// This file is part of birdtown-visit-counter, and is © 2024 Alexander Khodyrev.
//
// Full source code is available at https://git.akho.name/akho/birdtown-visit-counter
//
// birdtown-visit-counter is free software: you can redistribute it and/or
// modify it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the License,
// or (at your option) any later version.
//
// birdtown-visit-counter is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
// or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License
// for more details.
//
// You should have received a copy of the GNU Affero General Public License along
// with birdtown-visit-counter. If not, see <https://www.gnu.org/licenses/>.

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
}

func commonHeaders(w http.ResponseWriter) {
	// 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("Allow", "POST, GET, OPTIONS")
}

func (store *visitsStore) handleOptions(w http.ResponseWriter, r *http.Request) {
	commonHeaders(w)
	w.WriteHeader(http.StatusOK)
}

func (store *visitsStore) handlePost(w http.ResponseWriter, r *http.Request) {
	commonHeaders(w)
	w.Header().Set("content-type", "application/json")
	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(store.visit(s))))
}

func (store *visitsStore) handleGet(w http.ResponseWriter, r *http.Request) {
	commonHeaders(w)
	w.Header().Set("content-type", "application/json")
	store.Lock()
	jsonBytes, err := json.Marshal(store.m)
	store.Unlock()
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		w.Write([]byte("cannot serialize"))
		return
	}
	w.WriteHeader(http.StatusOK)
	w.Write(jsonBytes)
}

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{}}

	http.HandleFunc("GET /", store.handleGet)
	http.HandleFunc("POST /", store.handlePost)
	http.HandleFunc("OPTIONS /", store.handleOptions)

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