// Package web contains the web-related features: handlers and middleware
package web

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"os/signal"
	"regexp"
	"strconv"
	"time"

	"github.com/gorilla/mux"

	"code.osinet.fr/fgm/lbc/domain"
	"code.osinet.fr/fgm/lbc/internal"
)

type (
	ConfigKeyType string
	Config        struct {
		internal.Config
		Addr         string
		Base         string // Mount point for service
		WriteTimeout time.Duration
	}
	Server struct {
		BaseContext context.Context

		*Config
		*Stats
		*log.Logger
	}
)

func (c Config) String() string {
	return fmt.Sprintf("Addr: %s, Base URL: %s, Write Timeout: %v",
		c.Addr, c.Base, c.WriteTimeout)
}

const (
	ConfigKey ConfigKeyType = "config"

	// CT is the normalized representation of the content-type header.
	CT = "Content-Type"

	// JSON is the normalized representation of the JSON content-type per RFC4627 §6.
	JSON = "application/json; charset=utf-8"

	PathFB = "/{int1:[0-9]+}/{int2:[0-9]+}/{limit:[0-9]+}/{str1}/{str2}"
)

var (
	utf8       = regexp.MustCompile("utf-8")
	acceptJSON = regexp.MustCompile(`(application/json|\*/\*)`)
)

func argsFromRequest(r *http.Request) (int1, int2, limit int, str1, str2 string, status int) {
	var err error
	v := mux.Vars(r)
	status = http.StatusAccepted
	for k, p := range map[string]*int{"int1": &int1, "int2": &int2, "limit": &limit} {
		sn := v[k] // Cannot fail: route wouldn't match
		*p, err = strconv.Atoi(sn)
		if err != nil || *p < 2 {
			return 0, 0, 0, "", "", http.StatusNotFound
		}
	}
	for k, p := range map[string]*string{"str1": &str1, "str2": &str2} {
		// Cannot fail: route wouldn't match
		*p = v[k]
	}
	return
}

func isJSONAccepted(r *http.Request) bool {
	// Only return to clients accepting UTF-8 or not specifying a charset
	charset := r.Header.Get("Accept-Charset")
	if charset != "" && !utf8.MatchString(charset) {
		return false
	}
	// Only return to clients accepting JSON, anything, or not specifying Accept.
	accept := r.Header.Get("Accept")
	return accept == "" || acceptJSON.MatchString(accept)
}

func MakeFizzBuzz(logger *log.Logger, isVerbose bool) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		// Can we serve client at all?
		if !isJSONAccepted(r) {
			if isVerbose {
				logger.Println("request from client unable to accept JSON")
			}
			http.Error(w, "cannot offer an accepted content type: can only serve UTF-8 JSON", http.StatusNotAcceptable)
			return
		}

		int1, int2, limit, str1, str2, status := argsFromRequest(r)
		if status >= http.StatusBadRequest {
			logger.Printf("Invalid URL requested: %s", r.URL) // Dev-targeted actual error
			http.NotFound(w, r)                               // No human-targeted details to limit enumeration attacks
			return
		}

		if isVerbose {
			logger.Printf("request: %s", r.URL.Path)
		}
		fbs, err := domain.FizzBuzz(int1, int2, limit, str1, str2)
		if err != nil {
			logger.Println(err)                                                                            // Dev-targeted actual error
			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError) // Human-targeted error to user agent
			return
		}
		w.Header().Set(CT, JSON)
		enc := json.NewEncoder(w)
		enc.SetEscapeHTML(true) // Prevent XSS on str1/str2
		enc.SetIndent("", "\t") // Human-readable JSON
		err = enc.Encode(fbs)
		if err != nil {
			logger.Println(err)                                                                            // Dev-targeted actual error
			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError) // human-targeted error to user agent
			return
		}
	}
}

// SetupRouting configures the server route on the specified base path.
func SetupRouting(base string, logger *log.Logger, stats *Stats, isVerbose bool) (http.Handler, error) {
	restBase, err := url.JoinPath("/", base) // Perform base validation, including fixing "" as "/".
	if err != nil {
		return nil, fmt.Errorf("failed building the base path: %w", err)
	}
	sm := http.NewServeMux()
	router := mux.NewRouter()
	sub := router.PathPrefix(restBase).
		Name("base").
		Subrouter()
	sub.Handle(PathFB, stats.Middleware()(MakeFizzBuzz(logger, isVerbose))).
		Name("fizzbuzz")
	sub.Handle("/stats", stats.Handler())
	sm.Handle("/", sub)
	return sm, nil
}

// Serve starts the HTTP listening loop.
func (s *Server) Serve(serverClosed chan domain.Empty) error {
	// Provide access to the globally injected services in handlers.
	baseContext := func(listener net.Listener) context.Context {
		return s.BaseContext
	}
	r, err := SetupRouting(s.Base, s.Logger, s.Stats, s.Verbose)
	if err != nil {
		s.Logger.Fatalf("failed setting up routing: %v", err)
	}
	hs := http.Server{
		Addr:              s.Addr,
		BaseContext:       baseContext,
		ErrorLog:          s.Logger,
		Handler:           r,
		IdleTimeout:       100 * time.Millisecond, // Non-zero to improve benchmarks able to reuse connections.
		MaxHeaderBytes:    8192,                   // Cf. Apache as industry-standard practice.
		ReadHeaderTimeout: 10 * time.Millisecond,  // 8kB@1Mbps = 8 msec
		ReadTimeout:       10 * time.Millisecond,  // This server has no method expecting a body, so keep this small
		TLSConfig:         nil,
		WriteTimeout:      s.WriteTimeout, // 1Mbps*100msec = 100kB, which should be sufficient for this API
	}

	// Setup clean shutdown on signal.
	go func() {
		sigs := make(chan os.Signal, 1)
		signal.Notify(sigs, os.Interrupt)
		<-sigs
		if err := hs.Shutdown(context.Background()); err != nil {
			s.Logger.Printf("web server shutdown: %v", err)
		}
		close(serverClosed)
	}()

	if s.Verbose {
		s.Logger.Printf("listening with config %s", s.Config)
	}

	go func() {
		err = hs.ListenAndServe()
		switch err {
		case http.ErrServerClosed:
			if s.Verbose {
				s.Logger.Printf("http server shut down cleanly")
			}
			err = nil
		default:
			s.Logger.Printf("http server unexpected error: %v", err)
		}
	}()
	<-serverClosed
	return err
}