package main

import (
	"encoding/json"
	"flag"
	"log"
	"net/http"
	"os"
	"strconv"
	"sync"
)

type visitsStore struct {
	m map[string]int
	*sync.RWMutex
}

func (s *visitsStore) visit(ending string) int {
	s.Lock()
	defer s.Unlock()
	oldVisits, has := s.m[ending]
	var newVisits int
	if has {
		newVisits = oldVisits + 1
	} else {
		newVisits = 1
	}
	s.m[ending] = newVisits
	return newVisits
}

type visitsHandler struct {
	store *visitsStore
}

func (h *visitsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// CORS: this is expected to be called from a different domain,
	// hoops require jumping
	w.Header().Set("Access-Control-Allow-Origin", "*")
	w.Header().Set("Access-Control-Max-Age", "86400")
	w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS")
	w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization")

	w.Header().Set("content-type", "application/json")

	if r.Method == http.MethodPost {
		var s string
		if err := json.NewDecoder(r.Body).Decode(&s); err != nil {
			w.WriteHeader(http.StatusInternalServerError)
			w.Write([]byte("cannot parse data"))
			return
		}
		w.WriteHeader(http.StatusOK)
		w.Write(([]byte)(strconv.Itoa(h.store.visit(s))))
		return
	}
	if r.Method == http.MethodGet {
		jsonBytes, err := json.Marshal(h.store.m)
		if err != nil {
			w.WriteHeader(http.StatusInternalServerError)
			w.Write([]byte("cannot serialize"))
			return
		}
		w.WriteHeader(http.StatusOK)
		w.Write(jsonBytes)
		return
	}
	if r.Method == http.MethodOptions {
		w.WriteHeader(http.StatusOK)
		return
	}
	w.Header().Set("Allow", "POST, GET, OPTIONS")
	w.WriteHeader(http.StatusMethodNotAllowed)
}

func main() {
	addr := flag.String("addr", ":11568", "Listen address")
	initialFile := flag.String("file", "visits.json", "File with initial data in JSON format")
	flag.Parse()

	initialVisits := make(map[string]int)
	initialVisitsJson, err := os.ReadFile(*initialFile)
	if err == nil {
		json.Unmarshal(initialVisitsJson, &initialVisits)
	} else {
		log.Println("initial file", *initialFile, "could not be read, defaulting to empty data")
	}

	store := visitsStore{m: initialVisits, RWMutex: &sync.RWMutex{}}
	visitsH := &visitsHandler{store: &store}
	http.Handle("/visits", visitsH)

	log.Println("Starting web server...")
	http.ListenAndServe(*addr, nil)
}