// Copyright 2013 Gary Burd. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package websocket

import (
	"bufio"
	"errors"
	"net"
	"net/http"
	"strings"
)

// HandshakeError describes an error with the handshake from the peer.
type HandshakeError struct {
	message string
}

func (e HandshakeError) Error() string { return e.message }

// Upgrade upgrades the HTTP server connection to the WebSocket protocol.
//
// The application is responsible for checking the request origin before
// calling Upgrade. An example implementation of the same origin policy is:
//
//	if req.Header.Get("Origin") != "http://"+req.Host {
//		http.Error(w, "Origin not allowed", 403)
//		return
//	}
//
// If the endpoint supports WebSocket subprotocols, then the application is
// responsible for selecting a subprotocol that is acceptable to the client and
// echoing that value back to the client. Use the Subprotocols function to get
// the list of protocols specified by the client. Use the
// Sec-Websocket-Protocol response header to echo the selected protocol back to
// the client.
//
// Appilcations can set cookies by adding a Set-Cookie header to the response
// header.
//
// If the request is not a valid WebSocket handshake, then Upgrade returns an
// error of type HandshakeError. Applications should handle this error by
// replying to the client with an HTTP error response.
func Upgrade(w http.ResponseWriter, r *http.Request, responseHeader http.Header, readBufSize, writeBufSize int) (*Conn, error) {

	if values := r.Header["Sec-Websocket-Version"]; len(values) == 0 || values[0] != "13" {
		return nil, HandshakeError{"websocket: version != 13"}
	}

	if !tokenListContainsValue(r.Header, "Connection", "upgrade") {
		return nil, HandshakeError{"websocket: connection header != upgrade"}
	}

	if !tokenListContainsValue(r.Header, "Upgrade", "websocket") {
		return nil, HandshakeError{"websocket: upgrade != websocket"}
	}

	var challengeKey string
	values := r.Header["Sec-Websocket-Key"]
	if len(values) == 0 || values[0] == "" {
		return nil, HandshakeError{"websocket: key missing or blank"}
	}
	challengeKey = values[0]

	var (
		netConn net.Conn
		br      *bufio.Reader
		err     error
	)

	h, ok := w.(http.Hijacker)
	if !ok {
		return nil, errors.New("websocket: response does not implement http.Hijacker")
	}
	var rw *bufio.ReadWriter
	netConn, rw, err = h.Hijack()
	br = rw.Reader

	if br.Buffered() > 0 {
		netConn.Close()
		return nil, errors.New("websocket: client sent data before handshake is complete")
	}

	c := newConn(netConn, true, readBufSize, writeBufSize)

	p := c.writeBuf[:0]
	p = append(p, "HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-WebSocket-Accept: "...)
	p = append(p, computeAcceptKey(challengeKey)...)
	p = append(p, "\r\n"...)
	for k, vs := range responseHeader {
		for _, v := range vs {
			p = append(p, k...)
			p = append(p, ": "...)
			for i := 0; i < len(v); i++ {
				b := v[i]
				if b <= 31 {
					// prevent response splitting.
					b = ' '
				}
				p = append(p, b)
			}
			p = append(p, "\r\n"...)
		}
	}
	p = append(p, "\r\n"...)

	if _, err = netConn.Write(p); err != nil {
		netConn.Close()
		return nil, err
	}

	return c, nil
}

// Subprotocols returns the subprotocols requested by the client in the
// Sec-Websocket-Protocol header.
func Subprotocols(r *http.Request) []string {
	h := strings.TrimSpace(r.Header.Get("Sec-Websocket-Protocol"))
	if h == "" {
		return nil
	}
	protocols := strings.Split(h, ",")
	for i := range protocols {
		protocols[i] = strings.TrimSpace(protocols[i])
	}
	return protocols
}