// Package instance provides utilities for managing multi-agent instances.
package instance

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"os/exec"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/google/uuid"
	"github.com/kandev/kandev/internal/agentctl/server/config"
	"github.com/kandev/kandev/internal/agentctl/server/process"
	"github.com/kandev/kandev/internal/common/logger"
	"github.com/kandev/kandev/pkg/agent"
	"go.uber.org/zap"
)

// ServerFactory creates an HTTP handler for an instance given its config and process manager.
type ServerFactory func(cfg *config.InstanceConfig, procMgr *process.Manager, log *logger.Logger) http.Handler

const instanceHTTPShutdownGrace = 250 * time.Millisecond

// Manager manages multiple agent instances.
// It handles creation, tracking, and removal of agent instances,
// each with their own HTTP server on a dedicated port.
type Manager struct {
	config        *config.Config
	logger        *logger.Logger
	instances     map[string]*Instance
	portAlloc     *PortAllocator
	serverFactory ServerFactory
	mu            sync.RWMutex

	// reaperStop is closed by Shutdown to signal the idle reaper to exit.
	// reaperStopOnce serializes the close so concurrent Shutdown calls can't
	// double-close; sync.Once is used instead of m.mu because m.mu guards
	// the instances map and shouldn't gate a one-shot lifecycle signal.
	// reaperWG waits for the reaper goroutine to finish before Shutdown returns.
	reaperStop     chan struct{}
	reaperStopOnce sync.Once
	reaperWG       sync.WaitGroup
}

// NewManager creates a new instance manager.
// If cfg.IdleTimeout > 0, a background goroutine periodically reaps
// instances that have been idle (no in-flight HTTP requests and no
// activity) for the configured duration.
func NewManager(cfg *config.Config, log *logger.Logger) *Manager {
	// Clean up code-server processes orphaned by a previous session (safety net).
	process.CleanupOrphanedCodeServers(log)

	m := &Manager{
		config:     cfg,
		logger:     log.WithFields(zap.String("component", "instance-manager")),
		instances:  make(map[string]*Instance),
		portAlloc:  NewPortAllocator(cfg.Ports.Base, cfg.Ports.Max),
		reaperStop: make(chan struct{}),
	}

	if cfg.IdleTimeout > 0 {
		m.reaperWG.Add(1)
		go m.runIdleReaper(cfg.IdleTimeout, cfg.IdleReaperInterval)
	}

	return m
}

// SetServerFactory sets the factory function for creating HTTP handlers for instances.
// This must be called before creating any instances.
func (m *Manager) SetServerFactory(factory ServerFactory) {
	m.serverFactory = factory
}

// CreateInstance creates a new agent instance.
func (m *Manager) CreateInstance(ctx context.Context, req *CreateRequest) (*CreateResponse, error) {
	m.mu.Lock()
	defer m.mu.Unlock()

	id := req.ID
	if id == "" {
		id = uuid.New().String()
	}
	if _, exists := m.instances[id]; exists {
		return nil, fmt.Errorf("instance with ID %s already exists", id)
	}

	port, listener, err := m.allocatePortAndListener(id)
	if err != nil {
		return nil, err
	}

	agentCmd := m.resolveAgentCommand(req)
	autoStart := req.AutoStart
	mcpServers := m.buildMcpServerConfigs(req.McpServers)

	m.logger.Info("CreateInstance: received request",
		zap.String("req_protocol", req.Protocol),
		zap.String("workspace_path", req.WorkspacePath))

	overrides := &config.InstanceOverrides{
		InstanceID:             id,
		Protocol:               agent.Protocol(req.Protocol),
		AgentCommand:           agentCmd,
		WorkDir:                req.WorkspacePath,
		AutoStart:              &autoStart,
		Env:                    config.CollectAgentEnv(req.Env),
		AutoApprovePermissions: req.AutoApprovePermissions,
		AgentType:              req.AgentType,
		McpServers:             mcpServers,
		SessionID:              req.SessionID,
		TaskID:                 req.TaskID,
		DisableAskQuestion:     req.DisableAskQuestion,
		AssumeMcpSse:           req.AssumeMcpSse,
		AssumeMcpHttp:          req.AssumeMcpHttp,
		McpMode:                req.McpMode,
		RequiresProcessKill:    req.RequiresProcessKill,
		StripEnv:               req.StripEnv,
		BaseBranches:           req.BaseBranches,
	}

	m.logger.Info("CreateInstance: applying overrides",
		zap.String("override_protocol", string(overrides.Protocol)))

