package bm

import (
	"bytes"
	"crypto/sha256"
	"embed"
	"encoding/hex"
	"errors"
	"fmt"
	"io/fs"
	"mime"
	"net/http"
	"path"
	"strings"
	"time"

	"git.golem15.com/golem15/summercms/modules/backpack"
)

// publicFS holds the build output of the student app (frontend/ in the
// application repository). The all: prefix keeps dot-less underscore names
// that embed would otherwise drop.
//
//go:embed all:public
var publicFS embed.FS

const (
	// cacheImmutable is sent for content-hashed assets (Vite writes them to
	// assets/).
	cacheImmutable = "public, max-age=31536000, immutable"
	// cacheNoCache is sent for every other file; the ETag makes the
	// revalidation a cheap 304.
	cacheNoCache = "no-cache"
)

// defaultAdminPath is the admin mount used when backend.uri is not set.
const defaultAdminPath = "/bmadmin"

// reservedPrefixes are the path prefixes the student app never answers: the
// APIs of the plugins and the public uploads. The admin path is added from
// the configuration.
var reservedPrefixes = []string{"/_user", "/_bm", "/_quizzes", "/_presentations", "/api", "/storage"}

// errMissingIndex reports a tree without index.html (an unbuilt tree).
var errMissingIndex = errors.New("jz.bm: public/index.html missing; run the frontend build (cd frontend && npm ci && npm run build)")

// contentTypes is consulted before the platform MIME table, which lacks
// .woff2, .woff and .webmanifest on a minimal server.
var contentTypes = map[string]string{
	".html":        "text/html; charset=utf-8",
	".css":         "text/css; charset=utf-8",
	".js":          "text/javascript; charset=utf-8",
	".mjs":         "text/javascript; charset=utf-8",
	".json":        "application/json",
	".map":         "application/json",
	".txt":         "text/plain; charset=utf-8",
	".svg":         "image/svg+xml",
	".png":         "image/png",
	".jpg":         "image/jpeg",
	".webp":        "image/webp",
	".ico":         "image/x-icon",
	".woff2":       "font/woff2",
	".woff":        "font/woff",
	".webmanifest": "application/manifest+json",
}

func contentType(name string) string {
	ext := strings.ToLower(path.Ext(name))
	if ct, ok := contentTypes[ext]; ok {
		return ct
	}
	if ct := mime.TypeByExtension(ext); ct != "" {
		return ct
	}
	return "application/octet-stream"
}

type staticFile struct {
	body  []byte
	etag  string
	ctype string
}

// staticApp serves the built single-page app: files by path, index.html for
// every other extension-less GET outside the reserved prefixes.
type staticApp struct {
	files    map[string]*staticFile
	reserved []string
}

// newStaticApp loads every regular file of fsys whose path has no
// dot-segment. adminPath is the admin mount, kept out of the fallback.
func newStaticApp(fsys fs.FS, adminPath string) (*staticApp, error) {
	if info, err := fs.Stat(fsys, "index.html"); err != nil || !info.Mode().IsRegular() {
		return nil, errMissingIndex
	}
	s := &staticApp{files: map[string]*staticFile{}, reserved: append([]string{}, reservedPrefixes...)}
	if adminPath = "/" + strings.Trim(adminPath, "/"); adminPath != "/" {
		s.reserved = append(s.reserved, adminPath)
	}
	err := fs.WalkDir(fsys, ".", func(name string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if name != "." && strings.HasPrefix(d.Name(), ".") {
			if d.IsDir() {
				return fs.SkipDir
			}
			return nil
		}
		if !d.Type().IsRegular() {
			return nil
		}
		body, err := fs.ReadFile(fsys, name)
		if err != nil {
			return err
		}
		sum := sha256.Sum256(body)
		s.files[name] = &staticFile{
			body:  body,
			etag:  `"` + hex.EncodeToString(sum[:])[:16] + `"`,
			ctype: contentType(name),
		}
		return nil
	})
	if err != nil {
		return nil, fmt.Errorf("jz.bm: public: %w", err)
	}
	return s, nil
}

// isReserved reports whether p is one of the reserved prefixes or below one.
func (s *staticApp) isReserved(p string) bool {
	for _, r := range s.reserved {
		if p == r || strings.HasPrefix(p, r+"/") {
			return true
		}
	}
	return false
}

func (s *staticApp) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if strings.Contains(r.URL.Path, `\`) {
		http.NotFound(w, r)
		return
	}
	clean := path.Clean("/" + r.URL.Path)
	if s.isReserved(clean) {
		http.NotFound(w, r)
		return
	}
	name := strings.TrimPrefix(clean, "/")
	if name == "" {
		name = "index.html"
	}
	for _, seg := range strings.Split(name, "/") {
		if strings.HasPrefix(seg, ".") {
			http.NotFound(w, r)
			return
		}
	}
	if f, ok := s.files[name]; ok {
		s.serve(w, r, name, f)
		return
	}
	// A missing file with an extension is a missing asset, not a client route.
	if path.Ext(name) != "" {
		http.NotFound(w, r)
		return
	}
	s.serve(w, r, "index.html", s.files["index.html"])
}

func (s *staticApp) serve(w http.ResponseWriter, r *http.Request, name string, f *staticFile) {
	h := w.Header()
	h.Set("Content-Type", f.ctype)
	if strings.HasPrefix(name, "assets/") {
		h.Set("Cache-Control", cacheImmutable)
	} else {
		h.Set("Cache-Control", cacheNoCache)
	}
	h.Set("ETag", f.etag)
	h.Set("X-Content-Type-Options", "nosniff")
	h.Set("Referrer-Policy", "same-origin")
	http.ServeContent(w, r, name, time.Time{}, bytes.NewReader(f.body))
}

// newStaticHandler builds the handler over public (nil means the embedded
// public/ directory) with the admin path read from backend.uri.
func newStaticHandler(app *backpack.App, public fs.FS) (http.Handler, error) {
	if public == nil {
		sub, err := fs.Sub(publicFS, "public")
		if err != nil {
			return nil, fmt.Errorf("jz.bm: public: %w", err)
		}
		public = sub
	}
	admin := defaultAdminPath
	if app != nil && app.Config != nil {
		if v := strings.TrimSpace(app.Config.String("backend.uri")); v != "" {
			admin = v
		}
	}
	return newStaticApp(public, admin)
}