package httpapi

import (
	"bufio"
	"compress/gzip"
	"io"
	"io/fs"
	"net"
	"net/http"
	"path"
	"strconv"
	"strings"

	"github.com/mithral/vellum/web"
)

// mountWeb serves the embedded single-page app at the root path.
//
// Unknown non-API paths fall back to index.html so client-side routes work on
// reload. API paths never fall back: a typo in an endpoint returns JSON, not
// an HTML page.
func (s *Server) mountWeb() {
	sub, err := fs.Sub(web.Dist, "dist")
	if err != nil {
		s.mux.HandleFunc("/", s.handleNotFound)
		return
	}
	index, err := fs.ReadFile(sub, "index.html")
	if err != nil {
		// Frontend not built into this binary. Keep JSON 404s rather than
		// pretending the app exists.
		s.log.Warn("web UI not embedded; run the frontend build to include it")
		s.mux.HandleFunc("/", s.handleNotFound)
		return
	}

	files := http.FileServer(http.FS(sub))

	// Only the embedded static assets are wrapped for compression. API
	// responses and media are served by other handlers and are never touched.
	serveFiles := compressStatic(files.ServeHTTP)
	serveShell := compressStatic(func(w http.ResponseWriter, r *http.Request) {
		serveIndex(w, index)
	})

	s.mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if isAPIPath(r.URL.Path) {
			s.handleNotFound(w, r)
			return
		}
		if r.URL.Path == "/" || r.URL.Path == "" {
			serveShell(w, r)
			return
		}
		name := strings.TrimPrefix(path.Clean(r.URL.Path), "/")
		if f, err := sub.Open(name); err == nil {
			_ = f.Close()
			serveFiles(w, r)
			return
		}
		// SPA route: hand back the shell and let the client router decide.
		serveShell(w, r)
	})
}

func serveIndex(w http.ResponseWriter, index []byte) {
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.Header().Set("Cache-Control", "no-cache")
	_, _ = w.Write(index)
}

func isAPIPath(p string) bool {
	return p == "/healthz" || strings.HasPrefix(p, "/api/")
}

// ── Static-asset gzip ──────────────────────────────────────────────────
//
// The built SPA ships uncompressed: the JS bundle is ~456 KB and the CSS
// ~31 KB on every cold load. compressStatic wraps only the embedded static
// handlers with gzip negotiation; API responses and media are never wrapped.
//
// The narrow scope is deliberate. (a) Media is already compressed. (b) A Range
// response is used for seeking, and re-encoding it would invalidate the byte
// offsets. (c) server.go's statusRecorder was just fixed to forward
// http.Flusher and io.ReaderFrom for streaming; wrapping every response in a
// gzip writer would force all bodies through a compressor and undo that.
// Restricting the wrapper to the static file handler avoids all three.

// compressStatic gzips a static handler's response when the client accepts
// gzip and the response is a compressible text type. Close is called after the
// handler returns so the gzip trailer is always written.
func compressStatic(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		gz := newGzipResponseWriter(w, r)
		next(gz, r)
		_ = gz.Close()
	}
}

// compressibleType reports whether a Content-Type should be gzip-encoded. Only
// text-like types are listed; images, fonts, wasm and other media are already
// compressed, so re-encoding them wastes CPU for no gain.
func compressibleType(contentType string) bool {
	if i := strings.IndexByte(contentType, ';'); i >= 0 {
		contentType = contentType[:i] // drop "charset=utf-8" and friends
	}
	switch strings.ToLower(strings.TrimSpace(contentType)) {
	case "text/html",
		"text/css",
		"application/javascript",
		"text/javascript",
		"application/json",
		"image/svg+xml",
		"application/manifest+json":
		return true
	}
	return false
}

// acceptsGzip reports whether the client's Accept-Encoding allows gzip. An
// explicit "gzip;q=0" is a refusal, so it does not count.
func acceptsGzip(header string) bool {
	for _, part := range strings.Split(header, ",") {
		token, params, _ := strings.Cut(part, ";")
		if !strings.EqualFold(strings.TrimSpace(token), "gzip") {
			continue
		}
		for _, p := range strings.Split(params, ";") {
			if v, ok := strings.CutPrefix(strings.TrimSpace(p), "q="); ok {
				if q, err := strconv.ParseFloat(strings.TrimSpace(v), 64); err == nil && q == 0 {
					return false
				}
			}
		}
		return true
	}
	return false
}

