// Package novelfire provides a NovelScraper implementation for novelfire.net.
//
// Site structure (as of 2025):
//
//	Catalogue : https://novelfire.net/genre-all/sort-new/status-all/all-novel?page=N
//	Book page : https://novelfire.net/book/{slug}
//	Chapters  : https://novelfire.net/book/{slug}/chapters?page=N
//	Chapter   : https://novelfire.net/book/{slug}/{chapter-slug}
package novelfire

import (
	"context"
	"fmt"
	"log/slog"
	"net/url"
	"path"
	"strconv"
	"strings"
	"time"

	"github.com/libnovel/scraper/internal/browser"
	"github.com/libnovel/scraper/internal/scraper"
	"github.com/libnovel/scraper/internal/scraper/htmlutil"
	"golang.org/x/net/html"
)

const (
	baseURL       = "https://novelfire.net"
	cataloguePath = "/genre-all/sort-new/status-all/all-novel"
	rankingPath   = "/genre-all/sort-popular/status-all/all-novel"
)

// rejectResourceTypes lists Browserless resource types to block on every request.
// We keep: document (the page), script (JS renders the DOM), fetch/xhr (JS data calls).
// Everything else is safe to drop for HTML-only scraping.
var rejectResourceTypes = []string{
	"cspviolationreport",
	"eventsource",
	"fedcm",
	"font",
	"image",
	"manifest",
	"media",
	"other",
	"ping",
	"signedexchange",
	"stylesheet",
	"texttrack",
	"websocket",
}

// RankingStore is the subset of storage.Store consumed by ScrapeRanking.
type RankingStore interface {
	WriteRankingItem(ctx context.Context, item scraper.RankingItem) error
	RankingFreshEnough(ctx context.Context, maxAge time.Duration) (bool, error)
}

// Scraper is the novelfire.net implementation of scraper.NovelScraper.
// It uses the /content strategy by default (rendered HTML via Browserless).
type Scraper struct {
	client        browser.BrowserClient
	urlClient     browser.BrowserClient // separate client for URL retrieval (uses browserless content strategy)
	chapterClient browser.BrowserClient // direct HTTP client for chapter text (no JS rendering needed)
	rankingStore  RankingStore
	log           *slog.Logger
}

// New returns a new novelfire Scraper.
// client is used for catalogue/metadata/ranking fetching (Browserless).
// urlClient is used for chapter list pagination; falls back to client if nil.
// chapterClient is used for chapter text fetching (plain HTTP); falls back to client if nil.
// rankingStore is optional; pass nil to disable freshness checks and per-item persistence.
func New(client browser.BrowserClient, log *slog.Logger, urlClient browser.BrowserClient, chapterClient browser.BrowserClient, rankingStore RankingStore) *Scraper {
	if log == nil {
		log = slog.Default()
	}
	if urlClient == nil {
		urlClient = client
	}
	if chapterClient == nil {
		chapterClient = client
	}
	return &Scraper{client: client, urlClient: urlClient, chapterClient: chapterClient, rankingStore: rankingStore, log: log}
}

// SourceName implements NovelScraper.
func (s *Scraper) SourceName() string { return "novelfire.net" }

// ─── CatalogueProvider ───────────────────────────────────────────────────────

// ScrapeCatalogue streams all CatalogueEntry values across all pages.
func (s *Scraper) ScrapeCatalogue(ctx context.Context) (<-chan scraper.CatalogueEntry, <-chan error) {
	entries := make(chan scraper.CatalogueEntry, 64)
	errs := make(chan error, 16)

	go func() {
		defer close(entries)
		defer close(errs)

		pageURL := baseURL + cataloguePath
		page := 1

		for pageURL != "" {
			select {
			case <-ctx.Done():
				return
			default:
			}

			s.log.Info("scraping catalogue page", "page", page, "url", pageURL)
			s.log.Debug("catalogue page fetch starting",
				"page", page,
				"payload_url", pageURL,
				"payload_wait_selector", ".novel-item",
				"payload_wait_selector_timeout_ms", 5000,
			)

			html, err := s.client.GetContent(ctx, browser.ContentRequest{
				URL:                 pageURL,
				WaitFor:             &browser.WaitForSelector{Selector: ".novel-item", Timeout: 5000},
				RejectResourceTypes: rejectResourceTypes,
				GotoOptions:         &browser.GotoOptions{Timeout: 60000},
			})
			if err != nil {
				s.log.Debug("catalogue page fetch failed",
					"page", page,
					"url", pageURL,
					"err", err,
				)
				errs <- fmt.Errorf("catalogue page %d: %w", page, err)
				return
			}
			s.log.Debug("catalogue page fetch completed",
				"page", page,
				"url", pageURL,
				"response_bytes", len(html),
			)

			root, err := htmlutil.ParseHTML(html)
			if err != nil {
				errs <- fmt.Errorf("catalogue page %d parse: %w", page, err)
				return
			}

