package session

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/user"
	"path/filepath"
	"strings"
	"time"
)

// Lease is the on-disk format of .ctask/session.json. It identifies the ctask
// process holding this workspace open and records a heartbeat timestamp so
// that crashed sessions can be detected and cleaned up by a later session.
type Lease struct {
	SessionID       string    `json:"session_id"`
	PID             int       `json:"pid"`
	Hostname        string    `json:"hostname"`
	Username        string    `json:"username"`
	Agent           string    `json:"agent"`
	Mode            string    `json:"mode"`
	StartedAt       time.Time `json:"started_at"`
	LastHeartbeatAt time.Time `json:"last_heartbeat_at"`
	Terminal        string    `json:"terminal"`
}

// LeasePath returns the absolute path of the lease file for the given workspace.
func LeasePath(wsDir string) string {
	return filepath.Join(wsDir, ".ctask", "session.json")
}

// WriteLease marshals a lease to JSON and writes it to path.
// The .ctask/ directory is created if needed.
func WriteLease(path string, l *Lease) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return err
	}
	data, err := json.MarshalIndent(l, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(path, data, 0644)
}

// ReadLease reads and parses a lease file. A missing file returns the
// stdlib os.ErrNotExist (via os.ReadFile), which callers should detect
// with errors.Is.
func ReadLease(path string) (*Lease, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var l Lease
	if err := json.Unmarshal(data, &l); err != nil {
		return nil, fmt.Errorf("parsing lease: %w", err)
	}
	return &l, nil
}

// currentUsername returns the current OS username, falling back to "unknown"
// on error. Never panics.
func currentUsername() string {
	u, err := user.Current()
	if err != nil || u == nil {
		return "unknown"
	}
	if u.Username == "" {
		return "unknown"
	}
	return u.Username
}

// currentHostname returns the current hostname, falling back to "unknown".
func currentHostname() string {
	h, err := os.Hostname()
	if err != nil || h == "" {
		return "unknown"
	}
	return h
}

// currentTerminal is a best-effort terminal identifier based on common env vars.
// Returns "unknown" if none are set.
func currentTerminal() string {
	for _, k := range []string{"TERM_PROGRAM", "WT_SESSION", "TERM"} {
		if v := os.Getenv(k); v != "" {
			return v
		}
	}
	return "unknown"
}

// NewSessionID returns a session identifier in the format
// "<hostname>-<pid>-<YYYYMMDDHHMMSS>" using the given time in UTC.
func NewSessionID(hostname string, pid int, t time.Time) string {
	return fmt.Sprintf("%s-%d-%s", hostname, pid, t.UTC().Format("20060102150405"))
}

// NewLease constructs a fresh lease for the current process. startedAt is used
// for both StartedAt and LastHeartbeatAt.
func NewLease(startedAt time.Time, agent, mode string) *Lease {
	hostname := currentHostname()
	pid := os.Getpid()
	return &Lease{
		SessionID:       NewSessionID(hostname, pid, startedAt),
		PID:             pid,
		Hostname:        hostname,
		Username:        currentUsername(),
		Agent:           agent,
		Mode:            mode,
		StartedAt:       startedAt.UTC().Truncate(time.Second),
		LastHeartbeatAt: startedAt.UTC().Truncate(time.Second),
		Terminal:        currentTerminal(),
	}
}

// IsFresh returns true if the lease's last heartbeat is within threshold
// of now. A lease with a zero LastHeartbeatAt is treated as stale.
func IsFresh(l *Lease, now time.Time, threshold time.Duration) bool {
	if l == nil || l.LastHeartbeatAt.IsZero() {
		return false
	}
	return now.Sub(l.LastHeartbeatAt) <= threshold
}

// CleanupStaleLease inspects the lease at path:
//   - missing file: no-op, returns (nil, nil)
//   - corrupt / unparseable: removes the file, returns (nil, nil)
//   - stale (heartbeat older than staleAfter): removes the file, returns the parsed lease
//   - fresh: no-op, returns (nil, nil)
//
// An I/O error during removal is returned to the caller along with the
// parsed lease (if any), so the caller can log it explicitly.
func CleanupStaleLease(path string, staleAfter time.Duration) (*Lease, error) {
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	var l Lease
	if jsonErr := json.Unmarshal(data, &l); jsonErr != nil {
		if rmErr := os.Remove(path); rmErr != nil && !errors.Is(rmErr, os.ErrNotExist) {
			return nil, rmErr
		}
		return nil, nil
	}
	if IsFresh(&l, time.Now(), staleAfter) {
		return nil, nil
	}
	if rmErr := os.Remove(path); rmErr != nil && !errors.Is(rmErr, os.ErrNotExist) {
		return &l, rmErr
	}
	return &l, nil
}

// FormatActiveWarning renders the human-readable warning printed when a
// fresh active lease is detected on session start.
func FormatActiveWarning(l *Lease, now time.Time) string {
	startedAgo := now.Sub(l.StartedAt)
	lastSeenAgo := now.Sub(l.LastHeartbeatAt)

	var b strings.Builder
	b.WriteString("[ctask] This workspace has an active session:\n")
	fmt.Fprintf(&b, "  Session:   %s\n", l.SessionID)
	fmt.Fprintf(&b, "  Host:      %s\n", l.Hostname)
	fmt.Fprintf(&b, "  Agent:     %s\n", l.Agent)
	fmt.Fprintf(&b, "  Started:   %s (%s ago)\n",
		l.StartedAt.Local().Format("2006-01-02 15:04"),
		FormatAgo(startedAgo))
	fmt.Fprintf(&b, "  Last seen: %s ago\n", FormatAgoShort(lastSeenAgo))
	b.WriteString("\n")
	b.WriteString("  Opening a second session may cause conflicts.\n")
	b.WriteString("  Continue anyway? [y/N] ")
	return b.String()
}

// FormatStaleCleanupNotice renders the single-line notice printed when a
// stale lease has been removed during session start.
func FormatStaleCleanupNotice(l *Lease, now time.Time) string {
	return fmt.Sprintf("[ctask] Cleaned up stale session from %s (started %s, last seen %s ago)\n",
		l.Hostname,
		l.StartedAt.Local().Format("2006-01-02 15:04"),
		FormatAgo(now.Sub(l.LastHeartbeatAt)))
}

// FormatAgo renders a coarse "2h 15m" / "3m" / "45s" for the active warning.
func FormatAgo(d time.Duration) string {
	if d < 0 {
		d = 0
	}
	h := int(d.Hours())
	m := int(d.Minutes()) % 60
	if h > 0 {
		return fmt.Sprintf("%dh %dm", h, m)
	}
	if m > 0 {
		return fmt.Sprintf("%dm", m)
	}
	return fmt.Sprintf("%ds", int(d.Seconds()))
}

// FormatAgoShort renders "12 seconds" / "3 minutes" / "2 hours" for the
// Last seen line, which the spec shows with a noun suffix.
func FormatAgoShort(d time.Duration) string {
	if d < 0 {
		d = 0
	}
	s := int(d.Seconds())
	if s < 60 {
		if s == 1 {
			return "1 second"
		}
		return fmt.Sprintf("%d seconds", s)
	}
	m := int(d.Minutes())
	if m < 60 {
		if m == 1 {
			return "1 minute"
		}
		return fmt.Sprintf("%d minutes", m)
	}
	h := int(d.Hours())
	if h == 1 {
		return "1 hour"
	}
	return fmt.Sprintf("%d hours", h)
}