package backend

import (
	"fmt"
	"sort"
	"sync"
)

// Constructor builds a Backend from a name and its sub-block of raw config.
// The framework hands the adapter only its own slice of imagen.yaml, so the
// adapter owns its schema completely.
type Constructor func(name string, cfg map[string]any) (Backend, error)

// Registry holds the name → Constructor table. Adapters call Register from
// their package init() to make themselves available to the CLI.
type Registry struct {
	mu    sync.RWMutex
	ctors map[string]Constructor
}

// NewRegistry returns an empty registry.
func NewRegistry() *Registry {
	return &Registry{ctors: make(map[string]Constructor)}
}

// Register adds a constructor under typeName (e.g. "comfyui", "mock").
// Re-registering an existing type panics — names are global per binary.
func (r *Registry) Register(typeName string, ctor Constructor) {
	r.mu.Lock()
	defer r.mu.Unlock()
	if _, exists := r.ctors[typeName]; exists {
		panic(fmt.Sprintf("backend type %q already registered", typeName))
	}
	r.ctors[typeName] = ctor
}

// Build instantiates a backend of typeName using cfg. instanceName is the
// user-facing name from imagen.yaml (e.g. "flux-schnell-local").
func (r *Registry) Build(typeName, instanceName string, cfg map[string]any) (Backend, error) {
	r.mu.RLock()
	ctor, ok := r.ctors[typeName]
	r.mu.RUnlock()
	if !ok {
		return nil, fmt.Errorf("backend type %q not registered, available: %v", typeName, r.Types())
	}
	return ctor(instanceName, cfg)
}

// Types returns the registered backend type names, sorted.
func (r *Registry) Types() []string {
	r.mu.RLock()
	defer r.mu.RUnlock()
	out := make([]string, 0, len(r.ctors))
	for k := range r.ctors {
		out = append(out, k)
	}
	sort.Strings(out)
	return out
}

// Has reports whether typeName is registered.
func (r *Registry) Has(typeName string) bool {
	r.mu.RLock()
	defer r.mu.RUnlock()
	_, ok := r.ctors[typeName]
	return ok
}

// Default is the process-wide registry adapters register against.
var Default = NewRegistry()

// Register is shorthand for Default.Register.
func Register(typeName string, ctor Constructor) {
	Default.Register(typeName, ctor)
}