// Package process manages agent processes: discovery, start, stop, kill, stats.
// This is the impure shell layer — all I/O happens here.
package process

import (
	"bufio"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strconv"
	"strings"
	"syscall"
	"time"

	"github.com/enmanuel/agents/internal/config"
)

// AgentInfo holds metadata about an agent parsed from its config.
type AgentInfo struct {
	ID         string
	Name       string
	Version    string
	Desc       string
	ConfigPath string
	Enabled    bool
}

// AgentStatus combines agent metadata with runtime state.
type AgentStatus struct {
	AgentInfo
	Running bool
	PID     int
}

// ProcessStats holds resource usage for a running process.
type ProcessStats struct {
	PID        int
	UptimeSecs int64
	MemRSSKB   int64
	CPUPct     float64
	LogBytes   int64
}

// Manager handles agent process lifecycle.
type Manager struct {
	runDir     string
	agentsGlob string
	binPath    string
}

// NewManager creates a Manager. binPath can be empty for auto-detection.
func NewManager(runDir, agentsGlob, binPath string) *Manager {
	return &Manager{runDir: runDir, agentsGlob: agentsGlob, binPath: binPath}
}

// Scan discovers all agents from config files.
func (m *Manager) Scan() ([]AgentInfo, error) {
	matches, err := filepath.Glob(m.agentsGlob)
	if err != nil {
		return nil, err
	}

	var agents []AgentInfo
	for _, path := range matches {
		cfg, err := config.LoadMeta(path)
		if err != nil {
			continue
		}
		agents = append(agents, AgentInfo{
			ID:         cfg.Agent.ID,
			Name:       cfg.Agent.Name,
			Version:    cfg.Agent.Version,
			Desc:       cfg.Agent.Description,
			ConfigPath: path,
			Enabled:    cfg.Agent.Enabled,
		})
	}
	return agents, nil
}

// Status returns the runtime status for a single agent.
func (m *Manager) Status(info AgentInfo) AgentStatus {
	pid := m.readPID(info.ID)
	running := pid > 0 && m.isAlive(pid)
	return AgentStatus{AgentInfo: info, Running: running, PID: pid}
}

// StatusAll returns status for every discovered agent.
func (m *Manager) StatusAll() ([]AgentStatus, error) {
	agents, err := m.Scan()
	if err != nil {
		return nil, err
	}
	statuses := make([]AgentStatus, len(agents))
	for i, a := range agents {
		statuses[i] = m.Status(a)
	}
	return statuses, nil
}

// Start launches an agent process in the background.
func (m *Manager) Start(info AgentInfo) error {
	if err := os.MkdirAll(m.runDir, 0o755); err != nil {
		return fmt.Errorf("create run dir: %w", err)
	}

	logFile, err := os.OpenFile(m.logPath(info.ID), os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
	if err != nil {
		return fmt.Errorf("open log: %w", err)
	}

	bin := m.resolvedBin()
	var cmd *exec.Cmd
	if strings.HasPrefix(bin, "go run") {
		cmd = exec.Command("go", "run", "./cmd/launcher", "-c", info.ConfigPath)
	} else {
		cmd = exec.Command(bin, "-c", info.ConfigPath)
	}

	cmd.Stdout = logFile
	cmd.Stderr = logFile
	cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}

	if err := cmd.Start(); err != nil {
		logFile.Close()
		return fmt.Errorf("exec: %w", err)
	}

	if err := os.WriteFile(m.pidPath(info.ID), []byte(strconv.Itoa(cmd.Process.Pid)), 0o644); err != nil {
		return fmt.Errorf("write PID: %w", err)
	}

	go func() { _ = cmd.Wait() }()
	return nil
}

// Stop sends SIGTERM, waits up to 5s, then SIGKILL if needed.
func (m *Manager) Stop(id string) error {
	pid := m.readPID(id)
	if pid == 0 || !m.isAlive(pid) {
		return fmt.Errorf("agent %q is not running", id)
	}

	if err := syscall.Kill(pid, syscall.SIGTERM); err != nil {
		return fmt.Errorf("SIGTERM: %w", err)
	}

	// Wait up to 5 seconds for graceful shutdown.
	for i := 0; i < 10; i++ {
		if !m.isAlive(pid) {
			m.removePID(id)
			return nil
		}
		time.Sleep(500 * time.Millisecond)
	}

	// Force kill.
	if m.isAlive(pid) {
		_ = syscall.Kill(pid, syscall.SIGKILL)
	}
	m.removePID(id)
	return nil
}

