// Package boardwalk serves the embedded admin SPA build (D-01, D-02).
//
// The committed dist/ is path-agnostic: Vite builds it with a relative base
// and index.html carries the __SUMMER_ADMIN_BASE__ token. Handler rewrites
// index.html once for the configured backend.uri, serves hashed assets with
// long-lived caching, falls back to index.html for client-side routes and
// hands every unmatched api/ path back to the caller so API misses stay JSON.
//
// The handler also computes one build id when it is built (BuildID: the dist
// tree plus the binary's build information). Every response carries it in the
// X-Summer-Build header, {prefix}/build.json answers it, and index.html gets
// it in the summer-admin-build meta, so an open admin tab can tell that a new
// version was deployed.
package boardwalk

import (
	"bytes"
	"crypto/sha256"
	"embed"
	"encoding/hex"
	"errors"
	"fmt"
	"html"
	"io"
	"io/fs"
	"mime"
	"net/http"
	"path"
	"runtime/debug"
	"slices"
	"strconv"
	"strings"
	"time"
)

//go:embed all:dist
var distFS embed.FS

// BaseToken is replaced in dist/index.html by the configured admin prefix.
const BaseToken = "__SUMMER_ADMIN_BASE__"

// BuildHeader is the response header that carries the build id on every
// response the handler writes.
const BuildHeader = "X-Summer-Build"

// BuildToken is the token in dist/index.html's summer-admin-build meta that
// the handler replaces with the build id. An index without it is served
// unchanged and the SPA turns its version check off.
const BuildToken = "__SUMMER_ADMIN_BUILD__"

// BuildPath is the path under the prefix that answers the current build as
// JSON ({"build":"<id>"}, no-store). It wins over a dist file of that name.
const BuildPath = "build.json"

// buildIDLength matches the length cabana uses for plugin asset versions.
const buildIDLength = 12

// readBuildInfo reads the binary's build information; tests replace it.
var readBuildInfo = debug.ReadBuildInfo

// contentSecurityPolicy keeps the admin out of frames and allows scripts
// only from its own origin; the build contains no inline script. Injected
// markup can neither post a native form off-origin (form-action 'self') nor
// frame fetched content (frame-src 'none'); a sandboxed srcdoc preview is not
// fetched, so it still renders.
const contentSecurityPolicy = "frame-ancestors 'none'; base-uri 'none'; object-src 'none'; script-src 'self'; form-action 'self'; frame-src 'none'"

var contentTypes = map[string]string{
	".js":    "text/javascript; charset=utf-8",
	".mjs":   "text/javascript; charset=utf-8",
	".css":   "text/css; charset=utf-8",
	".html":  "text/html; charset=utf-8",
	".ico":   "image/x-icon",
	".woff2": "font/woff2",
	".woff":  "font/woff",
	".svg":   "image/svg+xml",
	".json":  "application/json",
}

// Dist returns the embedded build tree rooted at dist/.
func Dist() (fs.FS, error) {
	return fs.Sub(distFS, "dist")
}

// Handler serves the embedded SPA under prefix (for example /backend).
// notFoundAPI answers any request whose path under the prefix is api or
// starts with api/; it must write the admin API's JSON 404 envelope.
func Handler(prefix string, notFoundAPI http.Handler) (http.Handler, error) {
	root, err := Dist()
	if err != nil {
		return nil, err
	}
	return newHandler(root, prefix, notFoundAPI)
}

func newHandler(root fs.FS, prefix string, notFoundAPI http.Handler) (http.Handler, error) {
	if notFoundAPI == nil {
		return nil, errors.New("boardwalk: notFoundAPI handler is nil")
	}
	prefix = strings.TrimRight(prefix, "/")
	if !strings.HasPrefix(prefix, "/") {
		return nil, fmt.Errorf("boardwalk: prefix %q must start with /", prefix)
	}
	raw, err := fs.ReadFile(root, "index.html")
	if err != nil {
		return nil, fmt.Errorf("boardwalk: dist/index.html: %w", err)
	}
	index, err := RewriteIndex(raw, prefix)
	if err != nil {
		return nil, err
	}
	// The id is computed once, when the handler is built; it is hex, so it
	// needs no escaping in the meta.
	build, err := BuildID(root, buildInfo())
	if err != nil {
		return nil, err
	}
	index = bytes.ReplaceAll(index, []byte(BuildToken), []byte(build))
	return &handler{root: root, prefix: prefix, index: index, build: build, notFoundAPI: notFoundAPI}, nil
}

// buildInfo returns the binary's build information, or nil when it is not
// available.
func buildInfo() *debug.BuildInfo {
	info, ok := readBuildInfo()
	if !ok {
		return nil
	}
	return info
}

// BuildID identifies one deploy of the admin: the first 12 lowercase hex
// characters of a sha256 over
//
//   - the dist tree, raw (before any prefix rewrite): every regular file in
//     fs.WalkDir order as its slash path, its length and its contents;
//   - then, when info is not nil, the binary's build identity: the Go
//     version, the main module path, version and sum, the vcs.revision and
//     vcs.modified settings when present, and every dependency's path,
//     version and sum (and its replacement's), sorted by path.
//
// Other build settings (flags, GOOS and the like) are left out. With info nil
// the id is the dist hash alone. The same tree with the same info always
// gives the same id; a changed dist file or a different binary (a new commit,
// a dependency bump, a toolchain change) gives a new one.
func BuildID(root fs.FS, info *debug.BuildInfo) (string, error) {
	sum := sha256.New()
	err := fs.WalkDir(root, ".", func(name string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if !d.Type().IsRegular() {
			return nil
		}
		body, err := fs.ReadFile(root, name)
		if err != nil {
			return err
		}
		_, _ = io.WriteString(sum, name)
		_, _ = sum.Write([]byte{0})
		_, _ = io.WriteString(sum, strconv.Itoa(len(body)))
		_, _ = sum.Write([]byte{0})
		_, _ = sum.Write(body)
		return nil
	})
	if err != nil {
		return "", fmt.Errorf("boardwalk: build id: %w", err)
	}
	if info != nil {
		writeBuildInfo(sum, info)
	}
	return hex.EncodeToString(sum.Sum(nil))[:buildIDLength], nil
}

