// Package content loads and indexes markdown tutorial pages from disk.
//
// Layout on disk:
//
//	content/
//	  <locale>/             — e.g. en, cs
//	    <category>/         — e.g. install-client, registration, troubleshooting
//	      _index.md         — category landing page (optional)
//	      <slug>.md         — individual tutorial page
//
// Each markdown file may have YAML frontmatter delimited by `---`:
//
//	---
//	title: Install Minecraft Java Edition
//	order: 1
//	summary: Step-by-step for installing the official launcher.
//	---
//	# heading...
//
// Pages without frontmatter use the filename as title and order=999.
package content

import (
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"

	"gopkg.in/yaml.v3"
)

// Library is an in-memory index of all loaded pages. Construct via Load.
// Read-only after construction; the server swaps whole Library values on reload.
type Library struct {
	pages   map[string]*Page            // key: <locale>/<category>/<slug>
	byPath  map[string][]*Page          // key: <locale>/<category>, value: pages sorted by Order then Title
	locales []string                    // sorted, unique
	categs  map[string]map[string]*Page // <locale> → <category> → category-index page (or nil if no _index.md)
}

// maxPageBytes caps the size of a single markdown file. Loader skips bigger files with a stderr warning.
// Content is operator-controlled (volume mount), but this prevents a stray dump from ballooning memory.
const maxPageBytes = 1 << 20 // 1 MiB

// Page is one markdown document.
type Page struct {
	Locale   string // "en", "cs"
	Category string // "install-client"
	Slug     string // "java-edition", "_index"
	Title    string
	Summary  string
	Order    int
	Source   []byte // raw markdown body (no frontmatter)
	Path     string // <locale>/<category>/<slug>
}

type frontmatter struct {
	Title   string `yaml:"title"`
	Summary string `yaml:"summary"`
	Order   int    `yaml:"order"`
}

// Load walks contentDir and returns a populated Library.
// Returns an error only on filesystem-level failures; individual malformed pages are skipped with a warning written to stderr.
//
// Zero-trust posture: the content dir is volume-mounted in production. The loader
// resolves the absolute path of contentDir once and rejects any entry (dir or file)
// whose evaluated symlink target falls outside it. This prevents a misconfigured
// volume (or an operator slipping in a symlink) from reading host files.
func Load(contentDir string) (*Library, error) {
	info, err := os.Stat(contentDir)
	if err != nil {
		return nil, fmt.Errorf("stat content dir %q: %w", contentDir, err)
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("content path %q is not a directory", contentDir)
	}
	rootAbs, err := filepath.Abs(contentDir)
	if err != nil {
		return nil, fmt.Errorf("abs content dir: %w", err)
	}
	rootAbs, err = filepath.EvalSymlinks(rootAbs)
	if err != nil {
		return nil, fmt.Errorf("eval root symlinks: %w", err)
	}

	lib := &Library{
		pages:  make(map[string]*Page),
		byPath: make(map[string][]*Page),
		categs: make(map[string]map[string]*Page),
	}

	localeEntries, err := os.ReadDir(contentDir)
	if err != nil {
		return nil, fmt.Errorf("read content dir: %w", err)
	}
	for _, lEntry := range localeEntries {
		name := lEntry.Name()
		if strings.HasPrefix(name, ".") || !lEntry.IsDir() {
			continue
		}
		localeDir := filepath.Join(contentDir, name)
		if !withinRoot(rootAbs, localeDir) {
			fmt.Fprintf(os.Stderr, "[content] skipping locale %q: escapes content root\n", name)
			continue
		}
		lib.locales = append(lib.locales, name)
		lib.categs[name] = make(map[string]*Page)

		categEntries, err := os.ReadDir(localeDir)
		if err != nil {
			return nil, fmt.Errorf("read locale %q: %w", name, err)
		}
		for _, cEntry := range categEntries {
			cName := cEntry.Name()
			if strings.HasPrefix(cName, ".") || !cEntry.IsDir() {
				continue
			}
			categDir := filepath.Join(localeDir, cName)
			if !withinRoot(rootAbs, categDir) {
				fmt.Fprintf(os.Stderr, "[content] skipping %s/%s: escapes content root\n", name, cName)
				continue
			}
			// Register the category even if _index.md is missing; it'll show up
			// on the home page with a humanized name.
			if _, ok := lib.categs[name][cName]; !ok {
				lib.categs[name][cName] = nil
			}
			if err := lib.loadCategory(rootAbs, name, cName, categDir); err != nil {
				return nil, err
			}
		}
	}

	sort.Strings(lib.locales)
	for k := range lib.byPath {
		pages := lib.byPath[k]
		sort.SliceStable(pages, func(i, j int) bool {
			if pages[i].Order != pages[j].Order {
				return pages[i].Order < pages[j].Order
			}
			return pages[i].Title < pages[j].Title
		})
		lib.byPath[k] = pages
	}
	return lib, nil
}

