// RSS rendering for the recommendations feed: hand-rolled RSS 2.0 via
// encoding/xml, no feed library. The render source is the runs.jsonl tail;
// items appear newest batch first, kept order within a batch. Guid embeds
// the original Miniflux entry ID (mf:<id>) for traceability.
package cmds

import (
	"encoding/xml"
	"fmt"
	"time"
	"unicode/utf8"

	"fluxrec/core"
)

// Cap on the source-feed name in item titles: feed titles can be
// arbitrarily long, and the suffix must stay compact so list views keep
// showing the article title itself.
const maxFeedTitleRunes = 40

const rssXMLHeader = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"

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

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

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

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

// RenderRSS renders runs (oldest to newest, as read from runs.jsonl) as an
// RSS 2.0 document. Each item title gets the source feed as a rune-capped
// suffix (readers show the channel title as the source of every item, so
// this is the only in-list provenance), and the description body opens
// with feed + model score + the batch-wide median score. Item content was
// already cleaned when the run row was written.
func RenderRSS(runs []core.RunRow) ([]byte, error) {
	doc := rssDocument{
		Version: "2.0",
		Channel: rssChannel{
			Title:       "fluxrec recommendations",
			Description: "Top-scored Miniflux entries, refreshed on a schedule.",
		},
	}
	for i := len(runs) - 1; i >= 0; i-- {
		for _, k := range runs[i].Kept {
			doc.Channel.Items = append(doc.Channel.Items, rssItem{
				Title:       rssItemTitle(k),
				Link:        k.URL,
				GUID:        rssGUID{IsPermaLink: false, Value: fmt.Sprintf("mf:%d", k.EntryID)},
				PubDate:     k.PublishedAt.Format(time.RFC1123Z),
				Description: rssDescription(k, runs[i].ScoreMedian),
			})
		}
	}
	body, err := xml.MarshalIndent(doc, "", "  ")
	if err != nil {
		return nil, fmt.Errorf("marshaling RSS: %w", err)
	}
	return append([]byte(rssXMLHeader), body...), nil
}

// rssItemTitle builds the displayed title: article title first, then the
// (rune-capped) source feed as a suffix so narrow list views truncate the
// provenance, not the title itself.
func rssItemTitle(k core.KeptItem) string {
	feed := truncateRunes(k.FeedTitle, maxFeedTitleRunes)
	if feed == "" {
		return k.Title
	}
	return k.Title + " — " + feed
}

func truncateRunes(s string, max int) string {
	if utf8.RuneCountInString(s) <= max {
		return s
	}
	return string([]rune(s)[:max]) + "…"
}

// rssDescription builds the item body: first a headline line with the
// source feed, model score, and batch-wide median (omitted for run rows
// written before the median was logged), then the cleaned content (if any).
func rssDescription(k core.KeptItem, median *float64) string {
	head := fmt.Sprintf("score %.2f", k.Score)
	if median != nil {
		head += fmt.Sprintf(" · batch median %.2f", *median)
	}
	if k.FeedTitle != "" {
		head = k.FeedTitle + " · " + head
	}
	if k.Content == "" {
		return head
	}
	return head + "\n\n" + k.Content
}