package middleware

import (
	"bufio"
	"fmt"
	"net"
	"net/http"
	"regexp"
	"strconv"
	"strings"
	"time"

	"github.com/gorilla/mux"
	"github.com/prometheus/client_golang/prometheus"
)

// RouteMatcher matches routes
type RouteMatcher interface {
	Match(*http.Request, *mux.RouteMatch) bool
}

// Instrument is a Middleware which records timings for every HTTP request
type Instrument struct {
	RouteMatcher RouteMatcher
	Duration     *prometheus.HistogramVec
}

// IsWSHandshakeRequest returns true if the given request is a websocket handshake request.
func IsWSHandshakeRequest(req *http.Request) bool {
	if strings.ToLower(req.Header.Get("Upgrade")) == "websocket" {
		// Connection header values can be of form "foo, bar, ..."
		parts := strings.Split(strings.ToLower(req.Header.Get("Connection")), ",")
		for _, part := range parts {
			if strings.TrimSpace(part) == "upgrade" {
				return true
			}
		}
	}
	return false
}

// Wrap implements middleware.Interface
func (i Instrument) Wrap(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		begin := time.Now()
		isWS := strconv.FormatBool(IsWSHandshakeRequest(r))
		interceptor := &interceptor{ResponseWriter: w, statusCode: http.StatusOK}
		route := i.getRouteName(r)
		next.ServeHTTP(interceptor, r)
		var (
			status = strconv.Itoa(interceptor.statusCode)
			took   = time.Since(begin)
		)
		i.Duration.WithLabelValues(r.Method, route, status, isWS).Observe(took.Seconds())
	})
}

// Return a name identifier for ths request.  There are three options:
//   1. The request matches a gorilla mux route, with a name.  Use that.
//   2. The request matches an unamed gorilla mux router.  Munge the path
//      template such that templates like '/api/{org}/foo' come out as
//      'api_org_foo'.
//   3. The request doesn't match a mux route. Return "other"
// We do all this as we do not wish to emit high cardinality labels to
// prometheus.
func (i Instrument) getRouteName(r *http.Request) string {
	var routeMatch mux.RouteMatch
	if i.RouteMatcher != nil && i.RouteMatcher.Match(r, &routeMatch) {
		if name := routeMatch.Route.GetName(); name != "" {
			return name
		}
		if tmpl, err := routeMatch.Route.GetPathTemplate(); err == nil {
			return MakeLabelValue(tmpl)
		}
	}
	return "other"
}

var invalidChars = regexp.MustCompile(`[^a-zA-Z0-9]+`)

// MakeLabelValue converts a Gorilla mux path to a string suitable for use in
// a Prometheus label value.
func MakeLabelValue(path string) string {
	// Convert non-alnums to underscores.
	result := invalidChars.ReplaceAllString(path, "_")

	// Trim leading and trailing underscores.
	result = strings.Trim(result, "_")

	// Make it all lowercase
	result = strings.ToLower(result)

	// Special case.
	if result == "" {
		result = "root"
	}
	return result
}

// interceptor implements WriteHeader to intercept status codes. WriteHeader
// may not be called on success, so initialize statusCode with the status you
// want to report on success, i.e. http.StatusOK.
//
// interceptor also implements net.Hijacker, to let the downstream Handler
// hijack the connection. This is needed, for example, for working with websockets.
type interceptor struct {
	http.ResponseWriter
	statusCode int
	recorded   bool
}

func (i *interceptor) WriteHeader(code int) {
	if !i.recorded {
		i.statusCode = code
		i.recorded = true
	}
	i.ResponseWriter.WriteHeader(code)
}

func (i *interceptor) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	hj, ok := i.ResponseWriter.(http.Hijacker)
	if !ok {
		return nil, nil, fmt.Errorf("interceptor: can't cast parent ResponseWriter to Hijacker")
	}
	return hj.Hijack()
}