package session

import (
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"agentu/internal/llm"
)

// Session represents a persisted conversation.
type Session struct {
	ID        string        `json:"id"`
	Name      string        `json:"name"`
	CreatedAt time.Time     `json:"created_at"`
	UpdatedAt time.Time     `json:"updated_at"`
	Provider  string        `json:"provider"`
	Model     string        `json:"model"`
	Messages  []llm.Message `json:"messages"`
	LastUsage *llm.Usage    `json:"last_usage,omitempty"`
}

// SessionMeta is a lightweight summary for listing.
type SessionMeta struct {
	ID           string    `json:"id"`
	Name         string    `json:"name"`
	CreatedAt    time.Time `json:"created_at"`
	UpdatedAt    time.Time `json:"updated_at"`
	Provider     string    `json:"provider"`
	Model        string    `json:"model"`
	MessageCount int       `json:"message_count"`
}

// Store manages session files under a directory.
type Store struct {
	dir string
}

// NewStore creates a store, ensuring the directory exists.
func NewStore(dir string) (*Store, error) {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return nil, fmt.Errorf("create session dir: %w", err)
	}
	return &Store{dir: dir}, nil
}

// Save writes a session to disk.
func (s *Store) Save(sess *Session) error {
	if sess.ID == "" {
		return fmt.Errorf("session ID is required")
	}
	sess.UpdatedAt = time.Now().UTC()
	if sess.CreatedAt.IsZero() {
		sess.CreatedAt = sess.UpdatedAt
	}
	data, err := json.MarshalIndent(sess, "", "  ")
	if err != nil {
		return fmt.Errorf("marshal session: %w", err)
	}
	path := s.filePath(sess.ID)
	if err := os.WriteFile(path, data, 0o644); err != nil {
		return fmt.Errorf("write session: %w", err)
	}
	return nil
}

// Load reads a session by ID.
func (s *Store) Load(id string) (*Session, error) {
	path := s.filePath(id)
	data, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, fmt.Errorf("session %q not found", id)
		}
		return nil, fmt.Errorf("read session: %w", err)
	}
	var sess Session
	if err := json.Unmarshal(data, &sess); err != nil {
		return nil, fmt.Errorf("parse session: %w", err)
	}
	return &sess, nil
}

// List returns metadata for all sessions, sorted by updated_at descending.
func (s *Store) List() ([]SessionMeta, error) {
	entries, err := os.ReadDir(s.dir)
	if err != nil {
		return nil, fmt.Errorf("read session dir: %w", err)
	}
	var metas []SessionMeta
	for _, entry := range entries {
		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".json") {
			continue
		}
		path := filepath.Join(s.dir, entry.Name())
		data, err := os.ReadFile(path)
		if err != nil {
			continue
		}
		var sess Session
		if err := json.Unmarshal(data, &sess); err != nil {
			continue
		}
		metas = append(metas, SessionMeta{
			ID:           sess.ID,
			Name:         sess.Name,
			CreatedAt:    sess.CreatedAt,
			UpdatedAt:    sess.UpdatedAt,
			Provider:     sess.Provider,
			Model:        sess.Model,
			MessageCount: len(sess.Messages),
		})
	}
	sort.Slice(metas, func(i, j int) bool {
		return metas[i].UpdatedAt.After(metas[j].UpdatedAt)
	})
	return metas, nil
}

// Delete removes a session file.
func (s *Store) Delete(id string) error {
	path := s.filePath(id)
	if err := os.Remove(path); err != nil {
		if os.IsNotExist(err) {
			return fmt.Errorf("session %q not found", id)
		}
		return fmt.Errorf("delete session: %w", err)
	}
	return nil
}

func (s *Store) filePath(id string) string {
	return filepath.Join(s.dir, id+".json")
}

// GenerateID creates a random 8-character hex session ID.
func GenerateID() string {
	b := make([]byte, 4)
	_, _ = rand.Read(b)
	return hex.EncodeToString(b)
}

// DefaultName derives a session name from the first user message.
func DefaultName(messages []llm.Message) string {
	for _, msg := range messages {
		if msg.Role != llm.RoleUser {
			continue
		}
		text := strings.TrimSpace(msg.Content)
		if text == "" {
			continue
		}
		// Take first line, truncate at 40 chars at word boundary
		if idx := strings.IndexByte(text, '\n'); idx >= 0 {
			text = text[:idx]
		}
		text = strings.TrimSpace(text)
		if len(text) > 40 {
			text = text[:40]
			if idx := strings.LastIndexByte(text, ' '); idx > 20 {
				text = text[:idx]
			}
			text += "..."
		}
		return text
	}
	return ""
}