			// Extract novel cards: <div class="novel-item">
			cards := htmlutil.FindAll(root, scraper.Selector{Tag: "div", Class: "novel-item", Multiple: true})
			if len(cards) == 0 {
				s.log.Warn("no novel cards found, stopping pagination", "page", page)
				return
			}

			for _, card := range cards {
				// Title: <h3 class="novel-title"><a href="/book/slug">Title</a>
				titleNode := htmlutil.FindFirst(card, scraper.Selector{Tag: "h3", Class: "novel-title"})

				var title, href string
				if titleNode != nil {
					linkNode := htmlutil.FindFirst(titleNode, scraper.Selector{Tag: "a", Attr: "href"})
					if linkNode != nil {
						title = htmlutil.ExtractText(linkNode, scraper.Selector{})
						href = htmlutil.ExtractText(linkNode, scraper.Selector{Tag: "a", Attr: "href"})
					}
				}
				if href == "" || title == "" {
					continue
				}

				bookURL := resolveURL(baseURL, href)
				select {
				case <-ctx.Done():
					return
				case entries <- scraper.CatalogueEntry{Title: title, URL: bookURL}:
				}
			}

			// Find next page link: <a class="next" href="...">
			nextHref := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "a", Class: "next", Attr: "href"})
			if nextHref == "" {
				break
			}
			pageURL = resolveURL(baseURL, nextHref)
			page++
		}
	}()

	return entries, errs
}

// ─── MetadataProvider ────────────────────────────────────────────────────────

func (s *Scraper) ScrapeMetadata(ctx context.Context, bookURL string) (scraper.BookMeta, error) {
	s.log.Debug("metadata fetch starting",
		"payload_url", bookURL,
		"payload_wait_selector", ".novel-title",
		"payload_wait_selector_timeout_ms", 5000,
	)

	raw, err := s.client.GetContent(ctx, browser.ContentRequest{
		URL:                 bookURL,
		WaitFor:             &browser.WaitForSelector{Selector: ".novel-title", Timeout: 5000},
		RejectResourceTypes: rejectResourceTypes,
		GotoOptions:         &browser.GotoOptions{Timeout: 60000},
	})
	if err != nil {
		s.log.Debug("metadata fetch failed", "url", bookURL, "err", err)
		return scraper.BookMeta{}, fmt.Errorf("metadata fetch %s: %w", bookURL, err)
	}
	s.log.Debug("metadata fetch completed", "url", bookURL, "response_bytes", len(raw))

	root, err := htmlutil.ParseHTML(raw)
	if err != nil {
		return scraper.BookMeta{}, fmt.Errorf("metadata parse %s: %w", bookURL, err)
	}

	// <h1 class="novel-title">Title</h1>
	title := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "h1", Class: "novel-title"})
	// <span class="author"><a>Author Name</a></span>
	author := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "span", Class: "author"})
	// <figure class="cover"><img src="..."></figure>
	var cover string
	if figureCover := htmlutil.FindFirst(root, scraper.Selector{Tag: "figure", Class: "cover"}); figureCover != nil {
		cover = htmlutil.ExtractFirst(figureCover, scraper.Selector{Tag: "img", Attr: "src"})
		if cover != "" && !strings.HasPrefix(cover, "http") {
			cover = baseURL + cover
		}
	}
	// <span class="status">Ongoing</span>
	status := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "span", Class: "status"})

	// Genres: all <a> tags inside <div class="genres">
	genresNode := htmlutil.FindFirst(root, scraper.Selector{Tag: "div", Class: "genres"})
	var genres []string
	if genresNode != nil {
		genres = htmlutil.ExtractAll(genresNode, scraper.Selector{Tag: "a", Multiple: true})
	}