// gzipResponseWriter wraps a static response so its body can be gzip-encoded.
//
// Like statusRecorder it forwards Flush, Hijack and ReadFrom. While the body is
// being compressed those forwards must route through the compressor (or flush
// it first), or raw bytes would be written under a gzip Content-Encoding.
type gzipResponseWriter struct {
	http.ResponseWriter
	eligible    bool
	compress    bool
	wroteHeader bool
	gz          *gzip.Writer
}

func newGzipResponseWriter(w http.ResponseWriter, r *http.Request) *gzipResponseWriter {
	return &gzipResponseWriter{
		ResponseWriter: w,
		// A Range request must stay identity so its byte offsets remain valid.
		eligible: acceptsGzip(r.Header.Get("Accept-Encoding")) && r.Header.Get("Range") == "",
	}
}

// WriteHeader decides compression once the handler has set Content-Type. The
// decision has to happen before the status line is committed.
func (g *gzipResponseWriter) WriteHeader(code int) {
	if g.wroteHeader {
		return
	}
	g.wroteHeader = true

	h := g.Header()
	if g.eligible &&
		code == http.StatusOK &&
		h.Get("Content-Encoding") == "" &&
		h.Get("Content-Length") != "0" &&
		compressibleType(h.Get("Content-Type")) {
		g.compress = true
		h.Set("Content-Encoding", "gzip")
		h.Add("Vary", "Accept-Encoding")
		// The compressed length is not known until the body is written, and
		// the handler's original Content-Length is now wrong.
		h.Del("Content-Length")
	}
	g.ResponseWriter.WriteHeader(code)
}

// writer lazily creates the gzip stream on the first body byte. A HEAD request
// (or a bodyless response) never writes, so no empty gzip stream is emitted and
// net/http has nothing from which to fabricate a misleading Content-Length.
func (g *gzipResponseWriter) writer() *gzip.Writer {
	if g.gz == nil {
		g.gz = gzip.NewWriter(g.ResponseWriter)
	}
	return g.gz
}

func (g *gzipResponseWriter) Write(b []byte) (int, error) {
	if !g.wroteHeader {
		g.WriteHeader(http.StatusOK)
	}
	if g.compress {
		return g.writer().Write(b)
	}
	return g.ResponseWriter.Write(b)
}

// Flush forwards to the underlying http.Flusher, after flushing any buffered
// compressed bytes so the client sees a complete gzip block.
func (g *gzipResponseWriter) Flush() {
	if !g.wroteHeader {
		g.WriteHeader(http.StatusOK)
	}
	if g.compress && g.gz != nil {
		_ = g.gz.Flush()
	}
	if f, ok := g.ResponseWriter.(http.Flusher); ok {
		f.Flush()
	}
}

// Hijack forwards to the underlying http.Hijacker for protocols that take over
// the connection.
func (g *gzipResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	if hj, ok := g.ResponseWriter.(http.Hijacker); ok {
		return hj.Hijack()
	}
	return nil, nil, http.ErrNotSupported
}

// ReadFrom forwards to the underlying io.ReaderFrom so io.Copy keeps the
// sendfile fast path. When compressing, the bytes must pass through the gzip
// writer instead; the returned count is the source byte count.
func (g *gzipResponseWriter) ReadFrom(src io.Reader) (int64, error) {
	if !g.wroteHeader {
		g.WriteHeader(http.StatusOK)
	}
	if g.compress {
		return io.Copy(g.writer(), src)
	}
	if rf, ok := g.ResponseWriter.(io.ReaderFrom); ok {
		return rf.ReadFrom(src)
	}
	return io.Copy(g.ResponseWriter, src)
}

// Close flushes the gzip trailer. It must run after the static handler returns,
// or the body will be missing its checksum and the client will reject it.
func (g *gzipResponseWriter) Close() error {
	if g.compress && g.gz != nil {
		return g.gz.Close()
	}
	return nil
}