	// Create instance config using the unified method
	instanceCfg := m.config.NewInstanceConfig(port, overrides)

	m.logger.Info("CreateInstance: instance config created",
		zap.String("config_protocol", string(instanceCfg.Protocol)))

	// Create process manager
	procMgr := process.NewManager(instanceCfg, m.logger)

	// Start root + per-repo trackers so file-change events fire even in passthrough mode.
	procMgr.StartAllWorkspaceTrackers(context.Background())

	// Create instance up-front so the activity middleware can reference it.
	inst := &Instance{
		ID:            id,
		Port:          port,
		Status:        "running",
		WorkspacePath: req.WorkspacePath,
		AgentCommand:  agentCmd,
		Env:           req.Env,
		CreatedAt:     time.Now(),
		manager:       procMgr,
	}
	inst.MarkActivity()

	handler := activityMiddleware(inst)(m.buildHTTPHandler(instanceCfg, procMgr))
	httpServer := m.startHTTPServer(port, listener, handler, id)
	inst.server = httpServer
	m.instances[id] = inst

	m.logger.Info("created instance",
		zap.String("instance_id", id),
		zap.Int("port", port),
		zap.String("workspace", req.WorkspacePath))

	return &CreateResponse{
		ID:   id,
		Port: port,
	}, nil
}

// allocatePortAndListener allocates a free port and binds a TCP listener to it.
func (m *Manager) allocatePortAndListener(id string) (int, net.Listener, error) {
	maxAttempts := m.config.Ports.Max - m.config.Ports.Base + 1
	for attempt := 0; attempt < maxAttempts; attempt++ {
		allocated, err := m.portAlloc.Allocate(id)
		if err != nil {
			return 0, nil, fmt.Errorf("failed to allocate port: %w", err)
		}
		ln, err := net.Listen("tcp", fmt.Sprintf(":%d", allocated))
		if err != nil {
			if errors.Is(err, syscall.EADDRINUSE) || strings.Contains(err.Error(), "address already in use") {
				m.portAlloc.MarkUnavailable(allocated)
				m.logger.Warn("port already in use; retrying",
					zap.String("instance_id", id),
					zap.Int("port", allocated))
				continue
			}
			m.portAlloc.Release(allocated)
			return 0, nil, fmt.Errorf("failed to bind instance port %d: %w", allocated, err)
		}
		return allocated, ln, nil
	}
	return 0, nil, fmt.Errorf("failed to allocate an available port for instance %s", id)
}

// resolveAgentCommand returns the effective agent command for a create request.
func (m *Manager) resolveAgentCommand(req *CreateRequest) string {
	agentCmd := req.AgentCommand
	if agentCmd == "" {
		agentCmd = m.config.Defaults.AgentCommand
	}
	if req.WorkspacePath != "" && req.WorkspaceFlag != "" && !strings.Contains(agentCmd, req.WorkspaceFlag) {
		agentCmd = agentCmd + " " + req.WorkspaceFlag + " " + req.WorkspacePath
	}
	return agentCmd
}

