/* Copyright 2018 Martin Dosch
Licensed under the "MIT License" */

package main

import (
	"encoding/json"
	"hash/fnv"
	"log"
	"os"
	"os/user"
	"strconv"
	"strings"
	"time"

	"github.com/jaytaylor/html2text"
	"github.com/mmcdole/gofeed"
)

// Get new articles for specified feed.
func getArticles(feedURL string, max int, noExcerpt bool) (string, error) {

	type feedCache struct {
		LastChange string
	}

	var output, cachePath string
	var last time.Time
	var lastUpdate feedCache
	var file *os.File
	var updateTime time.Time

	// Get systems user cache path.
	osCacheDir := os.Getenv("$XDG_CACHE_HOME")
	if osCacheDir != "" {
		// Create configPath if not yet existing.
		cachePath = osCacheDir + "/feed-to-muc/"
		if _, err := os.Stat(cachePath); os.IsNotExist(err) {
			err = os.MkdirAll(cachePath, 0700)
			if err != nil {
				log.Fatal("Error: Can't create cache path:", err)
			}
		}

	} else { // Get the current user.
		curUser, err := user.Current()
		if err != nil {
			log.Fatal("Error: Can't get current user:", err)
			return "", err
		}
		// Get home directory.
		home := curUser.HomeDir

		if home == "" {
			log.Fatal("Error: No home directory available.")
			return "", err
		}

		// Create cachePath if not yet existing.
		cachePath = home + "/.cache/feed-to-muc/"
		if _, err := os.Stat(cachePath); os.IsNotExist(err) {
			err = os.MkdirAll(cachePath, 0700)
			if err != nil {
				log.Fatal("Error: Can't create cache path:", err)
			}
		}

	}

	// Create a hash as identifier for the feed.
	// The identifier will be used as filename for caching the update time.
	h := fnv.New32a()
	_, err := h.Write([]byte(feedURL))
	if err != nil {
		log.Fatal("Error: Can't create hash for", feedURL+":", err)
	}
	if _, err := os.Stat(cachePath); os.IsNotExist(err) {
		err = os.MkdirAll(cachePath, 0700)
		if err != nil {
			log.Fatal("Error: Can't create hash identifier for cache file:", err)
		}
	}

	cacheFile := cachePath + strconv.Itoa(int(h.Sum32()))

	if _, err := os.Stat(cacheFile); os.IsNotExist(err) {
		file, err = os.Create(cacheFile)
		if err != nil {
			log.Fatal("Error: Can't create cache file:", err)
		}
		defer file.Close()

		last = time.Now()

		lastUpdate.LastChange = last.Format(time.RFC3339)

		lastUpdateJSON, _ := json.MarshalIndent(lastUpdate, "", " ")
		_, err = file.Write(lastUpdateJSON)
		if err != nil {
			log.Fatal("Error: Can't write last update time stamp to cache file:", err)
		}

	} else {

		file, err = os.OpenFile(cacheFile, os.O_RDWR, 0600)
		if err != nil {
			log.Fatal("Error: Can't open cache file:", err)
		}
		defer file.Close()

		decoder := json.NewDecoder(file)
		lastUpdate := feedCache{}
		if err := decoder.Decode(&lastUpdate); err != nil {
			log.Fatal("Error: Can't decode laste updates time stamp:", err)
		}

		last, err = time.Parse(time.RFC3339, string(lastUpdate.LastChange))
		if err != nil {
			log.Fatal("Error: Can't parse last updates time stamp:", err)
		}
	}

	fp := gofeed.NewParser()
	feed, err := fp.ParseURL(feedURL)
	if err != nil {
		// Don't return an error, but log a message as the
		// bot should not crash when the feed is not available.
		log.Println(feedURL, ": Feed not available.")
		return "", nil
	}

	// If no publish date is offered try update date.
	// If both is not offered give up.
	if feed.Items[0].PublishedParsed == nil {
		if feed.Items[0].UpdatedParsed == nil {
			return "", err
		}
		// If cached timestamp is newer than the one of
		// the last article return.
		if last.After(*feed.Items[0].UpdatedParsed) {
			return "", err
		}
	} else {
		// If cached timestamp is newer than the one of
		// the last article return.
		if last.After(*feed.Items[0].PublishedParsed) {
			return "", err
		}
	}

	// Check last n (defined in config) articles for new ones.
	for i := max - 1; i >= 0; i-- {
		// Stop processing for article i if there are not so
		// many articles in the feed.
		if len(feed.Items) < i+1 {
			continue
		}
		article := *feed.Items[i]
		if err != nil {
			return "", err
		}

		if article.PublishedParsed == nil {
			updateTime = *article.UpdatedParsed
		} else {
			updateTime = *article.PublishedParsed
		}

		// If cached timestamp is not older than the article stop processing.
		// Note: Checking for cached timestamp being newer, instead of not older
		// lead to duplicate messages for the same article. Probably a corner
		// case when the time is identical.
		if !last.Before(updateTime) {
			continue
		}

		last = updateTime
		lastUpdate.LastChange = updateTime.Format(time.RFC3339)

		// Remove file with cached timestamp and create it
		// again with updated timestamp.
		// ToDo: Replace timestamp without deleting.
		err = os.Remove(cacheFile)
		if err != nil {
			log.Fatal("Error: Can't delete cache file:", err)
		}

		file, err = os.Create(cacheFile)
		if err != nil {
			log.Fatal("Error: Can't create cache file:", err)
		}
		defer file.Close()

		lastUpdateJSON, _ := json.MarshalIndent(lastUpdate, "", " ")
		_, err = file.Write(lastUpdateJSON)
		if err != nil {
			log.Fatal("Error: Can't write last update time stamp to cache file:", err)
		}

		// Remove redirects and tracking parameters from URL.
		cleanURL, _ := removeTracking(article.Link)
		// Don't process mastodon messages that are a reply
		// Strip HTML as we want to get plain text.
		mastodonContent := strings.Replace(article.Description, `</span><span class="ellipsis">`, "", -1)
		mastodonContent = strings.Replace(mastodonContent, `</span><span class="invisible">`, "", -1)
		mastodonContent = strings.Replace(mastodonContent, `</span><span class="">`, "", -1)
		mastodonContent = strings.Replace(mastodonContent, `</span>`, "", -1)
		mastodonContent = strings.Replace(mastodonContent, `<span>`, "", -1)
		mastodonContent, err = html2text.FromString(mastodonContent, html2text.Options{OmitLinks: true, TextOnly: true})
		if err != nil {
			return "", err
		}

		// If the content is empty after html2text (e.g. consisting of an image only)
		// then stop processing
		if strings.Replace(mastodonContent, " ", "", -1) == "" {
			continue
		}

		output = output + mastodonContent + "\n\n" + cleanURL

		if i > 0 {
			output = output + "\n\n---\n\n"
		}
	}

	return output, err
}