// Package handler handles incoming Gateway events. It reflects the function's
// first argument and caches that for use in each event.
//
// Performance
//
// Each call to the event would take 156 ns/op for roughly each handler. Scaling
// that up to 100 handlers is multiplying 156 ns by 100, which gives 15600 ns,
// or 0.0156 ms.
//
//    BenchmarkReflect-8  7260909  156 ns/op
//
// Usage
//
// Handler's usage is similar to discordgo, in that AddHandler expects a
// function with only one argument. The only argument must be a pointer to one
// of the events, or an interface{} which would accept all events.
//
// AddHandler would panic if the handler is invalid.
//
//    s.AddHandler(func(m *gateway.MessageCreateEvent) {
//         log.Println(m.Author.Username, "said", m.Content)
//    })
//
package handler

import (
	"context"
	"fmt"
	"reflect"
	"sync"

	"github.com/pkg/errors"
)

type Handler struct {
	// Synchronous controls whether to spawn each event handler in its own
	// goroutine. Default false (meaning goroutines are spawned).
	Synchronous bool

	handlers map[uint64]handler
	horders  []uint64
	hserial  uint64
	hmutex   sync.RWMutex
}

func New() *Handler {
	return &Handler{
		handlers: map[uint64]handler{},
	}
}

func (h *Handler) Call(ev interface{}) {
	var evV = reflect.ValueOf(ev)
	var evT = evV.Type()

	h.hmutex.RLock()
	defer h.hmutex.RUnlock()

	for _, order := range h.horders {
		handler, ok := h.handlers[order]
		if !ok {
			// This shouldn't ever happen, but we're adding this just in case.
			continue
		}

		if handler.not(evT) {
			continue
		}

		if h.Synchronous {
			handler.call(evV)
		} else {
			go handler.call(evV)
		}
	}
}

// WaitFor blocks until there's an event. It's advised to use ChanFor instead,
// as WaitFor may skip some events if it's not ran fast enough after the event
// arrived.
func (h *Handler) WaitFor(ctx context.Context, fn func(interface{}) bool) interface{} {
	var result = make(chan interface{})

	cancel := h.AddHandler(func(v interface{}) {
		if fn(v) {
			result <- v
		}
	})
	defer cancel()

	select {
	case r := <-result:
		return r
	case <-ctx.Done():
		return nil
	}
}

// ChanFor returns a channel that would receive all incoming events that match
// the callback given. The cancel() function removes the handler and drops all
// hanging goroutines.
func (h *Handler) ChanFor(fn func(interface{}) bool) (out <-chan interface{}, cancel func()) {
	result := make(chan interface{})
	closer := make(chan struct{})

	removeHandler := h.AddHandler(func(v interface{}) {
		if fn(v) {
			select {
			case result <- v:
			case <-closer:
			}
		}
	})

	// Only allow cancel to be called once.
	var once sync.Once
	cancel = func() {
		once.Do(func() {
			removeHandler()
			close(closer)
		})
	}
	out = result

	return
}

// AddHandler adds the handler, returning a function that would remove this
// handler when called.
func (h *Handler) AddHandler(handler interface{}) (rm func()) {
	rm, err := h.addHandler(handler)
	if err != nil {
		panic(err)
	}
	return rm
}

// AddHandlerCheck adds the handler, but safe-guards reflect panics with a
// recoverer, returning the error.
func (h *Handler) AddHandlerCheck(handler interface{}) (rm func(), err error) {
	// Reflect would actually panic if anything goes wrong, so this is just in
	// case.
	defer func() {
		if rec := recover(); rec != nil {
			if recErr, ok := rec.(error); ok {
				err = recErr
			} else {
				err = fmt.Errorf("%v", rec)
			}
		}
	}()

	return h.addHandler(handler)
}

func (h *Handler) addHandler(fn interface{}) (rm func(), err error) {
	// Reflect the handler
	r, err := reflectFn(fn)
	if err != nil {
		return nil, errors.Wrap(err, "Handler reflect failed")
	}

	h.hmutex.Lock()
	defer h.hmutex.Unlock()

	// Get the current counter value and increment the counter:
	serial := h.hserial
	h.hserial++

	// Create a map if there's none:
	if h.handlers == nil {
		h.handlers = map[uint64]handler{}
	}

	// Use the serial for the map:
	h.handlers[serial] = *r

	// Append the serial into the list of keys:
	h.horders = append(h.horders, serial)

	return func() {
		h.hmutex.Lock()
		defer h.hmutex.Unlock()

		// Delete the handler from the map:
		delete(h.handlers, serial)

		// Delete the key from the orders slice:
		for i, order := range h.horders {
			if order == serial {
				h.horders = append(h.horders[:i], h.horders[i+1:]...)
				break
			}
		}
	}, nil
}

type handler struct {
	event    reflect.Type
	callback reflect.Value
	isIface  bool
}

func reflectFn(function interface{}) (*handler, error) {
	fnV := reflect.ValueOf(function)
	fnT := fnV.Type()

	if fnT.Kind() != reflect.Func {
		return nil, errors.New("given interface is not a function")
	}

	if fnT.NumIn() != 1 {
		return nil, errors.New("function can only accept 1 event as argument")
	}

	if fnT.NumOut() > 0 {
		return nil, errors.New("function can't accept returns")
	}

	argT := fnT.In(0)
	kind := argT.Kind()

	// Accept either pointer type or interface{} type
	if kind != reflect.Ptr && kind != reflect.Interface {
		return nil, errors.New("first argument is not pointer")
	}

	return &handler{
		event:    argT,
		callback: fnV,
		isIface:  kind == reflect.Interface,
	}, nil
}

func (h handler) not(event reflect.Type) bool {
	if h.isIface {
		return !event.Implements(h.event)
	}

	return h.event != event
}

func (h handler) call(event reflect.Value) {
	h.callback.Call([]reflect.Value{event})
}