// Package beachcomber keeps search indexes in step with GORM models: after
// a write commits, the written row is reloaded and its document upserted
// into, or deleted from, the index of a pluggable engine. A failed sync is
// logged and never touches the write.
//
// Models implement Searchable. An engine package (for example
// beachcomber/typesense) is imported for its side effect of registering
// itself, and is chosen with search.driver.
package beachcomber

import (
	"database/sql"
	"fmt"
	"log/slog"
	"strings"
	"sync"

	"git.golem15.com/golem15/summercms/modules/backpack"
	"git.golem15.com/golem15/summercms/modules/lagoon"
	"gorm.io/gorm"
)

// DefaultDriver is the engine used when search.driver is empty.
const DefaultDriver = NullEngine

// Names of the GORM callbacks that sync Searchable models.
const (
	CallbackAfterCreate = "beachcomber:after_create"
	CallbackAfterUpdate = "beachcomber:after_update"
	CallbackAfterDelete = "beachcomber:after_delete"
)

// Service is the app-scoped search service. Get it with From.
type Service struct {
	app    *backpack.App
	engine Engine
	prefix string
	log    *slog.Logger

	mu   sync.RWMutex
	gate Gate
}

// From returns the app's Service, building and publishing it on first use.
// The first call reads search.driver (default "null") and search.prefix
// (default ""), builds the engine through its registered factory, and
// installs the sync GORM callbacks through lagoon.OnDatabase. An unknown
// driver name is an error.
func From(app *backpack.App) (*Service, error) {
	if app == nil {
		return nil, fmt.Errorf("beachcomber: app is nil")
	}
	if s, ok := app.Lookup[*Service](); ok && s != nil {
		return s, nil
	}
	svc := &Service{app: app, log: loggerFromApp(app)}
	name := DefaultDriver
	if app.Config != nil {
		if v := strings.TrimSpace(app.Config.String("search.driver")); v != "" {
			name = strings.ToLower(v)
		}
		svc.prefix = strings.TrimSpace(app.Config.String("search.prefix"))
	}
	factory, ok := engineFactory(name)
	if !ok {
		return nil, fmt.Errorf("beachcomber: unknown search.driver %q (registered: %s; an engine package must be imported to register itself)", name, strings.Join(engineNames(), ", "))
	}
	engine, err := factory(app)
	if err != nil {
		return nil, fmt.Errorf("beachcomber: engine %s: %w", name, err)
	}
	if engine == nil {
		return nil, fmt.Errorf("beachcomber: engine %s returned nil", name)
	}
	svc.engine = engine
	// The callbacks need the GORM handle, which serve publishes after
	// plugins boot.
	if err := lagoon.OnDatabase(app, func(_ *sql.DB, gdb *gorm.DB) error {
		return svc.installCallbacks(gdb)
	}); err != nil {
		return nil, fmt.Errorf("beachcomber: install sync callbacks: %w", err)
	}
	if err := app.Publish(svc); err != nil {
		if existing, ok := app.Lookup[*Service](); ok && existing != nil {
			return existing, nil
		}
		return nil, fmt.Errorf("beachcomber: %w", err)
	}
	return svc, nil
}

// SetGate installs the application kill-switch. With no gate, sync runs
// whenever the engine is configured and a database is published.
func (s *Service) SetGate(g Gate) {
	if s == nil {
		return
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	s.gate = g
}

func (s *Service) currentGate() Gate {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.gate
}

// Engine returns the engine selected by search.driver.
func (s *Service) Engine() Engine {
	if s == nil {
		return nil
	}
	return s.engine
}

// Prefix returns search.prefix.
func (s *Service) Prefix() string {
	if s == nil {
		return ""
	}
	return s.prefix
}

// IndexName is search.prefix followed by m.SearchableAs().
func (s *Service) IndexName(m Searchable) string {
	if m == nil {
		return s.Prefix()
	}
	return s.Prefix() + m.SearchableAs()
}

// Logger returns the app logger sync failures are logged through.
func (s *Service) Logger() *slog.Logger {
	if s == nil || s.log == nil {
		return slog.Default()
	}
	return s.log
}

// commitCallback is GORM's commit of a transaction it opened itself.
const commitCallback = "gorm:commit_or_rollback_transaction"

// installCallbacks registers the sync callbacks on gdb, replacing earlier
// ones so a handle shared by several apps syncs through the most recent
// service. Each runs after the model's own after hook and before GORM
// commits a single-statement write: GORM appends a callback that names only
// an After anchor to the end of the chain, past the commit and past
// lagoon's after-commit flush, where the registered work would never run.
func (s *Service) installCallbacks(gdb *gorm.DB) error {
	cb := gdb.Callback()
	if cb.Create().Get(CallbackAfterCreate) == nil {
		if err := cb.Create().After("gorm:after_create").Before(commitCallback).Register(CallbackAfterCreate, s.afterCreate); err != nil {
			return err
		}
	} else if err := cb.Create().Replace(CallbackAfterCreate, s.afterCreate); err != nil {
		return err
	}
	if cb.Update().Get(CallbackAfterUpdate) == nil {
		if err := cb.Update().After("gorm:after_update").Before(commitCallback).Register(CallbackAfterUpdate, s.afterUpdate); err != nil {
			return err
		}
	} else if err := cb.Update().Replace(CallbackAfterUpdate, s.afterUpdate); err != nil {
		return err
	}
	if cb.Delete().Get(CallbackAfterDelete) == nil {
		if err := cb.Delete().After("gorm:after_delete").Before(commitCallback).Register(CallbackAfterDelete, s.afterDelete); err != nil {
			return err
		}
	} else if err := cb.Delete().Replace(CallbackAfterDelete, s.afterDelete); err != nil {
		return err
	}
	return nil
}

func loggerFromApp(app *backpack.App) *slog.Logger {
	if app != nil {
		if log, ok := app.Lookup[*slog.Logger](); ok && log != nil {
			return log
		}
	}
	return slog.Default()
}