	// <div class="summary"><p>...</p></div>
	summary := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "div", Class: "summary"})
	// <span class="chapter-count">123 Chapters</span>
	totalStr := htmlutil.ExtractFirst(root, scraper.Selector{Tag: "span", Class: "chapter-count"})
	totalChapters := parseChapterCount(totalStr)

	slug := slugFromURL(bookURL)

	meta := scraper.BookMeta{
		Slug:          slug,
		Title:         title,
		Author:        author,
		Cover:         cover,
		Status:        status,
		Genres:        genres,
		Summary:       summary,
		TotalChapters: totalChapters,
		SourceURL:     bookURL,
	}
	s.log.Debug("metadata parsed",
		"url", bookURL,
		"slug", meta.Slug,
		"title", meta.Title,
		"author", meta.Author,
		"status", meta.Status,
		"genres", meta.Genres,
		"total_chapters", meta.TotalChapters,
	)
	return meta, nil
}

// ─── ChapterListProvider ─────────────────────────────────────────────────────

func (s *Scraper) ScrapeChapterList(ctx context.Context, bookURL string) ([]scraper.ChapterRef, error) {
	var refs []scraper.ChapterRef
	// Chapter list URL: {bookURL}/chapters?page=N
	baseChapterURL := strings.TrimRight(bookURL, "/") + "/chapters"
	page := 1

	for {
		select {
		case <-ctx.Done():
			return refs, ctx.Err()
		default:
		}

		pageURL := fmt.Sprintf("%s?page=%d", baseChapterURL, page)
		s.log.Info("scraping chapter list", "page", page, "url", pageURL)

		s.log.Debug("chapter list fetch starting",
			"page", page,
			"payload_url", pageURL,
			"strategy", s.urlClient.Strategy(),
		)

		raw, err := s.urlClient.GetContent(ctx, browser.ContentRequest{
			URL: pageURL,
		})
		if err != nil {
			s.log.Debug("chapter list fetch failed",
				"page", page,
				"url", pageURL,
				"err", err,
			)
			return refs, fmt.Errorf("chapter list page %d: %w", page, err)
		}
		s.log.Debug("chapter list fetch completed",
			"page", page,
			"url", pageURL,
			"response_bytes", len(raw),
		)

		root, err := htmlutil.ParseHTML(raw)
		if err != nil {
			return refs, fmt.Errorf("chapter list page %d parse: %w", page, err)
		}

		chapterList := htmlutil.FindFirst(root, scraper.Selector{Class: "chapter-list"})
		if chapterList == nil {
			// No chapter list container on this page — we've gone past the last page.
			s.log.Debug("chapter list container not found, stopping pagination", "page", page)
			break
		}

		// Each chapter row: <li class="chapter-item"><a href="...">Title</a></li>
		items := htmlutil.FindAll(chapterList, scraper.Selector{Tag: "li"})

		s.log.Debug("chapter list page parsed",
			"page", page,
			"url", pageURL,
			"chapters_on_page", len(items),
			"total_refs_so_far", len(refs),
		)

		// Zero items on this page means we've gone past the last page.
		if len(items) == 0 {
			s.log.Debug("no chapters on page, stopping pagination", "page", page)
			break
		}

		for _, item := range items {
			linkNode := htmlutil.FindFirst(item, scraper.Selector{Tag: "a"})
			if linkNode == nil {
				continue
			}
			href := htmlutil.ExtractText(linkNode, scraper.Selector{Attr: "href"})
			chTitle := htmlutil.ExtractText(linkNode, scraper.Selector{})
			if href == "" {
				continue
			}
			chURL := resolveURL(baseURL, href)
			num := chapterNumberFromURL(chURL)
			if num <= 0 {
				// Fall back to position if the URL has no parseable number.
				num = len(refs) + 1
				s.log.Warn("chapter number not parseable from URL, falling back to position",
					"url", chURL,
					"position", num,
				)
			}
			refs = append(refs, scraper.ChapterRef{
				Number: num,
				Title:  strings.TrimSpace(chTitle),
				URL:    chURL,
			})
		}

		page++
	}

	return refs, nil
}

// ScrapeChapterListPage fetches and parses a single chapter-list page URL and
// returns all chapter refs found on that page without following pagination.
// pageURL should be the full URL including query params, e.g.:
//
//	https://novelfire.net/book/shadow-slave/chapters?page=1
func (s *Scraper) ScrapeChapterListPage(ctx context.Context, pageURL string) ([]scraper.ChapterRef, error) {
	s.log.Info("scraping chapter list page (single)", "url", pageURL)

