// Copyright 2015 Matthew Holt and The Caddy Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package encode implements an encoder middleware for Caddy. The initial
// enhancements related to Accept-Encoding, minimum content length, and
// buffer/writer pools were adapted from https://github.com/xi2/httpgzip
// then modified heavily to accommodate modular encoders and fix bugs.
// Code borrowed from that repository is Copyright (c) 2015 The Httpgzip Authors.
package encode

import (
	"cmp"
	"fmt"
	"io"
	"math"
	"net/http"
	"slices"
	"strconv"
	"strings"
	"sync"

	"github.com/caddyserver/caddy/v2"
	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
)

const sseMediaType = "text/event-stream"

func init() {
	caddy.RegisterModule(Encode{})
}

// Encode is a middleware which can encode responses.
type Encode struct {
	// Selection of compression algorithms to choose from. The best one
	// will be chosen based on the client's Accept-Encoding header.
	EncodingsRaw caddy.ModuleMap `json:"encodings,omitempty" caddy:"namespace=http.encoders"`

	// If the client has no strong preference, choose these encodings in order.
	Prefer []string `json:"prefer,omitempty"`

	// Only encode responses that are at least this many bytes long.
	MinLength int `json:"minimum_length,omitempty"`

	// Only encode responses that match against this ResponseMatcher.
	// The default is a collection of text-based Content-Type headers.
	Matcher *caddyhttp.ResponseMatcher `json:"match,omitempty"`

	writerPools map[string]*sync.Pool // TODO: these pools do not get reused through config reloads...
}

// CaddyModule returns the Caddy module information.
func (Encode) CaddyModule() caddy.ModuleInfo {
	return caddy.ModuleInfo{
		ID:  "http.handlers.encode",
		New: func() caddy.Module { return new(Encode) },
	}
}

// Provision provisions enc.
func (enc *Encode) Provision(ctx caddy.Context) error {
	mods, err := ctx.LoadModule(enc, "EncodingsRaw")
	if err != nil {
		return fmt.Errorf("loading encoder modules: %v", err)
	}
	for modName, modIface := range mods.(map[string]any) {
		err = enc.addEncoding(modIface.(Encoding))
		if err != nil {
			return fmt.Errorf("adding encoding %s: %v", modName, err)
		}
	}
	if enc.MinLength == 0 {
		enc.MinLength = defaultMinLength
	}

	if enc.Matcher == nil {
		// common text-based content types
		// list based on https://developers.cloudflare.com/speed/optimization/content/brotli/content-compression/#compression-between-cloudflare-and-website-visitors
		enc.Matcher = &caddyhttp.ResponseMatcher{
			Headers: http.Header{
				"Content-Type": []string{
					"application/atom+xml*",
					"application/eot*",
					"application/font*",
					"application/geo+json*",
					"application/graphql+json*",
					"application/graphql-response+json*",
					"application/javascript*",
					"application/json*",
					"application/ld+json*",
					"application/manifest+json*",
					"application/opentype*",
					"application/otf*",
					"application/rss+xml*",
					"application/truetype*",
					"application/ttf*",
					"application/vnd.api+json*",
					"application/vnd.ms-fontobject*",
					"application/wasm*",
					"application/x-httpd-cgi*",
					"application/x-javascript*",
					"application/x-opentype*",
					"application/x-otf*",
					"application/x-perl*",
					"application/x-protobuf*",
					"application/x-ttf*",
					"application/xhtml+xml*",
					"application/xml*",
					"font/ttf*",
					"font/otf*",
					"image/svg+xml*",
					"image/vnd.microsoft.icon*",
					"image/x-icon*",
					"multipart/bag*",
					"multipart/mixed*",
					"text/*",
				},
			},
		}
	}

	if len(enc.Prefer) == 0 {
		for _, encName := range []string{"zstd", "br", "gzip"} {
			if _, ok := enc.writerPools[encName]; ok {
				enc.Prefer = append(enc.Prefer, encName)
			}
		}
	}

	return nil
}

// Validate ensures that enc's configuration is valid.
func (enc *Encode) Validate() error {
	check := make(map[string]bool)
	for _, encName := range enc.Prefer {
		if _, ok := enc.writerPools[encName]; !ok {
			return fmt.Errorf("encoding %s not enabled", encName)
		}

