package ratelimit

import (
	"errors"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	cache "github.com/go-pkgz/expirable-cache"

	"go.pkg.cx/middleware"
)

// Errors
var (
	ErrLimitReached = errors.New("limit reached")
)

// DefaultOptions represents default timeout middleware options
var DefaultOptions = Options(
	SetLimit(100),
	SetPeriod(time.Minute*1),
	SetKeyFn(defaultKeyFn),
	SetResponseHandler(RespondWithTooManyRequests),
)

// Options turns a list of option instances into an option
func Options(opts ...Option) Option {
	return func(l *limiter) {
		for _, opt := range opts {
			opt(l)
		}
	}
}

// Option configures timeout middleware
type Option func(l *limiter)

// SetLimit sets request limit
func SetLimit(limit int) Option {
	if limit < 1 {
		panic("rate limit middleware expects limit > 0")
	}

	return func(l *limiter) {
		l.limit = limit
	}
}

// SetPeriod sets limiter period
func SetPeriod(period time.Duration) Option {
	return func(l *limiter) {
		l.period = period
	}
}

// SetKeyFn sets limiter key extraction function
func SetKeyFn(fn func(r *http.Request) string) Option {
	return func(l *limiter) {
		l.keyFn = fn
	}
}

// SetResponseHandler sets response handler
func SetResponseHandler(fn middleware.ResponseHandle) Option {
	return func(l *limiter) {
		l.responseHandler = fn
	}
}

type info struct {
	limit     int
	remaining int
	reset     int64
	reached   bool
}

type entry struct {
	count      int
	expiration time.Time
}

type limiter struct {
	limit           int
	period          time.Duration
	keyFn           func(r *http.Request) string
	responseHandler middleware.ResponseHandle

	lock  sync.Mutex
	cache cache.Cache
}

func (s *limiter) initCache() {
	c, err := cache.NewCache(cache.TTL(s.period))
	if err != nil {
		panic(err)
	}
	s.cache = c
}

func (s *limiter) try(key string) info {
	s.lock.Lock()
	defer s.lock.Unlock()

	now := time.Now()

	if e, ok := s.cache.Get(key); ok {
		e.(*entry).count++
		s.cache.Set(key, e, 0)

		return s.infoFromEntry(e.(*entry))
	}

	e := &entry{count: 1, expiration: now.Add(s.period)}
	s.cache.Set(key, e, 0)

	return s.infoFromEntry(e)
}

func (s *limiter) infoFromEntry(e *entry) info {
	reached := true
	remaining := 0

	if e.count <= s.limit {
		reached = false
		remaining = s.limit - e.count
	}

	return info{
		limit:     s.limit,
		remaining: remaining,
		reset:     e.expiration.Unix(),
		reached:   reached,
	}
}

// Middleware is a rate limiter middleware
func Middleware(opts ...Option) func(next http.Handler) http.Handler {
	l := &limiter{}
	opts = append([]Option{DefaultOptions}, opts...)

	for _, opt := range opts {
		opt(l)
	}

	l.initCache()

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			info := l.try(l.keyFn(r))

			w.Header().Add("X-RateLimit-Limit", strconv.Itoa(info.limit))
			w.Header().Add("X-RateLimit-Remaining", strconv.Itoa(info.remaining))
			w.Header().Add("X-RateLimit-Reset", strconv.FormatInt(info.reset, 10))

			if info.reached {
				l.responseHandler(w, r, ErrLimitReached)
				return
			}

			next.ServeHTTP(w, r)
		})
	}
}

// RespondWithTooManyRequests is a default response handler
func RespondWithTooManyRequests(w http.ResponseWriter, r *http.Request, err error) {
	http.Error(w, http.StatusText(http.StatusTooManyRequests), http.StatusTooManyRequests)
}

func defaultKeyFn(r *http.Request) string {
	return strings.Split(r.RemoteAddr, ":")[0]
}