	raw, err := s.urlClient.GetContent(ctx, browser.ContentRequest{
		URL: pageURL,
	})
	if err != nil {
		return nil, fmt.Errorf("chapter list page fetch: %w", err)
	}

	root, err := htmlutil.ParseHTML(raw)
	if err != nil {
		return nil, fmt.Errorf("chapter list page parse: %w", err)
	}

	chapterList := htmlutil.FindFirst(root, scraper.Selector{Class: "chapter-list"})
	if chapterList == nil {
		return nil, fmt.Errorf("chapter list container not found in %s", pageURL)
	}

	items := htmlutil.FindAll(chapterList, scraper.Selector{Tag: "li"})
	var refs []scraper.ChapterRef
	for _, item := range items {
		linkNode := htmlutil.FindFirst(item, scraper.Selector{Tag: "a"})
		if linkNode == nil {
			continue
		}
		href := htmlutil.ExtractText(linkNode, scraper.Selector{Attr: "href"})
		chTitle := htmlutil.ExtractText(linkNode, scraper.Selector{})
		if href == "" {
			continue
		}
		chURL := resolveURL(baseURL, href)
		num := chapterNumberFromURL(chURL)
		if num <= 0 {
			num = len(refs) + 1
		}
		refs = append(refs, scraper.ChapterRef{
			Number: num,
			Title:  strings.TrimSpace(chTitle),
			URL:    chURL,
		})
	}
	return refs, nil
}

// ─── RankingProvider ───────────────────────────────────────────────────────────

// hasNextPageLink returns true if the HTML document contains a pagination link
// with rel="next". novelfire.net uses:
//
//	<a class="page-link" href="...?page=N" rel="next" ...>
func hasNextPageLink(root *html.Node) bool {
	links := htmlutil.FindAll(root, scraper.Selector{Tag: "a", Multiple: true})
	for _, a := range links {
		for _, attr := range a.Attr {
			if attr.Key == "rel" && attr.Val == "next" {
				return true
			}
		}
	}
	return false
}

// ScrapeRanking pages through up to maxPages pages of the popular-novels genre
// listing on novelfire.net (/genre-all/sort-popular/status-all/all-novel).
// Pages are fetched one at a time, strictly sequentially.
// maxPages <= 0 means "fetch all pages until no more are found".
//
// If a RankingStore was provided and the stored ranking is fresh (< 24 hours old),
// both channels are closed immediately without any network traffic.
func (s *Scraper) ScrapeRanking(ctx context.Context, maxPages int) (<-chan scraper.BookMeta, <-chan error) {
	entries := make(chan scraper.BookMeta, 32)
	errs := make(chan error, 16)

	go func() {
		defer close(entries)
		defer close(errs)

		// Freshness check: skip scraping if data is recent enough.
		if s.rankingStore != nil {
			fresh, err := s.rankingStore.RankingFreshEnough(ctx, 24*time.Hour)
			if err != nil {
				s.log.Warn("ranking freshness check failed, proceeding with scrape", "err", err)
			} else if fresh {
				s.log.Info("ranking data is fresh, skipping scrape")
				return
			}
		}

		rank := 1

		for page := 1; maxPages <= 0 || page <= maxPages; page++ {
			select {
			case <-ctx.Done():
				return
			default:
			}

			pageURL := fmt.Sprintf("%s%s?page=%d", baseURL, rankingPath, page)

			s.log.Info("scraping popular ranking page", "page", page, "url", pageURL)
			raw, err := s.client.GetContent(ctx, browser.ContentRequest{
				URL:                 pageURL,
				WaitFor:             &browser.WaitForSelector{Selector: ".novel-item", Timeout: 5000},
				RejectResourceTypes: rejectResourceTypes,
				GotoOptions:         &browser.GotoOptions{Timeout: 60000},
			})
			if err != nil {
				s.log.Debug("ranking page fetch failed", "page", page, "url", pageURL, "err", err)
				errs <- fmt.Errorf("ranking page %d: %w", page, err)
				return
			}

			root, err := htmlutil.ParseHTML(raw)
			if err != nil {
				errs <- fmt.Errorf("ranking page %d parse: %w", page, err)
				return
			}

