package main

import (
	"bytes"
	"encoding/xml"
	"fmt"
	"html"
	"log"
	"strings"
	"time"
)

const rssFilename = "rss.xml"

type rssDocument struct {
	XMLName xml.Name   `xml:"rss"`
	Version string     `xml:"version,attr"`
	Channel rssChannel `xml:"channel"`
}

type rssChannel struct {
	Title       string    `xml:"title"`
	Link        string    `xml:"link"`
	Description string    `xml:"description"`
	Items       []rssItem `xml:"item"`
}

type rssItem struct {
	Title       string  `xml:"title"`
	Link        string  `xml:"link"`
	Description string  `xml:"description,omitempty"`
	PubDate     string  `xml:"pubDate,omitempty"`
	GUID        rssGUID `xml:"guid"`
}

type rssGUID struct {
	IsPermaLink bool   `xml:"isPermaLink,attr"`
	Value       string `xml:",chardata"`
}

func generateRSS() {
	items := make([]rssItem, 0, len(config.Posts))
	for _, post := range config.Posts {
		source := post.Md
		if post.OutsideArticle != "" || source == "" {
			source = post.Thumb
		}

		title := markdownTitle(readFile(directory + "/" + config.PostsDir + source))
		link := post.OutsideArticle
		if link == "" {
			filename := strings.Split(post.Md, ".")[0] + ".html"
			link = joinURL(config.AbsoluteUrl, filename)
		}

		pubDate, err := rssDate(post.Date)
		if err != nil {
			log.Printf("invalid RSS date for %s: %v", source, err)
		}

		items = append(items, rssItem{
			Title:       title,
			Link:        link,
			Description: post.MetaDescr,
			PubDate:     pubDate,
			GUID:        rssGUID{IsPermaLink: true, Value: link},
		})
	}

	document := rssDocument{
		Version: "2.0",
		Channel: rssChannel{
			Title:       config.Title,
			Link:        joinURL(config.AbsoluteUrl),
			Description: config.MetaDescr,
			Items:       items,
		},
	}

	var output bytes.Buffer
	output.WriteString(xml.Header)
	encoder := xml.NewEncoder(&output)
	encoder.Indent("", "  ")
	check(encoder.Encode(document))
	writeFile(config.OutputDir+"/"+rssFilename, output.String())
}

func addRSSDiscovery(document string) string {
	feedURL := joinURL(config.AbsoluteUrl, rssFilename)
	tag := fmt.Sprintf(`<link rel="alternate" type="application/rss+xml" title="%s RSS" href="%s">`,
		html.EscapeString(config.Title), html.EscapeString(feedURL))
	if strings.Contains(document, tag) {
		return document
	}

	headEnd := strings.Index(strings.ToLower(document), "</head>")
	if headEnd == -1 {
		return document
	}
	return document[:headEnd] + tag + "\n" + document[headEnd:]
}

func markdownTitle(markdown string) string {
	firstLine := strings.SplitN(markdown, "\n", 2)[0]
	return strings.TrimSpace(strings.TrimLeft(firstLine, "#"))
}

func joinURL(parts ...string) string {
	if len(parts) == 0 {
		return ""
	}

	joined := strings.TrimRight(parts[0], "/")
	for _, part := range parts[1:] {
		part = strings.Trim(part, "/")
		if part == "" {
			continue
		}
		if joined == "" {
			joined = "/" + part
		} else {
			joined += "/" + part
		}
	}
	return joined
}

func rssDate(value string) (string, error) {
	if value == "" {
		return "", nil
	}

	formats := []string{time.RFC3339, "2006-01-02"}
	for _, format := range formats {
		parsed, err := time.Parse(format, value)
		if err == nil {
			return parsed.Format(time.RFC1123Z), nil
		}
	}
	return "", fmt.Errorf("%q must use RFC 3339 or YYYY-MM-DD format", value)
}