package session

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

// SessionSummary is the on-disk format of .ctask/last-session-summary.json.
// It records what the last session changed and what the workspace looked like
// at end-of-session, so the next session can (a) detect external modifications
// (Layer 3) and (b) display a launch-context banner (Layer 4).
type SessionSummary struct {
	SessionID       string    `json:"session_id"`
	Hostname        string    `json:"hostname"`
	Agent           string    `json:"agent"`
	Mode            string    `json:"mode"`
	StartedAt       time.Time `json:"started_at"`
	EndedAt         time.Time `json:"ended_at"`
	DurationSeconds int64     `json:"duration_seconds"`
	FilesAdded      []string  `json:"files_added"`
	FilesModified   []string  `json:"files_modified"`
	FilesDeleted    []string  `json:"files_deleted"`
	NotesUpdated    bool      `json:"notes_updated"`

	// EndReason is "child_exited" for direct mode, "tmux_session_ended" for
	// persistent mode (both owner-create and adopted reattach). Optional;
	// pre-v0.5.3 summaries omit it.
	EndReason string `json:"end_reason,omitempty"`

	// DetectedVia distinguishes the mechanism that observed session end:
	// "child_exit" for direct mode, "polling" for persistent mode.
	DetectedVia string `json:"detected_via,omitempty"`

	// SessionOwnership is "created" if this ctask process originated the
	// session (owner-create) or "adopted" if it took over an orphaned
	// persistent session. Omitted in direct mode.
	SessionOwnership string `json:"session_ownership,omitempty"`

	// AdoptedFromOrphanAt records the moment adoption took place. Set only
	// for adopted reattach.
	AdoptedFromOrphanAt *time.Time `json:"adopted_from_orphan_at,omitempty"`

	// EndManifest captures the workspace file list at end-of-session so the
	// next session can diff current state against it (Layer 3). This field
	// is ctask-internal; it is not part of the public summary format shown
	// in the spec examples, but it enables stale-workspace detection from a
	// single source file.
	EndManifest []FileEntry `json:"end_manifest"`
}

// SummaryPath returns the absolute path of the summary file for wsDir.
func SummaryPath(wsDir string) string {
	return filepath.Join(wsDir, ".ctask", "last-session-summary.json")
}

// WriteSummary serializes s to path with indented JSON. Creates .ctask/ if needed.
func WriteSummary(path string, s *SessionSummary) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return err
	}
	if s.FilesAdded == nil {
		s.FilesAdded = []string{}
	}
	if s.FilesModified == nil {
		s.FilesModified = []string{}
	}
	if s.FilesDeleted == nil {
		s.FilesDeleted = []string{}
	}
	if s.EndManifest == nil {
		s.EndManifest = []FileEntry{}
	}
	data, err := json.MarshalIndent(s, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(path, data, 0644)
}

// ReadSummary reads a summary from path.
//   - missing file: returns (nil, nil) — normal pre-v0.4 case
//   - corrupt JSON: returns (nil, wrapped err)
//   - other I/O errors: returned to caller as-is
func ReadSummary(path string) (*SessionSummary, error) {
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	var s SessionSummary
	if err := json.Unmarshal(data, &s); err != nil {
		return nil, fmt.Errorf("parsing summary: %w", err)
	}
	return &s, nil
}

// SummarizeFromDiff builds a SessionSummary from a session's identity,
// start/end times, the diff, and the end manifest.
func SummarizeFromDiff(
	sessionID, hostname, agent, mode string,
	startedAt, endedAt time.Time,
	diff *ManifestDiff,
	endManifest *Manifest,
) *SessionSummary {
	s := &SessionSummary{
		SessionID:       sessionID,
		Hostname:        hostname,
		Agent:           agent,
		Mode:            mode,
		StartedAt:       startedAt,
		EndedAt:         endedAt,
		DurationSeconds: int64(endedAt.Sub(startedAt).Seconds()),
		FilesAdded:      append([]string{}, diff.Added...),
		FilesModified:   append([]string{}, diff.Modified...),
		FilesDeleted:    append([]string{}, diff.Deleted...),
		NotesUpdated:    NotesUpdated(diff),
	}
	if endManifest != nil {
		s.EndManifest = append([]FileEntry{}, endManifest.Files...)
	} else {
		s.EndManifest = []FileEntry{}
	}
	return s
}

// FormatLaunchContext renders the short "Last session: ... Changed: ..." banner
// printed on session start (Layer 4). Returns empty string when s is nil.
func FormatLaunchContext(s *SessionSummary) string {
	if s == nil {
		return ""
	}
	var b strings.Builder

	fmt.Fprintf(&b, "[ctask] Last session: %s-%s (%s, %s)\n",
		s.StartedAt.Local().Format("2006-01-02 15:04"),
		s.EndedAt.Local().Format("15:04"),
		s.Hostname, s.Agent)

	changed := append([]string{}, s.FilesModified...)
	changed = append(changed, s.FilesAdded...)
	const maxShown = 2
	more := 0
	preview := changed
	if len(changed) > maxShown {
		preview = changed[:maxShown]
		more = len(changed) - maxShown
	}
	if len(preview) == 0 && len(s.FilesDeleted) == 0 {
		fmt.Fprintln(&b, "[ctask]   (no tracked file changes)")
		return b.String()
	}
	if len(preview) > 0 {
		fmt.Fprintf(&b, "[ctask]   Changed: %s", strings.Join(preview, ", "))
		if more > 0 {
			fmt.Fprintf(&b, " (+%d more)", more)
		}
		fmt.Fprintln(&b)
	}
	return b.String()
}