package gus

import "context"

// Handler is a function which can turn a request into a response.
//
// A Handler can return a nil response, in which case the Server is expected
// to build the protocol-appropriate "Not Found" response.
type Handler func(context.Context, *Request) *Response

// Middleware is a handler decorator.
//
// It returns a handler which may call the passed-in handler or not, or may
// transform the request or response in some way.
type Middleware func(Handler) Handler

// FallthroughHandler builds a handler which tries multiple child handlers.
//
// The returned handler will invoke each of the passed-in handlers in order,
// stopping when it receives a non-nil response.
func FallthroughHandler(handlers ...Handler) Handler {
	return func(ctx context.Context, request *Request) *Response {
		for _, handler := range handlers {
			if response := handler(ctx, request); response != nil {
				return response
			}
		}
		return nil
	}
}

// Filter builds a middleware which only calls the wrapped under a condition.
//
// When the condition function returns false it instead invokes the
// test-failure handler. The failure handler may also be nil, in which case
// the final handler will return a nil response whenever the condition fails.
func Filter(
	condition func(context.Context, *Request) bool,
	failure Handler,
) Middleware {
	return func(success Handler) Handler {
		return func(ctx context.Context, request *Request) *Response {
			if condition(ctx, request) {
				return success(ctx, request)
			}
			if failure == nil {
				return nil
			}
			return failure(ctx, request)
		}
	}
}