// Kill sends SIGKILL immediately.
func (m *Manager) Kill(id string) error {
	pid := m.readPID(id)
	if pid == 0 || !m.isAlive(pid) {
		return fmt.Errorf("agent %q is not running", id)
	}
	err := syscall.Kill(pid, syscall.SIGKILL)
	m.removePID(id)
	return err
}

// Stats gathers resource usage for a running agent from /proc.
func (m *Manager) Stats(id string) (ProcessStats, error) {
	pid := m.readPID(id)
	if pid == 0 || !m.isAlive(pid) {
		return ProcessStats{}, fmt.Errorf("agent %q is not running", id)
	}

	s := ProcessStats{PID: pid}

	// Uptime from /proc/<pid>/stat
	if data, err := os.ReadFile(fmt.Sprintf("/proc/%d/stat", pid)); err == nil {
		fields := strings.Fields(string(data))
		if len(fields) > 21 {
			startTicks, _ := strconv.ParseInt(fields[21], 10, 64)
			clkTck := int64(100) // sysconf(_SC_CLK_TCK) is 100 on Linux
			if raw, err := os.ReadFile("/proc/stat"); err == nil {
				for _, line := range strings.Split(string(raw), "\n") {
					if strings.HasPrefix(line, "btime ") {
						btime, _ := strconv.ParseInt(strings.Fields(line)[1], 10, 64)
						procStart := btime + startTicks/clkTck
						s.UptimeSecs = time.Now().Unix() - procStart
						break
					}
				}
			}
		}
	}

	// RSS from /proc/<pid>/status
	if data, err := os.ReadFile(fmt.Sprintf("/proc/%d/status", pid)); err == nil {
		for _, line := range strings.Split(string(data), "\n") {
			if strings.HasPrefix(line, "VmRSS:") {
				fields := strings.Fields(line)
				if len(fields) >= 2 {
					s.MemRSSKB, _ = strconv.ParseInt(fields[1], 10, 64)
				}
				break
			}
		}
	}

	// CPU% from ps (simpler than calculating from /proc/stat deltas)
	if out, err := exec.Command("ps", "-p", strconv.Itoa(pid), "-o", "pcpu=").Output(); err == nil {
		s.CPUPct, _ = strconv.ParseFloat(strings.TrimSpace(string(out)), 64)
	}

	// Log file size
	if info, err := os.Stat(m.logPath(id)); err == nil {
		s.LogBytes = info.Size()
	}

	return s, nil
}

// LogTail returns the last N lines of an agent's log.
func (m *Manager) LogTail(id string, lines int) ([]string, error) {
	f, err := os.Open(m.logPath(id))
	if err != nil {
		return nil, fmt.Errorf("open log: %w", err)
	}
	defer f.Close()

	// Read all lines and keep last N. For large files a reverse scanner
	// would be better, but agent logs are typically small.
	var all []string
	scanner := bufio.NewScanner(f)
	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
	for scanner.Scan() {
		all = append(all, scanner.Text())
	}
	if err := scanner.Err(); err != nil {
		return nil, err
	}

	if len(all) > lines {
		all = all[len(all)-lines:]
	}
	return all, nil
}

// IsRunning checks if an agent process is alive.
func (m *Manager) IsRunning(id string) bool {
	pid := m.readPID(id)
	return pid > 0 && m.isAlive(pid)
}

// ReadPID returns the PID from the PID file, or 0.
func (m *Manager) ReadPID(id string) int {
	return m.readPID(id)
}

// PidPath returns the path to the PID file for an agent.
func (m *Manager) PidPath(id string) string { return m.pidPath(id) }

// LogPath returns the path to the log file for an agent.
func (m *Manager) LogPath(id string) string { return m.logPath(id) }

// ── internal helpers ─────────────────────────────────────────────────────

func (m *Manager) pidPath(id string) string { return filepath.Join(m.runDir, id+".pid") }
func (m *Manager) logPath(id string) string { return filepath.Join(m.runDir, id+".log") }

func (m *Manager) readPID(id string) int {
	raw, err := os.ReadFile(m.pidPath(id))
	if err != nil {
		return 0
	}
	pid, _ := strconv.Atoi(strings.TrimSpace(string(raw)))
	return pid
}

func (m *Manager) isAlive(pid int) bool {
	return syscall.Kill(pid, 0) == nil
}

func (m *Manager) removePID(id string) {
	_ = os.Remove(m.pidPath(id))
}

func (m *Manager) resolvedBin() string {
	if m.binPath != "" {
		return m.binPath
	}
	if _, err := os.Stat("bin/launcher"); err == nil {
		return "bin/launcher"
	}
	return "go run ./cmd/launcher"
}