			// Real novelfire.net popular listing structure:
			//   <li class="novel-item">
			//     <a href="/book/slug" title="Title">
			//       <figure class="novel-cover"><img data-src="..."></figure>
			//       <h4 class="novel-title text2row">Title</h4>
			//     </a>
			//   </li>
			cards := htmlutil.FindAll(root, scraper.Selector{Tag: "li", Class: "novel-item", Multiple: true})
			if len(cards) == 0 {
				s.log.Debug("no novel cards found, stopping pagination", "page", page)
				break
			}

			for _, card := range cards {
				// The outer <a> carries the href and title attribute.
				linkNode := htmlutil.FindFirst(card, scraper.Selector{Tag: "a"})
				if linkNode == nil {
					continue
				}
				href := htmlutil.ExtractText(linkNode, scraper.Selector{Tag: "a", Attr: "href"})
				bookURL := resolveURL(baseURL, href)
				if bookURL == "" {
					continue
				}

				// Title: prefer <h4 class="novel-title"> text; fall back to <a title="...">
				title := strings.TrimSpace(htmlutil.ExtractFirst(card, scraper.Selector{Tag: "h4", Class: "novel-title"}))
				if title == "" {
					title = strings.TrimSpace(htmlutil.ExtractText(linkNode, scraper.Selector{Tag: "a", Attr: "title"}))
				}
				if title == "" {
					continue
				}

				// Cover: <figure class="novel-cover"><img data-src="...">
				var cover string
				if fig := htmlutil.FindFirst(card, scraper.Selector{Tag: "figure", Class: "novel-cover"}); fig != nil {
					cover = htmlutil.ExtractFirst(fig, scraper.Selector{Tag: "img", Attr: "data-src"})
					if cover == "" {
						cover = htmlutil.ExtractFirst(fig, scraper.Selector{Tag: "img", Attr: "src"})
					}
					// Filter out base64 placeholder images.
					if strings.HasPrefix(cover, "data:") {
						cover = ""
					}
					if cover != "" && !strings.HasPrefix(cover, "http") {
						cover = baseURL + cover
					}
				}

				bookSlug := slugFromURL(bookURL)

				meta := scraper.BookMeta{
					Slug:      bookSlug,
					Title:     title,
					Cover:     cover,
					SourceURL: bookURL,
					Ranking:   rank,
				}
				rank++

				// Persist item to store immediately.
				if s.rankingStore != nil {
					item := scraper.RankingItem{
						Rank:      meta.Ranking,
						Slug:      meta.Slug,
						Title:     meta.Title,
						Cover:     meta.Cover,
						SourceURL: meta.SourceURL,
					}
					if werr := s.rankingStore.WriteRankingItem(ctx, item); werr != nil {
						s.log.Warn("ranking item write failed", "slug", meta.Slug, "err", werr)
					}
				}

				select {
				case <-ctx.Done():
					return
				case entries <- meta:
				}
			}

			// Stop if no next-page link exists.
			// The real pagination uses <a rel="next" ...> inside .pagination.
			if !hasNextPageLink(root) {
				s.log.Debug("no next-page link found, stopping pagination", "page", page)
				break
			}
		}
	}()

	return entries, errs
}

// ─── ChapterTextProvider ─────────────────────────────────────────────────────

// retryGetContent calls client.GetContent up to maxAttempts times, backing off
// exponentially between retries. Only errors that look like transient Browserless
// failures (timeouts, 5xx responses) are retried; context cancellation and
// permanent errors are returned immediately.
func retryGetContent(
	ctx context.Context,
	log *slog.Logger,
	client browser.BrowserClient,
	req browser.ContentRequest,
	maxAttempts int,
	baseDelay time.Duration,
) (string, error) {
	var lastErr error
	delay := baseDelay
	for attempt := 1; attempt <= maxAttempts; attempt++ {
		html, err := client.GetContent(ctx, req)
		if err == nil {
			return html, nil
		}
		lastErr = err

