// Package content reads a site root into bundles. It knows the disk and nothing about HTTP.
//
// Every read goes through an [os.Root] (ADR-0031), so no path — from a filename or later from a
// request — can escape the site root, even through a symlink.
package content

import (
	"bytes"
	"fmt"
	"io/fs"
	"log/slog"
	"os"
	"path"
	"strings"

	"golang.org/x/text/unicode/norm"
	"gopkg.in/yaml.v3"
)

// DefaultLang is the locale served at the root of the URL space; every other language is served under a
// prefix (ADR-0009). A filename with no language suffix means this locale (ADR-0021).
const DefaultLang = "en"

// Bundle is one addressable piece of content in one language.
//
// Only Title is lifted out of frontmatter; every other key lands in Extra, so a template or a later
// feature can read a field the parser has never heard of (ADR-0002). Absence is always the zero value,
// never an error.
type Bundle struct {
	// Key identifies the bundle across languages: its path under content/, without language suffix or
	// extension, NFC-normalised. This is the identity in ADR-0004 and never contains a language.
	Key string
	// Lang is always set; a file with no suffix reports DefaultLang.
	Lang string
	// Path is the file this bundle was read from, relative to the site root.
	Path string
	// Title is empty when frontmatter omits it. Whether that is legal depends on the type, which
	// nothing decides yet, so the parser accepts it.
	Title string
	// Body is everything after the frontmatter, unrendered.
	Body []byte
	// Extra holds every frontmatter key other than title, exactly as YAML parsed it.
	Extra map[string]any
}

// OpenSite opens a site root for reading.
//
// The returned fs.FS is backed by [os.Root], which refuses any name that would resolve outside dir,
// including through a symlink — unlike os.DirFS, which does not (ADR-0031). The root is held for the
// life of the process.
func OpenSite(dir string) (fs.FS, error) {
	root, err := os.OpenRoot(dir)
	if err != nil {
		return nil, fmt.Errorf("open site root %s: %w", dir, err)
	}
	return root.FS(), nil
}

// Scan reads every bundle under content/ in fsys.
//
// A bundle that cannot be parsed, or that collides with another on the same key and language, is logged
// at error level and left out; neither is fatal, because one mistyped colon must not take down a site
// (ADR-0029). An error is returned only when the walk itself fails.
func Scan(fsys fs.FS) ([]Bundle, error) {
	var found []Bundle
	err := fs.WalkDir(fsys, "content", func(p string, d fs.DirEntry, err error) error {
		switch {
		case err != nil:
			return err
		case d.IsDir():
			if skipDir(path.Base(p)) {
				return fs.SkipDir
			}
			return nil
		case !strings.HasSuffix(p, ".md"):
			return nil
		}
		data, err := fs.ReadFile(fsys, p)
		if err != nil {
			slog.Error("skipping unreadable bundle", "path", p, "err", err)
			return nil
		}
		b, err := Parse(strings.TrimPrefix(p, "content/"), data)
		if err != nil {
			slog.Error("skipping unparseable bundle", "path", p, "err", err)
			return nil
		}
		b.Path = p
		found = append(found, b)
		return nil
	})
	if err != nil {
		return nil, fmt.Errorf("scan content: %w", err)
	}
	return dropCollisions(found), nil
}

// Parse reads one bundle from the bytes of a file, named relative to content/.
func Parse(name string, data []byte) (Bundle, error) {
	key, lang, ok := splitName(name)
	if !ok {
		return Bundle{}, fmt.Errorf("not a bundle filename: %s", name)
	}
	front, body := splitFrontmatter(data)
	b := Bundle{Key: key, Lang: lang, Body: body, Extra: map[string]any{}}
	if len(front) > 0 {
		if err := yaml.Unmarshal(front, &b.Extra); err != nil {
			return Bundle{}, fmt.Errorf("frontmatter: %w", err)
		}
	}
	if t, isStr := b.Extra["title"].(string); isStr {
		b.Title = t
	}
	delete(b.Extra, "title")
	return b, nil
}

// Normalise puts s into NFC.
//
// Every identifier goes through this: Bengali conjuncts have several byte encodings for text that looks
// identical, and macOS hands back NFD, so without it two indistinguishable files take different keys
// (ADR-0015).
func Normalise(s string) string { return norm.NFC.String(s) }

// splitName derives a bundle key and language from a path under content/.
//
// index.md and _index.md name the directory they sit in; anything else names itself. A trailing
// two- or three-letter lowercase segment is a language suffix.
func splitName(name string) (key, lang string, ok bool) {
	if !strings.HasSuffix(name, ".md") || name == "" {
		return "", "", false
	}
	dir, base := path.Split(strings.TrimSuffix(name, ".md"))
	dir = strings.TrimSuffix(dir, "/")
	if base == "" {
		return "", "", false
	}
	lang = DefaultLang
	if i := strings.LastIndex(base, "."); i > 0 && isLangTag(base[i+1:]) {
		lang, base = base[i+1:], base[:i]
	}
	if base == "index" || base == "_index" {
		key = dir
	} else {
		key = path.Join(dir, base)
	}
	return Normalise(key), lang, true
}

// isLangTag reports whether s looks like a language suffix: two or three lowercase letters.
func isLangTag(s string) bool {
	if len(s) < 2 || len(s) > 3 {
		return false
	}
	for _, r := range s {
		if r < 'a' || r > 'z' {
			return false
		}
	}
	return true
}

// skipDir reports whether a directory is not content: hidden, or underscore-prefixed.
func skipDir(base string) bool {
	return strings.HasPrefix(base, ".") && base != "." || strings.HasPrefix(base, "_")
}

// splitFrontmatter separates a leading --- delimited YAML block from the body. A file without one is
// all body.
func splitFrontmatter(data []byte) (front, body []byte) {
	const fence = "---"
	rest, hasFence := trimLeadingFence(data, fence)
	if !hasFence {
		return nil, data
	}
	if i := bytes.Index(rest, []byte("\n"+fence)); i >= 0 {
		front = rest[:i+1]
		body = rest[i+1+len(fence):]
		return front, bytes.TrimLeft(body, "\r\n")
	}
	return nil, data
}

// trimLeadingFence removes an opening fence line, reporting whether one was there.
func trimLeadingFence(data []byte, fence string) ([]byte, bool) {
	s := bytes.TrimLeft(data, "\ufeff")
	if !bytes.HasPrefix(s, []byte(fence)) {
		return data, false
	}
	s = s[len(fence):]
	s = bytes.TrimLeft(s, "\r")
	if !bytes.HasPrefix(s, []byte("\n")) {
		return data, false
	}
	return s[1:], true
}

// dropCollisions removes every bundle sharing a key and language with another.
//
// Two spellings of the same variant — about.md and about.en.md, or about.md and about/index.md — are
// ambiguous rather than harmless, so none of them is served (ADR-0021).
func dropCollisions(all []Bundle) []Bundle {
	seen := map[string]int{}
	for _, b := range all {
		seen[b.Key+"\x00"+b.Lang]++
	}
	kept := make([]Bundle, 0, len(all))
	for _, b := range all {
		if seen[b.Key+"\x00"+b.Lang] > 1 {
			slog.Error("skipping ambiguous bundle: two files claim one key and language",
				"key", b.Key, "lang", b.Lang, "path", b.Path)
			continue
		}
		kept = append(kept, b)
	}
	return kept
}