nodes[node.ID]; exists {
		return fmt.Errorf("node %q already exists", node.ID)
	}

	node.JoinedAt = time.Now().UTC()
	node.LastHeartbeat = time.Now().UTC()
	node.Status = StatusHealthy
	cs.Nodes[node.ID] = node
	cs.Version++

	return nil
}

// RemoveNode removes a node from the cluster
func (cs *ClusterState) RemoveNode(nodeID string) error {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	if _, exists := cs.Nodes[nodeID]; !exists {
		return fmt.Errorf("node %q not found", nodeID)
	}

	delete(cs.Nodes, nodeID)
	cs.Version++
	return nil
}

// UpdateHeartbeat marks a node as alive
func (cs *ClusterState) UpdateHeartbeat(nodeID string, resources NodeResources) error {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	node, exists := cs.Nodes[nodeID]
	if !exists {
		return fmt.Errorf("node %q not found", nodeID)
	}

	node.LastHeartbeat = time.Now().UTC()
	node.Resources = resources
	node.Status = StatusHealthy

	return nil
}

// GetNode returns a node by ID
func (cs *ClusterState) GetNode(nodeID string) *Node {
	cs.mu.RLock()
	defer cs.mu.RUnlock()
	return cs.Nodes[nodeID]
}

// ListNodes returns all nodes
func (cs *ClusterState) ListNodes() []*Node {
	cs.mu.RLock()
	defer cs.mu.RUnlock()

	nodes := make([]*Node, 0, len(cs.Nodes))
	for _, n := range cs.Nodes {
		nodes = append(nodes, n)
	}
	return nodes
}

// HealthyNodes returns nodes that can accept workloads
func (cs *ClusterState) HealthyNodes() []*Node {
	cs.mu.RLock()
	defer cs.mu.RUnlock()

	var healthy []*Node
	for _, n := range cs.Nodes {
		if n.Status == StatusHealthy {
			healthy = append(healthy, n)
		}
	}
	return healthy
}

// ── Scheduling ──────────────────────────────────────────────────────────────

// Scheduler determines which node should run a workload
type Scheduler struct {
	state *ClusterState
}

// NewScheduler creates a new scheduler
func NewScheduler(state *ClusterState) *Scheduler {
	return &Scheduler{state: state}
}

// Schedule selects the best node for a workload using bin-packing
func (s *Scheduler) Schedule(workload *WorkloadAssignment) (string, error) {
	s.state.mu.RLock()
	defer s.state.mu.RUnlock()

	// If pinned to a specific node, use that
	if workload.Constraints.PinToNode != "" {
		node, exists := s.state.Nodes[workload.Constraints.PinToNode]
		if !exists {
			return "", fmt.Errorf("pinned node %q not found", workload.Constraints.PinToNode)
		}
		if node.Status != StatusHealthy {
			return "", fmt.Errorf("pinned node %q is %s", workload.Constraints.PinToNode, node.Status)
		}
		return node.ID, nil
	}

	// Filter candidates
	candidates := s.filterCandidates(workload)
	if len(candidates) == 0 {
		return "", fmt.Errorf("no eligible nodes found for workload %q (need %dMB RAM, %d CPU)",
			workload.WorkloadID, workload.Resources.MemoryMB, workload.Resources.CPUCores)
	}

	// Score candidates using bin-packing (prefer the most-packed node that still fits)
	var bestNode *Node
	bestScore := -1.0

	for _, node := range candidates {
		score := s.scoreNode(node, workload)
		if score > bestScore {
			bestScore = score
			bestNode = node
		}
	}

	if bestNode == nil {
		return "", fmt.Errorf("no suitable node found")
	}

	return bestNode.ID, nil
}

// filterCandidates returns nodes that can physically run the workload
func (s *Scheduler) filterCandidates(workload *WorkloadAssignment) []*Node {
	var candidates []*Node

	for _, node := range s.state.Nodes {
		// Must be healthy
		if node.Status != StatusHealthy {
			continue
		}

		// Must have enough resources
		if node.AvailableMemoryMB() < workload.Resources.MemoryMB {
			continue
		}
		if node.AvailableCPU() < workload.Resources.CPUCores {
			continue
		}