// withinRoot returns true if p, after resolving symlinks, is rootAbs or a descendant.
// Used to reject volume entries that point outside the content directory.
func withinRoot(rootAbs, p string) bool {
	abs, err := filepath.Abs(p)
	if err != nil {
		return false
	}
	resolved, err := filepath.EvalSymlinks(abs)
	if err != nil {
		return false
	}
	rel, err := filepath.Rel(rootAbs, resolved)
	if err != nil {
		return false
	}
	return rel == "." || (!strings.HasPrefix(rel, "..") && !filepath.IsAbs(rel))
}

func (l *Library) loadCategory(rootAbs, locale, category, dir string) error {
	files, err := os.ReadDir(dir)
	if err != nil {
		return err
	}
	pathKey := locale + "/" + category
	for _, f := range files {
		name := f.Name()
		if strings.HasPrefix(name, ".") || f.IsDir() || !strings.HasSuffix(name, ".md") {
			continue
		}
		fp := filepath.Join(dir, name)
		if !withinRoot(rootAbs, fp) {
			fmt.Fprintf(os.Stderr, "[content] skipping %s: escapes content root\n", fp)
			continue
		}
		// Use Stat on the resolved path to enforce size cap before reading.
		info, err := os.Stat(fp)
		if err != nil {
			fmt.Fprintf(os.Stderr, "[content] stat %s: %v\n", fp, err)
			continue
		}
		if info.Size() > maxPageBytes {
			fmt.Fprintf(os.Stderr, "[content] skipping %s: %d bytes exceeds %d cap\n", fp, info.Size(), maxPageBytes)
			continue
		}
		slug := strings.TrimSuffix(name, ".md")
		raw, err := os.ReadFile(fp)
		if err != nil {
			return err
		}
		page, err := parsePage(raw, locale, category, slug)
		if err != nil {
			fmt.Fprintf(os.Stderr, "[content] skipping %s: %v\n", fp, err)
			continue
		}
		key := page.Path
		l.pages[key] = page
		if slug == "_index" {
			l.categs[locale][category] = page
		} else {
			l.byPath[pathKey] = append(l.byPath[pathKey], page)
		}
	}
	return nil
}

func parsePage(raw []byte, locale, category, slug string) (*Page, error) {
	body := raw
	fm := frontmatter{Order: 999}
	if bytes := raw; len(bytes) >= 4 && string(bytes[:3]) == "---" {
		// Find closing fence on its own line.
		rest := bytes[3:]
		idx := strings.Index(string(rest), "\n---")
		if idx >= 0 {
			head := rest[:idx]
			if err := yaml.Unmarshal(head, &fm); err != nil {
				return nil, fmt.Errorf("frontmatter: %w", err)
			}
			body = rest[idx+4:]
			body = trimLeadingNewline(body)
		}
	}
	if fm.Title == "" {
		fm.Title = slugToTitle(slug)
	}
	return &Page{
		Locale:   locale,
		Category: category,
		Slug:     slug,
		Title:    fm.Title,
		Summary:  fm.Summary,
		Order:    fm.Order,
		Source:   body,
		Path:     locale + "/" + category + "/" + slug,
	}, nil
}

func trimLeadingNewline(b []byte) []byte {
	if len(b) > 0 && b[0] == '\n' {
		return b[1:]
	}
	return b
}

func slugToTitle(slug string) string {
	s := strings.ReplaceAll(slug, "-", " ")
	s = strings.ReplaceAll(s, "_", " ")
	if s == "" {
		return ""
	}
	return strings.ToUpper(s[:1]) + s[1:]
}

// --- Accessors ---

func (l *Library) Locales() []string  { return append([]string(nil), l.locales...) }
func (l *Library) PageCount() int     { return len(l.pages) }
func (l *Library) HasLocale(loc string) bool {
	_, ok := l.categs[loc]
	return ok
}

// Categories returns the categories in a locale, sorted by their _index.Order then name.
type Category struct {
	Name    string
	Index   *Page // may be nil if no _index.md
	Pages   []*Page
}

func (l *Library) Categories(locale string) []Category {
	cats, ok := l.categs[locale]
	if !ok {
		return nil
	}
	var out []Category
	for name := range cats {
		idx := cats[name]
		out = append(out, Category{
			Name:  name,
			Index: idx,
			Pages: l.byPath[locale+"/"+name],
		})
	}
	// Stable order: by index Order, then name.
	sort.SliceStable(out, func(i, j int) bool {
		oi, oj := 999, 999
		if out[i].Index != nil {
			oi = out[i].Index.Order
		}
		if out[j].Index != nil {
			oj = out[j].Index.Order
		}
		if oi != oj {
			return oi < oj
		}
		return out[i].Name < out[j].Name
	})
	return out
}

// Page returns the page for a path or nil.
func (l *Library) Page(locale, category, slug string) *Page {
	return l.pages[locale+"/"+category+"/"+slug]
}

// AllPages returns every page; used by the search index endpoint.
func (l *Library) AllPages() []*Page {
	out := make([]*Page, 0, len(l.pages))
	for _, p := range l.pages {
		out = append(out, p)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Path < out[j].Path })
	return out
}