// writeBuildInfo writes the deploy-identifying part of info, one line per
// item, after a NUL-separated marker.
func writeBuildInfo(w io.Writer, info *debug.BuildInfo) {
	_, _ = io.WriteString(w, "\x00buildinfo\x00")
	fmt.Fprintf(w, "go %s\n", info.GoVersion)
	fmt.Fprintf(w, "main %s %s %s\n", info.Main.Path, info.Main.Version, info.Main.Sum)
	for _, key := range []string{"vcs.revision", "vcs.modified"} {
		for _, s := range info.Settings {
			if s.Key == key {
				fmt.Fprintf(w, "setting %s=%s\n", s.Key, s.Value)
				break
			}
		}
	}
	deps := slices.Clone(info.Deps)
	slices.SortStableFunc(deps, func(a, b *debug.Module) int {
		return strings.Compare(a.Path, b.Path)
	})
	for _, d := range deps {
		fmt.Fprintf(w, "dep %s %s %s", d.Path, d.Version, d.Sum)
		if d.Replace != nil {
			fmt.Fprintf(w, " => %s %s %s", d.Replace.Path, d.Replace.Version, d.Replace.Sum)
		}
		_, _ = io.WriteString(w, "\n")
	}
}

// RewriteIndex points relative asset URLs at prefix and injects the prefix
// into the summer-admin-base meta. It fails when the token is absent, which
// catches a stale or hand-edited dist at boot.
func RewriteIndex(raw []byte, prefix string) ([]byte, error) {
	if !bytes.Contains(raw, []byte(BaseToken)) {
		return nil, errors.New("boardwalk: dist/index.html has no " + BaseToken + " token; rebuild the admin SPA")
	}
	escaped := html.EscapeString(prefix)
	out := bytes.ReplaceAll(raw, []byte(`="./`), []byte(`="`+escaped+`/`))
	out = bytes.ReplaceAll(out, []byte(BaseToken), []byte(escaped))
	return out, nil
}

type handler struct {
	root        fs.FS
	prefix      string
	index       []byte
	build       string
	notFoundAPI http.Handler
}

func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	SetSecurityHeaders(w.Header())
	w.Header().Set(BuildHeader, h.build)
	rel := strings.TrimPrefix(r.URL.Path, h.prefix)
	rel = strings.TrimPrefix(rel, "/")
	if rel == "api" || strings.HasPrefix(rel, "api/") {
		h.notFoundAPI.ServeHTTP(w, r)
		return
	}
	name := strings.TrimPrefix(path.Clean("/"+rel), "/")
	if name == BuildPath {
		h.serveBuild(w)
		return
	}
	if name == "" || name == "index.html" {
		h.serveIndex(w, r)
		return
	}
	if info, err := fs.Stat(h.root, name); err == nil {
		if info.Mode().IsRegular() {
			h.serveFile(w, r, name)
			return
		}
		// A directory is never listed; it is treated as a client route.
		h.serveIndex(w, r)
		return
	}
	if path.Ext(name) != "" {
		http.NotFound(w, r)
		return
	}
	h.serveIndex(w, r)
}

func (h *handler) serveIndex(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.Header().Set("Cache-Control", "no-store")
	http.ServeContent(w, r, "index.html", time.Time{}, bytes.NewReader(h.index))
}

// serveBuild answers the handler's build id as {"build":"<id>"}; the id is
// hex, so it is written without marshalling.
func (h *handler) serveBuild(w http.ResponseWriter) {
	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Cache-Control", "no-store")
	_, _ = io.WriteString(w, `{"build":"`+h.build+`"}`)
}

func (h *handler) serveFile(w http.ResponseWriter, r *http.Request, name string) {
	body, err := fs.ReadFile(h.root, name)
	if err != nil {
		http.NotFound(w, r)
		return
	}
	w.Header().Set("Content-Type", ContentType(name))
	if strings.HasPrefix(name, "assets/") {
		w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
	} else {
		w.Header().Set("Cache-Control", "no-cache")
	}
	http.ServeContent(w, r, path.Base(name), time.Time{}, bytes.NewReader(body))
}

// ContentType is the Content-Type the admin serves for a file name: explicit
// UTF-8 JavaScript and CSS types (module scripts and nosniff'd stylesheets
// need them), then the platform MIME table, then application/octet-stream.
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"
}

// SetSecurityHeaders sets the admin response headers: nosniff, a same-origin
// referrer policy, no framing, the admin Content-Security-Policy
// (script-src 'self', form-action 'self', frame-src 'none': no native form
// post off-origin, no framed content) and noindex.
func SetSecurityHeaders(h http.Header) {
	h.Set("X-Content-Type-Options", "nosniff")
	h.Set("Referrer-Policy", "same-origin")
	h.Set("X-Frame-Options", "DENY")
	h.Set("Content-Security-Policy", contentSecurityPolicy)
	h.Set("X-Robots-Tag", "noindex, nofollow")
}