package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"time"
)

// Example log output:
// 127.0.0.1 - - [28/Oct/2016:18:35:05 -0400] "GET / HTTP/1.1" 200 13 "" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36"
// 127.0.0.1 - - [28/Oct/2016:18:35:05 -0400] "GET /favicon.ico HTTP/1.1" 404 10 "http://localhost:8080/" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36"
func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path != "/" {
			http.Error(w, "Not Found", http.StatusNotFound)
			return
		}
		fmt.Fprintln(w, "Hello World!")
	})
	log.Println("Serving...")
	// Logger takes an io.Writer and an http.Handler function to wrap:
	http.ListenAndServe(":8080", Logger(os.Stderr, http.DefaultServeMux))
}

// Logs incoming requests, including response status.
func Logger(out io.Writer, h http.Handler) http.Handler {
	logger := log.New(out, "", 0)
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		o := &responseObserver{ResponseWriter: w}
		h.ServeHTTP(o, r)
		addr := r.RemoteAddr
		if i := strings.LastIndex(addr, ":"); i != -1 {
			addr = addr[:i]
		}
		logger.Printf("%s - - [%s] %q %d %d %q %q",
			addr,
			time.Now().Format("02/Jan/2006:15:04:05 -0700"),
			fmt.Sprintf("%s %s %s", r.Method, r.URL, r.Proto),
			o.status,
			o.written,
			r.Referer(),
			r.UserAgent())
	})
}

type responseObserver struct {
	http.ResponseWriter
	status      int
	written     int64
	wroteHeader bool
}

func (o *responseObserver) Write(p []byte) (n int, err error) {
	if !o.wroteHeader {
		o.WriteHeader(http.StatusOK)
	}
	n, err = o.ResponseWriter.Write(p)
	o.written += int64(n)
	return
}

func (o *responseObserver) WriteHeader(code int) {
	o.ResponseWriter.WriteHeader(code)
	if o.wroteHeader {
		return
	}
	o.wroteHeader = true
	o.status = code
}