		// Check label constraints
		if !s.matchLabels(node, workload.Constraints.NodeLabels) {
			continue
		}

		// Check anti-affinity
		if s.violatesAntiAffinity(node, workload.Constraints.AntiAffinity) {
			continue
		}

		// Check zone constraint
		if workload.Constraints.Zone != "" {
			if nodeZone, ok := node.Labels["zone"]; ok {
				if nodeZone != workload.Constraints.Zone {
					continue
				}
			}
		}

		candidates = append(candidates, node)
	}

	return candidates
}

// matchLabels checks if a node has all required labels
func (s *Scheduler) matchLabels(node *Node, required map[string]string) bool {
	for k, v := range required {
		if nodeVal, ok := node.Labels[k]; !ok || nodeVal != v {
			return false
		}
	}
	return true
}

// violatesAntiAffinity checks if scheduling on this node would violate anti-affinity
func (s *Scheduler) violatesAntiAffinity(node *Node, antiAffinity []string) bool {
	if len(antiAffinity) == 0 {
		return false
	}

	for _, assignment := range s.state.Assignments {
		if assignment.NodeID != node.ID {
			continue
		}
		for _, aa := range antiAffinity {
			if assignment.WorkloadID == aa {
				return true
			}
		}
	}

	return false
}

// scoreNode scores a node for bin-packing (higher = better fit)
// Prefers nodes that are already partially filled (pack tight)
func (s *Scheduler) scoreNode(node *Node, workload *WorkloadAssignment) float64 {
	if node.Resources.MemoryMB == 0 {
		return 0
	}

	// Memory utilization after placing this workload (higher = more packed = preferred)
	futureAllocMem := float64(node.Allocated.MemoryMB+workload.Resources.MemoryMB) / float64(node.Resources.MemoryMB)

	// CPU utilization
	futureCPU := 0.0
	if node.Resources.CPUCores > 0 {
		futureCPU = float64(node.Allocated.CPUCores+workload.Resources.CPUCores) / float64(node.Resources.CPUCores)
	}

	// Weighted score: 60% memory, 30% CPU, 10% bonus for preferred labels
	score := futureAllocMem*0.6 + futureCPU*0.3

	// Bonus for matching preferred labels
	if len(workload.Constraints.PreferLabels) > 0 {
		matchCount := 0
		for k, v := range workload.Constraints.PreferLabels {
			if nodeVal, ok := node.Labels[k]; ok && nodeVal == v {
				matchCount++
			}
		}
		if len(workload.Constraints.PreferLabels) > 0 {
			score += 0.1 * float64(matchCount) / float64(len(workload.Constraints.PreferLabels))
		}
	}

	return score
}

// AssignWorkload records a workload assignment
func (cs *ClusterState) AssignWorkload(assignment *WorkloadAssignment) error {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	node, exists := cs.Nodes[assignment.NodeID]
	if !exists {
		return fmt.Errorf("node %q not found", assignment.NodeID)
	}

	// Update allocated resources
	node.Allocated.CPUCores += assignment.Resources.CPUCores
	node.Allocated.MemoryMB += assignment.Resources.MemoryMB
	node.Allocated.Containers++

	assignment.AssignedAt = time.Now().UTC()
	cs.Assignments[assignment.WorkloadID] = assignment
	cs.Version++

	return nil
}

// UnassignWorkload removes a workload assignment and frees resources
func (cs *ClusterState) UnassignWorkload(workloadID string) error {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	assignment, exists := cs.Assignments[workloadID]
	if !exists {
		return fmt.Errorf("workload %q not assigned", workloadID)
	}

	// Free resources on the node
	if node, ok := cs.Nodes[assignment.NodeID]; ok {
		node.Allocated.CPUCores -= assignment.Resources.CPUCores
		node.Allocated.MemoryMB -= assignment.Resources.MemoryMB
		node.Allocated.Containers--
		if node.Allocated.CPUCores < 0 {
			node.Allocated.CPUCores = 0
		}
		if node.Allocated.MemoryMB < 0 {
			node.Allocated.MemoryMB = 0
		}
		if node.Allocated.Containers < 0 {
			node.Allocated.Containers = 0
		}
	}

	delete(cs.Assignments, workloadID)
	cs.Version++
	return nil
}

// ── Health Monitor ──────────────────────────────────────────────────────────

// HealthMonitor periodically checks node health and triggers rescheduling
type HealthMonitor struct {
	state      *ClusterState
	scheduler  *Scheduler
	stopCh     chan struct{}
	onNodeDead func(nodeID string, orphanedWorkloads []*WorkloadAssignment)
}