		if _, ok := check[encName]; ok {
			return fmt.Errorf("encoding %s is duplicated in prefer", encName)
		}
		check[encName] = true
	}

	return nil
}

func isEncodeAllowed(h http.Header) bool {
	return !strings.Contains(h.Get("Cache-Control"), "no-transform")
}

func (enc *Encode) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
	if isEncodeAllowed(r.Header) {
		for _, encName := range AcceptedEncodings(r, enc.Prefer) {
			if _, ok := enc.writerPools[encName]; !ok {
				continue // encoding not offered
			}
			w = enc.openResponseWriter(encName, w, r.Method == http.MethodConnect)
			defer w.(*responseWriter).Close()

			// to comply with RFC 9110 section 8.8.3(.3), we modify the Etag when encoding
			// by appending a hyphen and the encoder name; the problem is, the client will
			// send back that Etag in an If-None-Match header, but upstream handlers that set
			// the Etag in the first place don't know that we appended to their Etag! so here
			// we have to strip our addition so the upstream handlers can still honor client
			// caches without knowing about our changes...
			if etag := r.Header.Get("If-None-Match"); etag != "" && !strings.HasPrefix(etag, "W/") {
				ourSuffix := "-" + encName + `"`
				if before, ok := strings.CutSuffix(etag, ourSuffix); ok {
					etag = before + `"`
					r.Header.Set("If-None-Match", etag)
				}
			}

			break
		}
	}

	err := next.ServeHTTP(w, r)
	// If there was an error, disable encoding completely
	// This prevents corruption when handle_errors processes the response
	if err != nil {
		if ew, ok := w.(*responseWriter); ok {
			ew.disabled = true
		}
	}

	return err
}

func (enc *Encode) addEncoding(e Encoding) error {
	ae := e.AcceptEncoding()
	if ae == "" {
		return fmt.Errorf("encoder does not specify an Accept-Encoding value")
	}
	if _, ok := enc.writerPools[ae]; ok {
		return fmt.Errorf("encoder already added: %s", ae)
	}
	if enc.writerPools == nil {
		enc.writerPools = make(map[string]*sync.Pool)
	}
	enc.writerPools[ae] = &sync.Pool{
		New: func() any {
			return e.NewEncoder()
		},
	}
	return nil
}

// openResponseWriter creates a new response writer that may (or may not)
// encode the response with encodingName. The returned response writer MUST
// be closed after the handler completes.
func (enc *Encode) openResponseWriter(encodingName string, w http.ResponseWriter, isConnect bool) *responseWriter {
	var rw responseWriter
	return enc.initResponseWriter(&rw, encodingName, w, isConnect)
}

// initResponseWriter initializes the responseWriter instance
// allocated in openResponseWriter, enabling mid-stack inlining.
func (enc *Encode) initResponseWriter(rw *responseWriter, encodingName string, wrappedRW http.ResponseWriter, isConnect bool) *responseWriter {
	if rww, ok := wrappedRW.(*caddyhttp.ResponseWriterWrapper); ok {
		rw.ResponseWriter = rww
	} else {
		rw.ResponseWriter = &caddyhttp.ResponseWriterWrapper{ResponseWriter: wrappedRW}
	}
	rw.encodingName = encodingName
	rw.config = enc
	rw.isConnect = isConnect

	return rw
}

// responseWriter writes to an underlying response writer
// using the encoding represented by encodingName and
// configured by config.
type responseWriter struct {
	http.ResponseWriter
	encodingName string
	w            Encoder
	config       *Encode
	statusCode   int
	wroteHeader  bool
	isConnect    bool
	disabled     bool // disable encoding for this response
}

// WriteHeader stores the status to write when the time comes
// to actually write the header.
func (rw *responseWriter) WriteHeader(status int) {
	rw.statusCode = status
	if status == http.StatusPartialContent {
		rw.disabled = true // partial representations must not be dynamically re-encoded
	}

	h := rw.Header()