// buildMcpServerConfigs converts instance McpServerConfig entries to
// config.McpServerConfig. Stdio entries whose Command can't be resolved
// (binary missing from PATH, no longer installed, etc.) are dropped with
// a warning so the agent doesn't spawn a permanently-broken child for an
// MCP it can never invoke. See GH issue #1247 — the `/snap/bin/brave`
// stale-MCP repro.
func (m *Manager) buildMcpServerConfigs(mcpServers []McpServerConfig) []config.McpServerConfig {
	result := make([]config.McpServerConfig, 0, len(mcpServers))
	for _, mcp := range mcpServers {
		if reason := mcpStdioValidationError(mcp); reason != "" {
			m.logger.Warn("dropping MCP server: stdio command unavailable",
				zap.String("mcp_name", mcp.Name),
				zap.String("command", mcp.Command),
				zap.String("reason", reason))
			continue
		}
		result = append(result, config.McpServerConfig{
			Name:    mcp.Name,
			URL:     mcp.URL,
			Type:    mcp.Type,
			Command: mcp.Command,
			Args:    mcp.Args,
			Env:     mcp.Env,
			Headers: mcp.Headers,
		})
	}
	return result
}

// mcpStdioValidationError returns an empty string when the MCP entry is
// either non-stdio (URL transport) or its stdio Command resolves on PATH.
// Otherwise it returns a human-readable reason the entry should be dropped.
//
// Caveat: PATH is resolved against the agentctl process's environment.
// In Docker and SSH executor modes the agent may launch in a different
// environment than agentctl, so a binary that lives only inside the
// container/remote host but not on the agentctl host will be dropped
// here even though it would have worked at agent runtime. For Standalone
// and Sprites this is unambiguously correct; for Docker/SSH it's an
// acceptable false positive — surfacing the warn log is better than
// spawning a permanently broken child every session (the `/snap/bin/brave`
// repro in GH issue #1247).
func mcpStdioValidationError(mcp McpServerConfig) string {
	// Non-stdio transports (sse, http, streamable_http) carry their endpoint
	// in URL — nothing to validate locally.
	if mcp.URL != "" {
		return ""
	}
	if mcp.Command == "" {
		return "stdio MCP entry has neither URL nor Command"
	}
	// Tolerate a compound `Command` string like "python3 -m mcp_server"
	// where the user collapsed Command+Args into Command. The first token
	// is the actual binary to look up; everything else is argv that the
	// agent will splice in later. This is more permissive than the
	// schema strictly allows, but it matches what real configs look like.
	bin := mcp.Command
	if i := strings.IndexAny(bin, " \t"); i > 0 {
		bin = bin[:i]
	}
	if _, err := exec.LookPath(bin); err != nil {
		return err.Error()
	}
	return ""
}

// buildHTTPHandler creates the HTTP handler for an instance.
func (m *Manager) buildHTTPHandler(instanceCfg *config.InstanceConfig, procMgr *process.Manager) http.Handler {
	if m.serverFactory != nil {
		return m.serverFactory(instanceCfg, procMgr, m.logger)
	}
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusServiceUnavailable)
		if _, err := w.Write([]byte("server factory not configured")); err != nil {
			m.logger.Debug("failed to write default response", zap.Error(err))
		}
	})
}

// startHTTPServer creates and starts an HTTP server on the given listener.
func (m *Manager) startHTTPServer(port int, listener net.Listener, handler http.Handler, id string) *http.Server {
	httpServer := &http.Server{
		Addr:    fmt.Sprintf(":%d", port),
		Handler: handler,
	}
	go func() {
		if err := httpServer.Serve(listener); err != nil && err != http.ErrServerClosed {
			m.logger.Error("instance server error",
				zap.String("instance_id", id),
				zap.Error(err))
		}
	}()
	return httpServer
}

// GetInstance returns an instance by ID.
func (m *Manager) GetInstance(id string) (*Instance, bool) {
	m.mu.RLock()
	defer m.mu.RUnlock()
	inst, ok := m.instances[id]
	return inst, ok
}

// ListInstances returns info for all instances.
func (m *Manager) ListInstances() []*InstanceInfo {
	m.mu.RLock()
	defer m.mu.RUnlock()

	result := make([]*InstanceInfo, 0, len(m.instances))
	for _, inst := range m.instances {
		result = append(result, inst.Info())
	}
	return result
}

// StopInstance stops and removes an instance by ID.
func (m *Manager) StopInstance(ctx context.Context, id string) error {
	// Get instance and remove from map under lock (quick operation)
	m.mu.Lock()
	inst, ok := m.instances[id]
	if !ok {
		m.mu.Unlock()
		return fmt.Errorf("instance %s not found", id)
	}
	// Remove from map immediately so new instances aren't blocked
	delete(m.instances, id)
	m.mu.Unlock()

	m.logger.Debug("stopping instance", zap.String("instance_id", id))