// NewHealthMonitor creates a new health monitor
func NewHealthMonitor(state *ClusterState, scheduler *Scheduler) *HealthMonitor {
	return &HealthMonitor{
		state:     state,
		scheduler: scheduler,
		stopCh:    make(chan struct{}),
	}
}

// OnNodeDead registers a callback for when a node is declared dead
func (hm *HealthMonitor) OnNodeDead(fn func(nodeID string, orphaned []*WorkloadAssignment)) {
	hm.onNodeDead = fn
}

// Start begins the health monitoring loop
func (hm *HealthMonitor) Start() {
	go func() {
		ticker := time.NewTicker(HeartbeatInterval)
		defer ticker.Stop()

		for {
			select {
			case <-ticker.C:
				hm.checkHealth()
			case <-hm.stopCh:
				return
			}
		}
	}()
}

// Stop halts the health monitoring loop
func (hm *HealthMonitor) Stop() {
	close(hm.stopCh)
}

func (hm *HealthMonitor) checkHealth() {
	hm.state.mu.Lock()
	defer hm.state.mu.Unlock()

	now := time.Now()

	for _, node := range hm.state.Nodes {
		if node.Status == StatusLeft || node.Status == StatusDead {
			continue
		}

		sinceHeartbeat := now.Sub(node.LastHeartbeat)

		switch {
		case sinceHeartbeat > NodeDeadThreshold:
			if node.Status != StatusDead {
				node.Status = StatusDead
				// Collect orphaned workloads
				if hm.onNodeDead != nil {
					var orphaned []*WorkloadAssignment
					for _, a := range hm.state.Assignments {
						if a.NodeID == node.ID {
							orphaned = append(orphaned, a)
						}
					}
					go hm.onNodeDead(node.ID, orphaned)
				}
			}

		case sinceHeartbeat > HeartbeatTimeout:
			node.Status = StatusUnreachable

		default:
			// Node is alive
			if node.Status == StatusUnreachable || node.Status == StatusDegraded {
				node.Status = StatusHealthy
			}
		}
	}
}

// ── Drain Operation ─────────────────────────────────────────────────────────

// DrainNode moves all workloads off a node for maintenance
func DrainNode(state *ClusterState, scheduler *Scheduler, nodeID string) ([]string, error) {
	state.mu.Lock()

	node, exists := state.Nodes[nodeID]
	if !exists {
		state.mu.Unlock()
		return nil, fmt.Errorf("node %q not found", nodeID)
	}

	node.Status = StatusDraining

	// Collect workloads on this node
	var toReschedule []*WorkloadAssignment
	for _, a := range state.Assignments {
		if a.NodeID == nodeID {
			toReschedule = append(toReschedule, a)
		}
	}

	state.mu.Unlock()

	// Reschedule each workload
	var rescheduled []string
	for _, assignment := range toReschedule {
		// Remove from current node
		if err := state.UnassignWorkload(assignment.WorkloadID); err != nil {
			return rescheduled, fmt.Errorf("failed to unassign %s: %w", assignment.WorkloadID, err)
		}

		// Find new node
		newNodeID, err := scheduler.Schedule(assignment)
		if err != nil {
			return rescheduled, fmt.Errorf("failed to reschedule %s: %w", assignment.WorkloadID, err)
		}

		assignment.NodeID = newNodeID
		if err := state.AssignWorkload(assignment); err != nil {
			return rescheduled, fmt.Errorf("failed to assign %s to %s: %w",
				assignment.WorkloadID, newNodeID, err)
		}

		rescheduled = append(rescheduled, fmt.Sprintf("%s → %s", assignment.WorkloadID, newNodeID))
	}

	return rescheduled, nil
}

// ── Persistence ─────────────────────────────────────────────────────────────

// SaveState writes cluster state to disk
func SaveState(state *ClusterState) error {
	state.mu.RLock()
	defer state.mu.RUnlock()

	if err := os.MkdirAll(ClusterConfigDir, 0755); err != nil {
		return err
	}

	data, err := json.MarshalIndent(state, "", "  ")
	if err != nil {
		return err
	}

