// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package feeds renders the RSS 2.0, Atom 1.0, JSON Feed 1.1 and
// sitemap documents for the public API.
package feeds

import (
	"bytes"
	json "encoding/json/v2"
	"fmt"
	"html"
	"log/slog"
	"os"
	"strings"
	"time"

	"sourcedock.dev/petrbalvin/volumen/internal/config"
	"sourcedock.dev/petrbalvin/volumen/internal/post"
)

// FeedItemLimit caps the number of items in a feed.
const FeedItemLimit = 20

// SitemapURLLimit caps the URLs in one sitemap document: the sitemaps.org
// protocol defines 50 000 as the maximum a single document may carry, so
// a larger site truncates to the newest posts rather than shipping a
// document consumers may refuse whole.
const SitemapURLLimit = 50_000

// language returns the site language, defaulting to English.
func language(site config.Site) string {
	if site.Language != "" {
		return site.Language
	}
	return DefaultLanguage
}

// DefaultLanguage is the feed language when [site].language is empty.
const DefaultLanguage = "en"

// RenderRSSFeed renders an RSS 2.0 XML feed for the given posts.
// selfPath is the path of the feed being rendered, so a reader can see
// which document it fetched.
func RenderRSSFeed(posts []*post.Post, site config.Site, baseURL, selfPath string) string {
	items := make([]string, 0, min(len(posts), FeedItemLimit))
	for _, p := range posts[:min(len(posts), FeedItemLimit)] {
		items = append(items, rssItem(p, baseURL))
	}
	return fmt.Sprintf(`<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:atom="http://www.w3.org/2005/Atom">
  <channel>
    <title>%s</title>
    <link>%s</link>
    <description>%s</description>
    <language>%s</language>
    <atom:link rel="self" type="application/rss+xml" href="%s"/>
%s
  </channel>
</rss>`,
		html.EscapeString(site.Title),
		html.EscapeString(baseURL),
		html.EscapeString(site.Description),
		html.EscapeString(language(site)),
		html.EscapeString(baseURL+selfPath),
		strings.Join(items, "\n"))
}

func rssItem(p *post.Post, baseURL string) string {
	permalink := baseURL + "/" + p.Slug()
	var extra strings.Builder
	if dateStr := p.DateString(); dateStr != "" {
		fmt.Fprintf(&extra, "\n      <pubDate>%s</pubDate>", html.EscapeString(rfc822Date(dateStr)))
	}
	if creator := p.FediverseCreator(); creator != "" {
		fmt.Fprintf(&extra, "\n      <dc:creator>%s</dc:creator>", html.EscapeString(creator))
	}
	if lang := p.Lang(); lang != "" {
		fmt.Fprintf(&extra, "\n      <dc:language>%s</dc:language>", html.EscapeString(lang))
	}
	return fmt.Sprintf(`    <item>
      <title>%s</title>
      <link>%s</link>
      <guid>%s</guid>
      <description>%s</description>%s
    </item>`,
		html.EscapeString(p.Title()),
		html.EscapeString(permalink),
		html.EscapeString(permalink),
		html.EscapeString(p.Excerpt()),
		extra.String())
}

// rfc822Date formats a YYYY-MM-DD string as an RFC 822 timestamp, or
// returns it unchanged when unparseable.
func rfc822Date(value string) string {
	t, err := time.Parse("2006-01-02", value)
	if err != nil {
		return value
	}
	return t.UTC().Format("Mon, 02 Jan 2006 15:04:05") + " GMT"
}

// rfc3339Date formats a YYYY-MM-DD string as RFC 3339 at UTC midnight,
// or returns it unchanged when it cannot be parsed.
func rfc3339Date(value string) string {
	t, err := time.Parse("2006-01-02", value)
	if err != nil {
		return value
	}
	return t.UTC().Format("2006-01-02T15:04:05-07:00")
}

// RenderAtomFeed renders an Atom 1.0 XML feed for the given posts.
// selfPath is the path of the feed being rendered, so the rel=self link
// names the document the client actually fetched rather than always the
// site-wide feed.
func RenderAtomFeed(posts []*post.Post, site config.Site, baseURL, selfPath string) string {
	title := html.EscapeString(site.Title)
	link := html.EscapeString(baseURL)
	feedURL := html.EscapeString(baseURL + selfPath)
	description := html.EscapeString(site.Description)
	updated := atomUpdated(posts)

	items := make([]string, 0, min(len(posts), FeedItemLimit))
	for _, p := range posts[:min(len(posts), FeedItemLimit)] {
		items = append(items, atomEntry(p, baseURL))
	}
	return fmt.Sprintf(`<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>%s</title>
  <link rel="alternate" type="text/html" href="%s"/>
  <link rel="self" type="application/atom+xml" href="%s"/>
  <id>%s/</id>
  <updated>%s</updated>
  <subtitle>%s</subtitle>
%s
</feed>`,
		title, link, feedURL, link,
		html.EscapeString(updated), description,
		strings.Join(items, "\n"))
}

func atomUpdated(posts []*post.Post) string {
	for _, p := range posts {
		if dateStr := p.DateString(); dateStr != "" {
			return rfc3339Date(dateStr)
		}
	}
	// No post carries a date, so the feed has no update time of its own.
	// The epoch is used rather than the current time, because a document
	// that changes on every request defeats every cache in front of it.
	return "1970-01-01T00:00:00Z"
}