	// See #5849 and RFC 9110 section 15.4.5 (https://www.rfc-editor.org/rfc/rfc9110.html#section-15.4.5) - 304
	// Not Modified must have certain headers set as if it was a 200 response, and according to the issue
	// we would miss the Vary header in this case when compression was also enabled; note that we set this
	// header in the responseWriter.init() method but that is only called if we are writing a response body
	if status == http.StatusNotModified && !hasVaryValue(h, "Accept-Encoding") {
		h.Add("Vary", "Accept-Encoding")
	}

	// write status immediately if status is 2xx and the request is CONNECT
	// since it means the response is successful.
	// see: https://github.com/caddyserver/caddy/issues/6733#issuecomment-2525058845
	if rw.isConnect && 200 <= status && status <= 299 {
		rw.ResponseWriter.WriteHeader(status)
		rw.wroteHeader = true
	}

	// write status immediately when status code is informational
	// see: https://caddy.community/t/disappear-103-early-hints-response-with-encode-enable-caddy-v2-7-6/23081/5
	if 100 <= status && status <= 199 {
		rw.ResponseWriter.WriteHeader(status)
	}

	// write header immediately for server-sent events responses, since the
	// body may not be written for a while and the client needs the headers
	// to establish the event stream; see #6293
	if !rw.wroteHeader && (status < 100 || status > 199) && isSSE(h.Get("Content-Type")) {
		rw.init()
		rw.ResponseWriter.WriteHeader(status)
		rw.wroteHeader = true
	}
}

func isSSE(contentType string) bool {
	if len(contentType) < len(sseMediaType) || !strings.EqualFold(contentType[:len(sseMediaType)], sseMediaType) {
		return false
	}

	// After the media type, allow only optional whitespace followed by the
	// end of the value or a parameter separator, so a longer type such as
	// "text/event-streamfoo" or garbage like "text/event-stream nonsense"
	// does not match. TrimLeft on the tail does not allocate.
	rest := strings.TrimLeft(contentType[len(sseMediaType):], " \t")

	return rest == "" || rest[0] == ';'
}

// Match determines, if encoding should be done based on the ResponseMatcher.
func (enc *Encode) Match(rw *responseWriter) bool {
	return enc.Matcher.Match(rw.statusCode, rw.Header())
}

// FlushError is an alternative Flush returning an error. It delays the actual Flush of the underlying
// ResponseWriterWrapper until headers were written.
func (rw *responseWriter) FlushError() error {
	// WriteHeader wasn't called and is a CONNECT request, treat it as a success.
	// otherwise, wait until header is written.
	if rw.isConnect && !rw.wroteHeader && rw.statusCode == 0 {
		rw.WriteHeader(http.StatusOK)
	}

	if !rw.wroteHeader {
		// flushing the underlying ResponseWriter will write header and status code,
		// but we need to delay that until we can determine if we must encode and
		// therefore add the Content-Encoding header; this happens in the first call
		// to rw.Write (see bug in #4314)
		return nil
	}
	// also flushes the encoder, if any
	// see: https://github.com/jjiang-stripe/caddy-slow-gzip
	if rw.w != nil {
		err := rw.w.Flush()
		if err != nil {
			return err
		}
	}
	//nolint:bodyclose
	return http.NewResponseController(rw.ResponseWriter).Flush()
}

// Write writes to the response. If the response qualifies,
// it is encoded using the encoder, which is initialized
// if not done so already.
func (rw *responseWriter) Write(p []byte) (int, error) {
	// ignore zero data writes, probably head request
	if len(p) == 0 {
		return 0, nil
	}

	// WriteHeader wasn't called and is a CONNECT request, treat it as a success.
	// otherwise, determine if the response should be compressed.
	if rw.isConnect && !rw.wroteHeader && rw.statusCode == 0 {
		rw.WriteHeader(http.StatusOK)
	}

	// sniff content-type and determine content-length
	if !rw.wroteHeader && rw.config.MinLength > 0 {
		var gtMinLength bool
		if len(p) > rw.config.MinLength {
			gtMinLength = true
		} else if cl, err := strconv.Atoi(rw.Header().Get("Content-Length")); err == nil && cl > rw.config.MinLength {
			gtMinLength = true
		}

		if gtMinLength {
			if rw.Header().Get("Content-Type") == "" {
				rw.Header().Set("Content-Type", http.DetectContentType(p))
			}
			rw.init()
		}
	}