		// Stop immediately on context cancellation.
		if ctx.Err() != nil {
			return "", err
		}

		if attempt < maxAttempts {
			log.Warn("chapter fetch failed, retrying",
				"url", req.URL,
				"attempt", attempt,
				"max_attempts", maxAttempts,
				"retry_in", delay,
				"err", err,
			)
			select {
			case <-ctx.Done():
				return "", ctx.Err()
			case <-time.After(delay):
			}
			delay *= 2
		}
	}
	return "", lastErr
}

func (s *Scraper) ScrapeChapterText(ctx context.Context, ref scraper.ChapterRef) (scraper.Chapter, error) {
	s.log.Debug("chapter text fetch starting",
		"chapter", ref.Number,
		"title", ref.Title,
		"payload_url", ref.URL,
		"payload_wait_selector", "#content",
		"payload_wait_selector_timeout_ms", 5000,
	)

	raw, err := retryGetContent(ctx, s.log, s.chapterClient, browser.ContentRequest{
		URL: ref.URL,
	}, 9, 6*time.Second)
	if err != nil {
		s.log.Debug("chapter text fetch failed",
			"chapter", ref.Number,
			"url", ref.URL,
			"err", err,
		)
		return scraper.Chapter{}, fmt.Errorf("chapter %d fetch: %w", ref.Number, err)
	}
	if len(raw) > 0 {
		preview := raw
		if len(preview) > 500 {
			preview = preview[:500]
		}
		s.log.Debug("chapter text fetch partial content",
			"chapter", ref.Number,
			"url", ref.URL,
			"response_bytes", len(raw),
			"preview", preview,
		)
	}
	s.log.Debug("chapter text fetch completed",
		"chapter", ref.Number,
		"url", ref.URL,
		"response_bytes", len(raw),
	)

	root, err := htmlutil.ParseHTML(raw)
	if err != nil {
		return scraper.Chapter{}, fmt.Errorf("chapter %d parse: %w", ref.Number, err)
	}

	// <div id="content">…</div>
	container := htmlutil.FindFirst(root, scraper.Selector{ID: "content"})
	if container == nil {
		return scraper.Chapter{}, fmt.Errorf("chapter %d: #content container not found in %s", ref.Number, ref.URL)
	}

	text := htmlutil.NodeToMarkdown(container)

	s.log.Debug("chapter text parsed",
		"chapter", ref.Number,
		"url", ref.URL,
		"text_bytes", len(text),
	)

	return scraper.Chapter{
		Ref:  ref,
		Text: text,
	}, nil
}

// ─── helpers ─────────────────────────────────────────────────────────────────

func resolveURL(base, href string) string {
	if strings.HasPrefix(href, "http://") || strings.HasPrefix(href, "https://") {
		return href
	}
	b, err := url.Parse(base)
	if err != nil {
		return base + href
	}
	ref, err := url.Parse(href)
	if err != nil {
		return base + href
	}
	return b.ResolveReference(ref).String()
}

func slugFromURL(bookURL string) string {
	u, err := url.Parse(bookURL)
	if err != nil {
		return bookURL
	}
	parts := strings.Split(strings.Trim(u.Path, "/"), "/")
	if len(parts) >= 2 && parts[0] == "book" {
		return parts[1]
	}
	if len(parts) > 0 {
		return parts[len(parts)-1]
	}
	return ""
}

func parseChapterCount(s string) int {
	// Formats: "123 Chapters", "1,234 Chapters", "123"
	s = strings.ReplaceAll(s, ",", "")
	fields := strings.Fields(s)
	if len(fields) == 0 {
		return 0
	}
	n, _ := strconv.Atoi(fields[0])
	return n
}

// chapterNumberFromURL extracts the chapter number from a novelfire chapter URL.
//
// URL pattern: https://novelfire.net/book/{book-slug}/chapter-{N}
// The last path segment is expected to be "chapter-{N}" or "{N}".
// Returns 0 if no number can be parsed.
func chapterNumberFromURL(chapterURL string) int {
	u, err := url.Parse(chapterURL)
	if err != nil {
		return 0
	}
	seg := path.Base(u.Path) // e.g. "chapter-42" or "42"
	// Strip a "chapter-" prefix if present.
	seg = strings.TrimPrefix(seg, "chapter-")
	// Also handle "chap-", "ch-" variants used by some sites.
	seg = strings.TrimPrefix(seg, "chap-")
	seg = strings.TrimPrefix(seg, "ch-")
	// Take only the leading digits (handles slugs like "42-title-text").
	digits := strings.FieldsFunc(seg, func(r rune) bool {
		return r < '0' || r > '9'
	})
	if len(digits) == 0 {
		return 0
	}
	n, _ := strconv.Atoi(digits[0])
	return n
}