func atomEntry(p *post.Post, baseURL string) string {
	permalink := baseURL + "/" + p.Slug()
	updated, published := "", ""
	if dateStr := p.DateString(); dateStr != "" {
		updated = rfc3339Date(dateStr)
		published = updated
	}
	authorTag := ""
	if creator := p.FediverseCreator(); creator != "" {
		authorTag = fmt.Sprintf("\n    <author><name>%s</name></author>", html.EscapeString(creator))
	}
	langAttr := ""
	if lang := p.Lang(); lang != "" {
		langAttr = fmt.Sprintf(` xml:lang="%s"`, html.EscapeString(lang))
	}
	return fmt.Sprintf(`  <entry>
    <title%s>%s</title>
    <link rel="alternate" type="text/html" href="%s"/>
    <id>%s</id>
    <updated>%s</updated>
    <published>%s</published>
    <summary>%s</summary>%s
  </entry>`,
		langAttr, html.EscapeString(p.Title()),
		html.EscapeString(permalink), html.EscapeString(permalink),
		updated, published,
		html.EscapeString(p.Excerpt()), authorTag)
}

// JSONFeed is a JSON Feed 1.1 document. Empty optional fields are
// omitted, matching the feed specification.
type JSONFeed struct {
	Version     string     `json:"version"`
	Title       string     `json:"title"`
	HomePageURL string     `json:"home_page_url"`
	FeedURL     string     `json:"feed_url"`
	Description string     `json:"description"`
	Language    string     `json:"language"`
	Items       []JSONItem `json:"items,omitempty"`
}

// JSONItem is one JSON Feed item. Empty optional fields are omitted.
type JSONItem struct {
	ID            string       `json:"id"`
	URL           string       `json:"url"`
	Title         string       `json:"title"`
	ContentHTML   string       `json:"content_html"`
	Summary       string       `json:"summary,omitempty"`
	DatePublished string       `json:"date_published,omitempty"`
	Tags          []string     `json:"tags,omitempty"`
	Authors       []JSONAuthor `json:"authors,omitempty"`
}

// JSONAuthor is the author entry of a JSON Feed item.
type JSONAuthor struct {
	Name string `json:"name"`
}

// RenderJSONFeed builds a JSON Feed 1.1 document for the given posts.
// selfPath is the path of the feed being rendered, for feed_url.
func RenderJSONFeed(posts []*post.Post, site config.Site, baseURL, selfPath string) JSONFeed {
	limit := min(len(posts), FeedItemLimit)
	items := make([]JSONItem, 0, limit)
	for _, p := range posts[:limit] {
		items = append(items, jsonFeedItem(p, baseURL, site))
	}
	return JSONFeed{
		Version:     "https://jsonfeed.org/version/1.1",
		Title:       site.Title,
		HomePageURL: baseURL,
		FeedURL:     baseURL + selfPath,
		Description: site.Description,
		Language:    language(site),
		Items:       items,
	}
}

func jsonFeedItem(p *post.Post, baseURL string, site config.Site) JSONItem {
	permalink := baseURL + "/" + p.Slug()
	htmlOut, err := p.HTML()
	if err != nil {
		// The item still goes out (a feed with a body-less entry beats a
		// feed that 500s for one bad post), but not silently: the detail
		// endpoint fails loudly for the same post, and the feed should
		// leave the same trace.
		slog.Warn("feeds: cannot render post for the JSON feed", "slug", p.Slug(), "error", err)
		htmlOut = ""
	}
	item := JSONItem{
		ID:            permalink,
		URL:           permalink,
		Title:         p.Title(),
		ContentHTML:   htmlOut,
		Summary:       p.Excerpt(),
		DatePublished: p.DateString(),
		Tags:          p.Tags(),
	}
	author := p.FediverseCreator()
	if author == "" {
		author = site.FediverseCreator
	}
	if author != "" {
		item.Authors = []JSONAuthor{{Name: author}}
	}
	return item
}

// MarshalJSONFeed serialises a feed. encoding/json/v2 escapes only what
// JSON requires, so the HTML inside content_html reaches the client as the
// post was rendered rather than with every angle bracket escaped.
func MarshalJSONFeed(feed JSONFeed) ([]byte, error) {
	var buf bytes.Buffer
	if err := json.MarshalWrite(&buf, feed, json.Deterministic(true)); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// RenderSitemap renders an XML sitemap, preferring the file
// modification time for <lastmod>. At most SitemapURLLimit URLs are
// included, newest posts first (the caller lists them that way).
func RenderSitemap(posts []*post.Post, baseURL string) string {
	var urls []string
	for _, p := range posts[:min(len(posts), SitemapURLLimit)] {
		entry := "  <url><loc>" + html.EscapeString(baseURL+"/"+p.Slug()) + "</loc>"
		if lastmod := lastmodFor(p); lastmod != "" {
			entry += "<lastmod>" + html.EscapeString(lastmod) + "</lastmod>"
		}
		entry += "</url>"
		urls = append(urls, entry)
	}
	return `<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
` + strings.Join(urls, "\n") + `
</urlset>`
}

func lastmodFor(p *post.Post) string {
	if p.Path != "" {
		if info, err := os.Stat(p.Path); err == nil {
			return info.ModTime().UTC().Format("2006-01-02")
		}
	}
	return p.DateString()
}