	// Stop the process manager (potentially slow, done without lock)
	if inst.manager != nil {
		if err := inst.manager.Stop(ctx); err != nil {
			m.logger.Warn("error stopping process manager",
				zap.String("instance_id", id),
				zap.Error(err))
		}
	}

	m.logger.Debug("StopInstance: shutting down HTTP server",
		zap.String("instance_id", id),
		zap.Int("port", inst.Port))

	var stopErr error
	if inst.server != nil {
		if err := m.stopHTTPServer(ctx, id, inst.Port, inst.server); err != nil {
			stopErr = err
		}
	}

	m.logger.Debug("StopInstance: releasing port",
		zap.String("instance_id", id),
		zap.Int("port", inst.Port))

	// Release port (quick operation, re-acquire lock)
	m.mu.Lock()
	m.portAlloc.Release(inst.Port)
	m.mu.Unlock()

	if stopErr != nil {
		return stopErr
	}

	m.logger.Info("StopInstance completed",
		zap.String("instance_id", id),
		zap.Int("port", inst.Port))

	return nil
}

type instanceHTTPServer interface {
	Shutdown(context.Context) error
	Close() error
}

func (m *Manager) stopHTTPServer(ctx context.Context, id string, port int, server instanceHTTPServer) error {
	serverCtx, cancel := context.WithTimeout(ctx, instanceHTTPShutdownGrace)
	err := server.Shutdown(serverCtx)
	cancel()
	if err == nil || errors.Is(err, http.ErrServerClosed) {
		return nil
	}
	m.logger.Debug("StopInstance: HTTP server graceful shutdown expired, closing active connections",
		zap.String("instance_id", id),
		zap.Int("port", port),
		zap.Duration("grace", instanceHTTPShutdownGrace),
		zap.Error(err))
	if closeErr := server.Close(); closeErr != nil && !errors.Is(closeErr, http.ErrServerClosed) {
		m.logger.Warn("error closing HTTP server",
			zap.String("instance_id", id),
			zap.Error(closeErr))
		return fmt.Errorf("close HTTP server for instance %s: %w", id, closeErr)
	}
	if !errors.Is(err, context.DeadlineExceeded) && !errors.Is(err, context.Canceled) {
		return fmt.Errorf("shutdown HTTP server for instance %s: %w", id, err)
	}
	return nil
}

// Shutdown stops all instances gracefully.
func (m *Manager) Shutdown(ctx context.Context) error {
	// Stop the idle reaper first so it doesn't fire StopInstance concurrently
	// with our explicit per-instance shutdown loop.
	m.stopReaperOnce()

	m.mu.Lock()
	ids := make([]string, 0, len(m.instances))
	for id := range m.instances {
		ids = append(ids, id)
	}
	m.mu.Unlock()

	var lastErr error
	for _, id := range ids {
		if err := m.StopInstance(ctx, id); err != nil {
			m.logger.Error("error stopping instance during shutdown",
				zap.String("instance_id", id),
				zap.Error(err))
			lastErr = err
		}
	}

	return lastErr
}

// stopReaperOnce closes the reaper stop channel exactly once and waits for
// the reaper goroutine to drain. Safe to call multiple times.
//
// Note: reaperWG.Wait() blocks until the reaper finishes whatever sweep
// is currently in flight. Each instance in that sweep gets its own bounded
// context (idleReaperShutdownTimeout) independent of any caller deadline,
// so a Shutdown(ctx) caller with a tight deadline can find that the
// reaper drain consumed it before the main shutdown loop starts. The
// reaper polls reaperStop between instances, so the worst-case drain is
// one StopInstance round (15s), not N×timeout.
func (m *Manager) stopReaperOnce() {
	m.reaperStopOnce.Do(func() {
		close(m.reaperStop)
	})
	m.reaperWG.Wait()
}