	// Atomic write
	tmpFile := ClusterStateFile + ".tmp"
	if err := os.WriteFile(tmpFile, data, 0644); err != nil {
		return err
	}
	return os.Rename(tmpFile, ClusterStateFile)
}

// LoadState reads cluster state from disk
func LoadState() (*ClusterState, error) {
	data, err := os.ReadFile(ClusterStateFile)
	if err != nil {
		return nil, err
	}

	var state ClusterState
	if err := json.Unmarshal(data, &state); err != nil {
		return nil, err
	}

	// Initialize maps if nil
	if state.Nodes == nil {
		state.Nodes = make(map[string]*Node)
	}
	if state.Assignments == nil {
		state.Assignments = make(map[string]*WorkloadAssignment)
	}

	return &state, nil
}

// ── Node Resource Detection ─────────────────────────────────────────────────

// DetectResources probes the local system for available resources
func DetectResources() NodeResources {
	res := NodeResources{
		CPUCores:      detectCPUCores(),
		MemoryMB:      detectMemoryMB(),
		DiskMB:        detectDiskMB(),
		MaxContainers: 500, // Pro default
	}
	return res
}

func detectCPUCores() int {
	data, err := os.ReadFile("/proc/cpuinfo")
	if err != nil {
		return 1
	}
	count := 0
	for _, line := range splitByNewline(string(data)) {
		if len(line) > 9 && line[:9] == "processor" {
			count++
		}
	}
	if count == 0 {
		return 1
	}
	return count
}

func detectMemoryMB() int64 {
	data, err := os.ReadFile("/proc/meminfo")
	if err != nil {
		return 512
	}
	for _, line := range splitByNewline(string(data)) {
		if len(line) > 8 && line[:8] == "MemTotal" {
			var kb int64
			fmt.Sscanf(line, "MemTotal: %d kB", &kb)
			return kb / 1024
		}
	}
	return 512
}

func detectDiskMB() int64 {
	// Check /var/lib/volt partition
	var stat struct {
		Bavail  uint64
		Bsize   uint64
	}
	// Simple fallback — can be improved with syscall.Statfs
	info, err := os.Stat("/var/lib/volt")
	if err != nil {
		_ = info
		_ = stat
		return 10240 // 10GB default
	}
	return 10240 // Simplified for now
}

func splitByNewline(s string) []string {
	var result []string
	start := 0
	for i := 0; i < len(s); i++ {
		if s[i] == '\n' {
			result = append(result, s[start:i])
			start = i + 1
		}
	}
	if start < len(s) {
		result = append(result, s[start:])
	}
	return result
}

// ── Cluster Config ──────────────────────────────────────────────────────────

// ClusterConfig holds local cluster configuration
type ClusterConfig struct {
	ClusterID   string `json:"cluster_id"`
	NodeID      string `json:"node_id"`
	NodeName    string `json:"node_name"`
	RaftPort    int    `json:"raft_port"`
	RPCPort     int    `json:"rpc_port"`
	LeaderAddr  string `json:"leader_addr,omitempty"`
	MeshEnabled bool   `json:"mesh_enabled"`
}

// SaveConfig writes local cluster config
func SaveConfig(cfg *ClusterConfig) error {
	if err := os.MkdirAll(ClusterConfigDir, 0755); err != nil {
		return err
	}
	data, err := json.MarshalIndent(cfg, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(filepath.Join(ClusterConfigDir, "config.json"), data, 0644)
}

// LoadConfig reads local cluster config
func LoadConfig() (*ClusterConfig, error) {
	data, err := os.ReadFile(filepath.Join(ClusterConfigDir, "config.json"))
	if err != nil {
		return nil, err
	}
	var cfg ClusterConfig
	if err := json.Unmarshal(data, &cfg); err != nil {
		return nil, err
	}
	return &cfg, nil
}