// Package persist stores the set of user-created top-level command
// processes for a project so they can be re-spawned after patterm
// restarts. SPEC §2 keeps everything ephemeral within one run; this
// state file is the exception — it survives the process tear-down so a
// user who fires up `bun run dev` and `tail -F log` doesn't have to
// re-spawn them every time patterm relaunches.
//
// Only top-level command entries (ParentID == "") are recorded.
// Agents, terminals, and orchestrator-spawned commands stay ephemeral.
// The file lives at
// $XDG_DATA_HOME/patterm/projects/<projectKey>/processes.json — the
// same parent directory the trust store uses.
package persist

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"sync"
)

// Entry is one persisted top-level command process. ID matches the
// session-minted process id; on restore Session.Spawn mints a fresh
// id, so ID is treated as opaque (used only to key Save/Remove).
type Entry struct {
	ID          string   `json:"id"`
	Name        string   `json:"name"`
	Argv        []string `json:"argv"`
	WorkDir     string   `json:"working_dir,omitempty"`
	PresetRef   string   `json:"preset_ref,omitempty"`
	AutoRestart bool     `json:"auto_restart,omitempty"`
}

// Store is one project's persisted-process file. Safe for concurrent
// use.
type Store struct {
	path string

	mu      sync.Mutex
	entries map[string]Entry
	order   []string
}

// Open loads (or creates) the processes file for projectKey. Missing
// file is not an error — it simply means nothing has been spawned
// yet.
func Open(projectKey string) (*Store, error) {
	if projectKey == "" {
		return nil, errors.New("persist.Open: empty project key")
	}
	base, err := dataDir()
	if err != nil {
		return nil, err
	}
	dir := filepath.Join(base, "projects", projectKey)
	if err := os.MkdirAll(dir, 0o700); err != nil {
		return nil, fmt.Errorf("persist: mkdir %s: %w", dir, err)
	}
	path := filepath.Join(dir, "processes.json")
	s := &Store{path: path, entries: make(map[string]Entry)}
	if err := s.loadLocked(); err != nil {
		return nil, err
	}
	return s, nil
}

func dataDir() (string, error) {
	if h := os.Getenv("XDG_DATA_HOME"); h != "" {
		return filepath.Join(h, "patterm"), nil
	}
	home, err := os.UserHomeDir()
	if err != nil {
		return "", err
	}
	return filepath.Join(home, ".local", "share", "patterm"), nil
}

// Path returns the on-disk file path. Used by tests / diagnostics.
func (s *Store) Path() string { return s.path }

// Save inserts or updates an entry, keyed by Entry.ID. Empty ID is an
// error.
func (s *Store) Save(e Entry) error {
	if e.ID == "" {
		return errors.New("persist.Save: empty entry id")
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	if _, exists := s.entries[e.ID]; !exists {
		s.order = append(s.order, e.ID)
	}
	s.entries[e.ID] = e
	return s.saveLocked()
}

// Remove drops an entry by ID. No-op if the entry doesn't exist.
func (s *Store) Remove(id string) error {
	if id == "" {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	if _, exists := s.entries[id]; !exists {
		return nil
	}
	delete(s.entries, id)
	for i, oid := range s.order {
		if oid == id {
			s.order = append(s.order[:i], s.order[i+1:]...)
			break
		}
	}
	return s.saveLocked()
}

// List returns entries in the order they were first saved.
func (s *Store) List() []Entry {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make([]Entry, 0, len(s.order))
	for _, id := range s.order {
		if e, ok := s.entries[id]; ok {
			out = append(out, e)
		}
	}
	return out
}

type fileShape struct {
	Processes []Entry `json:"processes"`
}

func (s *Store) loadLocked() error {
	b, err := os.ReadFile(s.path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return nil
		}
		return fmt.Errorf("persist: read %s: %w", s.path, err)
	}
	if len(b) == 0 {
		return nil
	}
	var f fileShape
	if err := json.Unmarshal(b, &f); err != nil {
		return fmt.Errorf("persist: parse %s: %w", s.path, err)
	}
	for _, e := range f.Processes {
		if e.ID == "" {
			continue
		}
		if _, exists := s.entries[e.ID]; !exists {
			s.order = append(s.order, e.ID)
		}
		s.entries[e.ID] = e
	}
	// Stable serialization order across re-saves.
	sort.SliceStable(s.order, func(i, j int) bool { return s.order[i] < s.order[j] })
	return nil
}

func (s *Store) saveLocked() error {
	out := make([]Entry, 0, len(s.entries))
	for _, id := range s.order {
		if e, ok := s.entries[id]; ok {
			out = append(out, e)
		}
	}
	body, err := json.MarshalIndent(fileShape{Processes: out}, "", "  ")
	if err != nil {
		return err
	}
	body = append(body, '\n')
	tmp := s.path + ".tmp"
	if err := os.WriteFile(tmp, body, 0o600); err != nil {
		return fmt.Errorf("persist: write %s: %w", tmp, err)
	}
	if err := os.Rename(tmp, s.path); err != nil {
		return fmt.Errorf("persist: rename %s: %w", s.path, err)
	}
	return nil
}