package sliderule

import "context"

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

type handlerFunc func(context.Context, *Request) *Response

// HandlerFunc is a wrapper to allow using a function as a Handler.
func HandlerFunc(f func(context.Context, *Request) *Response) Handler {
	return handlerFunc(f)
}

// Handle implements Handler.
func (f handlerFunc) Handle(ctx context.Context, request *Request) *Response {
	return f(ctx, request)
}

// 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 HandlerFunc(func(ctx context.Context, request *Request) *Response {
		for _, handler := range handlers {
			if response := handler.Handle(ctx, request); response != nil {
				return response
			}
		}
		return nil
	})
}

// Filter builds a middleware which only calls the wrapped Handler 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 HandlerFunc(func(ctx context.Context, request *Request) *Response {
			if condition(ctx, request) {
				return success.Handle(ctx, request)
			}
			if failure == nil {
				return nil
			}
			return failure.Handle(ctx, request)
		})
	}
}

// VirtualHosts builds a handler which dispatches to site handlers by hostname.
//
// The 'catchall' argument may be used to specify a handler for use when no hostname
// match is found. If the catchall is nil and no match is found, the VirtualHosts
// handler returns a nil response.
func VirtualHosts(hosts map[string]Handler, catchall Handler) Handler {
	return HandlerFunc(func(ctx context.Context, request *Request) *Response {
		if h := hosts[request.Hostname()]; h != nil {
			return h.Handle(ctx, request)
		}
		if catchall != nil {
			return catchall.Handle(ctx, request)
		}
		return nil
	})
}