	// before we write to the response, we need to make
	// sure the header is written exactly once; we do
	// that by checking if a status code has been set,
	// and if so, that means we haven't written the
	// header OR the default status code will be written
	// by the standard library
	if !rw.wroteHeader {
		if rw.statusCode != 0 {
			rw.ResponseWriter.WriteHeader(rw.statusCode)
		}
		rw.wroteHeader = true
	}

	if rw.w != nil {
		return rw.w.Write(p)
	} else {
		return rw.ResponseWriter.Write(p)
	}
}

// used to mask ReadFrom method
type writerOnly struct {
	io.Writer
}

// copied from stdlib
const sniffLen = 512

// ReadFrom will try to use sendfile to copy from the reader to the response writer.
// It's only used if the response writer implements io.ReaderFrom and the data can't be compressed.
// It's based on the standard library HTTP/1.1 response writer implementation.
// https://github.com/golang/go/blob/f4e3ec3dbe3b8e04a058d266adf8e048bab563f2/src/net/http/server.go#L586
func (rw *responseWriter) ReadFrom(r io.Reader) (int64, error) {
	rf, ok := rw.ResponseWriter.(io.ReaderFrom)
	// sendfile can't be used anyway
	if !ok {
		// mask ReadFrom to avoid infinite recursion
		return io.Copy(writerOnly{rw}, r)
	}

	var ns int64
	// try to sniff the content type and determine if the response should be compressed
	if !rw.wroteHeader && rw.config.MinLength > 0 {
		var (
			err error
			buf [sniffLen]byte
		)
		// mask ReadFrom to let Write determine if the response should be compressed
		ns, err = io.CopyBuffer(writerOnly{rw}, io.LimitReader(r, sniffLen), buf[:])
		if err != nil || ns < sniffLen {
			return ns, err
		}
	}

	// the response will be compressed, no sendfile support
	if rw.w != nil {
		nr, err := io.Copy(rw.w, r)
		return nr + ns, err
	}
	nr, err := rf.ReadFrom(r)
	return nr + ns, err
}

// Close writes any remaining buffered response and
// deallocates any active resources.
func (rw *responseWriter) Close() error {
	// didn't write, probably head request
	if !rw.wroteHeader {
		cl, err := strconv.Atoi(rw.Header().Get("Content-Length"))
		if err == nil && cl > rw.config.MinLength {
			rw.init()
		}

		// issue #5059, don't write status code if not set explicitly.
		if rw.statusCode != 0 {
			rw.ResponseWriter.WriteHeader(rw.statusCode)
		}
		rw.wroteHeader = true
	}

	var err error
	if rw.w != nil {
		err = rw.w.Close()
		rw.w.Reset(nil)
		rw.config.writerPools[rw.encodingName].Put(rw.w)
		rw.w = nil
	}
	return err
}

// Unwrap returns the underlying ResponseWriter.
func (rw *responseWriter) Unwrap() http.ResponseWriter {
	return rw.ResponseWriter
}

// init should be called before we write a response, if rw.buf has contents.
func (rw *responseWriter) init() {
	// Don't initialize encoder for responses that must not be encoded.
	if rw.disabled {
		return
	}

	hdr := rw.Header()

	if hdr.Get("Content-Encoding") == "" && isEncodeAllowed(hdr) &&
		rw.config.Match(rw) {
		rw.w = rw.config.writerPools[rw.encodingName].Get().(Encoder)
		rw.w.Reset(rw.ResponseWriter)
		hdr.Del("Content-Length") // https://github.com/golang/go/issues/14975
		hdr.Set("Content-Encoding", rw.encodingName)
		if !hasVaryValue(hdr, "Accept-Encoding") {
			hdr.Add("Vary", "Accept-Encoding")
		}
		hdr.Del("Accept-Ranges") // we don't know ranges for dynamically-encoded content

