package promptkit

import (
	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
)

// BackendOpenRouter is the reserved ID of Promptkit's built-in OpenRouter
// backend.
const BackendOpenRouter = backend.OpenRouterID

// Backend configures one engine-scoped OpenAI-compatible backend.
//
// Backend has no stable JSON representation. Use keyed literals so additions
// to this configuration value do not break source compatibility.
type Backend struct {
	// ID is the stable, case-sensitive registry key. NewEngine trims it and
	// requires a non-blank value. BackendOpenRouter is reserved.
	ID string
	// Endpoint is the OpenAI-compatible base endpoint. NewEngine trims it and
	// requires an absolute HTTP or HTTPS URL with a host and without user
	// information, a query string, or a fragment. Paths are allowed.
	Endpoint string
	// APIKeyEnv optionally names the environment variable containing the API
	// key. NewEngine trims it and requires the portable form
	// [A-Za-z_][A-Za-z0-9_]*. Store only the name, never a credential value.
	APIKeyEnv string
	// ExtraParams contains backend-wide request defaults. Values must be
	// JSON-compatible, finite, acyclic, and keyed by non-empty strings. Keys
	// must not be model, session_id, messages, temperature, max_tokens, top_p,
	// service_tier, reasoning_effort, or response_format. An empty map supplies
	// no defaults. NewEngine deeply copies the map.
	ExtraParams map[string]any
	// ConcurrencyLimit is the maximum number of simultaneous model-generation
	// calls allowed for this backend within one Engine. Zero leaves the backend
	// unlimited. A negative value makes NewEngine fail with ErrInvalidConfig.
	ConcurrencyLimit int
	// QueueCapacity controls how many additional Run calls may be admitted
	// beyond ConcurrencyLimit. Nil uses 1024 when ConcurrencyLimit is positive;
	// a pointer uses its exact value, including zero. The pointed-to value must
	// be non-negative, and QueueCapacity must be nil when ConcurrencyLimit is
	// zero. Their sum must fit in an int. WithBackend copies the value and does
	// not retain the pointer.
	QueueCapacity *int
}

// WithBackend adds one Backend registration to the constructed Engine.
//
// Registrations accumulate in option order. Every normalized ID must be unique
// across consumer registrations and built-ins; a duplicate or invalid
// definition makes NewEngine fail with ErrInvalidConfig. In particular,
// BackendOpenRouter cannot be replaced. The immutable registration is scoped
// to the resulting Engine and cannot be enumerated, replaced, removed, or
// mutated after construction. WithBackend does not install package-global
// state.
func WithBackend(backend Backend) Option {
	queueCapacity := 0
	queueCapacitySet := backend.QueueCapacity != nil
	if queueCapacitySet {
		queueCapacity = *backend.QueueCapacity
	}
	return optionFunc(func(options *engineOptions) error {
		options.backends = append(options.backends, domain.Backend{
			ID:               backend.ID,
			Endpoint:         backend.Endpoint,
			APIKeyEnv:        backend.APIKeyEnv,
			ExtraParams:      backend.ExtraParams,
			ConcurrencyLimit: backend.ConcurrencyLimit,
			QueueCapacity:    queueCapacity,
			QueueCapacitySet: queueCapacitySet,
		})
		return nil
	})
}