htwkalender/service/ical/ical.go

package ical

import (
	"bytes"
	"crypto/rand"
	"fmt"
	"github.com/jordic/goics"
	"github.com/labstack/echo/v5"
	"github.com/pocketbase/pocketbase"
	"htwk-planner/service/db"
	"log"
	"net/http"
	"time"
)

const expirationTime = 5 * time.Minute

var cache = make(map[string]*FeedModel)

func FeedURL(c echo.Context, app *pocketbase.PocketBase) error {
	token := randomToken(20)
	_, err := createFeedForToken(app, token)
	if err != nil {
		return err
	}
	return c.JSON(http.StatusOK, fmt.Sprintf("FeedToken: %s", token))
}

func Feed(c echo.Context, app *pocketbase.PocketBase) error {

	var result string
	var responseWriter = c.Response().Writer
	token := c.QueryParam("token")
	log.Print("iCal feed Token: " + token)
	feed, ok := cache[token]
	if !ok || feed == nil {
		return c.JSON(http.StatusNotFound, "No FeedModel for this Token")
	}

	result = feed.Content
	if feed.ExpiresAt.Before(time.Now()) {
		newFeed, err := createFeedForToken(app, token)
		if err != nil {
			return c.JSON(http.StatusInternalServerError, err)
		}
		result = newFeed.Content
	}

	responseWriter.Header().Set("Content-type", "text/calendar")
	responseWriter.Header().Set("charset", "utf-8")
	responseWriter.Header().Set("Content-Disposition", "inline")
	responseWriter.Header().Set("filename", "calendar.ics")
	writeSuccess(result, responseWriter)
	c.Response().Writer = responseWriter
	return nil
}

func createFeedForToken(app *pocketbase.PocketBase, token string) (*FeedModel, error) {
	res := db.GetPlanForCourseAndSemester(app, "22INM", "ws")
	b := bytes.Buffer{}
	goics.NewICalEncode(&b).Encode(res)
	feed := &FeedModel{Content: b.String(), ExpiresAt: time.Now().Add(expirationTime)}
	cache[token] = feed
	return feed, nil
}

func randomToken(len int) string {
	b := make([]byte, len)
	read, err := rand.Read(b)
	if err != nil {
		return ""
	}
	return fmt.Sprintf("%x", read)
}

func writeSuccess(message string, w http.ResponseWriter) {
	w.WriteHeader(http.StatusOK)
	w.Write([]byte(message))
}