		// strong ETags need to be distinct depending on the encoding ("selected representation")
		// see RFC 9110 section 8.8.3.3:
		// https://www.rfc-editor.org/rfc/rfc9110.html#name-example-entity-tags-varying
		// I don't know a great way to do this... how about appending? That's a neat trick!
		// (We have to strip the value we append from If-None-Match headers before
		// sending subsequent requests back upstream, however, since upstream handlers
		// don't know about our appending to their Etag since they've already done their work)
		if etag := hdr.Get("Etag"); etag != "" && !strings.HasPrefix(etag, "W/") {
			etag = fmt.Sprintf(`%s-%s"`, strings.TrimSuffix(etag, `"`), rw.encodingName)
			hdr.Set("Etag", etag)
		}
	}
}

func hasVaryValue(hdr http.Header, target string) bool {
	for _, vary := range hdr.Values("Vary") {
		for val := range strings.SplitSeq(vary, ",") {
			if strings.EqualFold(strings.TrimSpace(val), target) {
				return true
			}
		}
	}
	return false
}

// AcceptedEncodings returns the list of encodings that the
// client supports, in descending order of preference.
// The client preference via q-factor and the server
// preference via Prefer setting are taken into account. If
// the Sec-WebSocket-Key header is present then non-identity
// encodings are not considered. See
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html.
func AcceptedEncodings(r *http.Request, preferredOrder []string) []string {
	acceptEncHeader := r.Header.Get("Accept-Encoding")
	if acceptEncHeader == "" {
		return []string{}
	}
	websocketKey := r.Header.Get("Sec-Websocket-Key")

	prefs := make([]encodingPreference, 0, strings.Count(acceptEncHeader, ",")+1)

	for accepted := range strings.SplitSeq(acceptEncHeader, ",") {
		encName, params, found := strings.Cut(accepted, ";")
		encName = strings.ToLower(strings.TrimSpace(encName))

		// determine q-factor
		qFactor := 1.0
		if found {
			qFactorStr, _, _ := strings.Cut(params, ";")
			qFactorStr = strings.TrimSpace(qFactorStr)
			if len(qFactorStr) >= 2 && strings.EqualFold(qFactorStr[:2], "q=") {
				if qFactorFloat, err := strconv.ParseFloat(qFactorStr[2:], 32); err == nil {
					if qFactorFloat >= 0 && qFactorFloat <= 1 {
						qFactor = qFactorFloat
					}
				}
			}
		}

		// encodings with q-factor of 0 are not accepted;
		// use a small threshold to account for float precision
		if qFactor < 0.00001 {
			continue
		}

		// don't encode WebSocket handshakes
		if websocketKey != "" && encName != "identity" {
			continue
		}

		// set server preference
		prefOrder := slices.Index(preferredOrder, encName)
		if prefOrder > -1 {
			prefOrder = len(preferredOrder) - prefOrder
		}

		prefs = append(prefs, encodingPreference{
			encoding:    encName,
			q:           qFactor,
			preferOrder: prefOrder,
		})
	}

	// sort preferences by descending q-factor first, then by preferOrder
	slices.SortStableFunc(prefs, func(a, b encodingPreference) int {
		if math.Abs(a.q-b.q) < 0.00001 {
			return cmp.Compare(b.preferOrder, a.preferOrder)
		}
		return cmp.Compare(b.q, a.q)
	})

	prefEncNames := make([]string, len(prefs))
	for i := range prefs {
		prefEncNames[i] = prefs[i].encoding
	}

	return prefEncNames
}

// encodingPreference pairs an encoding with its q-factor.
type encodingPreference struct {
	encoding    string
	q           float64
	preferOrder int
}

// Encoder is a type which can encode a stream of data.
type Encoder interface {
	io.WriteCloser
	Reset(io.Writer)
	Flush() error // encoder by default buffers data to maximize compressing rate
}

// Encoding is a type which can create encoders of its kind
// and return the name used in the Accept-Encoding header.
type Encoding interface {
	AcceptEncoding() string
	NewEncoder() Encoder
}

// Precompressed is a type which returns filename suffix of precompressed
// file and Accept-Encoding header to use when serving this file.
type Precompressed interface {
	AcceptEncoding() string
	Suffix() string
}

// defaultMinLength is the minimum length at which to compress content.
const defaultMinLength = 512

// Interface guards
var (
	_ caddy.Provisioner           = (*Encode)(nil)
	_ caddy.Validator             = (*Encode)(nil)
	_ caddyhttp.MiddlewareHandler = (*Encode)(nil)
)
