You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: server.go
    path of file: ./repos/websocket
    
    the code of the file until where you have to start completion: // Copyright 2013 The Gorilla WebSocket Authors. 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"
	"io"
	"log"
	"net/http"
	"net/url"
	"strings"
	"time"
)

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

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

// Upgrader specifies parameters for upgrading an HTTP connection to a
// WebSocket connection.
//
// It is safe to call Upgrader's methods concurrently.
type Upgrader struct {
	// HandshakeTimeout specifies the duration for the handshake to complete.
	HandshakeTimeout time.Duration

	// ReadBufferSize and WriteBufferSize specify I/O buffer sizes in bytes. If a buffer
	// size is zero, then buffers allocated by the HTTP server are used. The
	// I/O buffer sizes do not limit the size of the messages that can be sent
	// or received.
	// The default value is 4096 bytes, 4kb.
	ReadBufferSize, WriteBufferSize int

	// WriteBufferPool is a pool of buffers for write operations. If the value
	// is not set, then write buffers are allocated to the connection for the
	// lifetime of the connection.
	//
	// A pool is most useful when the application has a modest volume of writes
	// across a large number of connections.
	//
	// Applications should use a single pool for each unique value of
	// WriteBufferSize.
	WriteBufferPool BufferPool

	// Subprotocols specifies the server's supported protocols in order of
	// preference. If this field is not nil, then the Upgrade method negotiates a
	// subprotocol by selecting the first match in this list with a protocol
	// requested by the client. If there's no match, then no protocol is
	// negotiated (the Sec-Websocket-Protocol header is not included in the
	// handshake response).
	Subprotocols []string

	// Error specifies the function for generating HTTP error responses. If Error
	// is nil, then http.Error is used to generate the HTTP response.
	Error func(w http.ResponseWriter, r *http.Request, status int, reason error)

	// CheckOrigin returns true if the request Origin header is acceptable. If
	// CheckOrigin is nil, then a safe default is used: return false if the
	// Origin request header is present and the origin host is not equal to
	// request Host header.
	//
	// A CheckOrigin function should carefully validate the request origin to
	// prevent cross-site request forgery.
	CheckOrigin func(r *http.Request) bool

	// EnableCompression specify if the server should attempt to negotiate per
	// message compression (RFC 7692). Setting this value to true does not
	// guarantee that compression will be supported. Currently only "no context
	// takeover" modes are supported.
	EnableCompression bool
}

func (u *Upgrader) returnError(w http.ResponseWriter, r *http.Request, status int, reason string) (*Conn, error) {
	err := HandshakeError{reason}
	if u.Error != nil {
		u.Error(w, r, status, err)
	} else {
		w.Header().Set("Sec-Websocket-Version", "13")
		http.Error(w, http.StatusText(status), status)
	}
	return nil, err
}

// checkSameOrigin returns true if the origin is not set or is equal to the request host.
func checkSameOrigin(r *http.Request) bool {
	origin := r.Header["Origin"]
	if len(origin) == 0 {
		return true
	}
	u, err := url.Parse(origin[0])
	if err != nil {
		return false
	}
	return equalASCIIFold(u.Host, r.Host)
}

func (u *Upgrader) selectSubprotocol(r *http.Request, responseHeader http.Header) string {
	if u.Subprotocols != nil {
		clientProtocols := Subprotocols(r)
		for _, clientProtocol := range clientProtocols {
			for _, serverProtocol := range u.Subprotocols {
				if clientProtocol == serverProtocol {
					return clientProtocol
				}
			}
		}
	} else if responseHeader != nil {
		return responseHeader.Get("Sec-Websocket-Protocol")
	}
	return ""
}

// Upgrade upgrades the HTTP
    
func (u *Upgrader) Upgrade(w http.ResponseWriter, r *http.Request, responseHeader http.Header) (*Conn, error) {
	const badHandshake = "websocket: the client is not using the websocket protocol: "

	if !tokenListContainsValue(r.Header, "Connection", "upgrade") {
		return u.returnError(w, r, http.StatusBadRequest, badHandshake+"'upgrade' token not found in 'Connection' header")
	}

	if !tokenListContainsValue(r.Header, "Upgrade", "websocket") {
		return u.returnError(w, r, http.StatusBadRequest, badHandshake+"'websocket' token not found in 'Upgrade' header")
	}

	if r.Method != http.MethodGet {
		return u.returnError(w, r, http.StatusMethodNotAllowed, badHandshake+"request method is not GET")
	}

	if !tokenListContainsValue(r.Header, "Sec-Websocket-Version", "13") {
		return u.returnError(w, r, http.StatusBadRequest, "websocket: unsupported version: 13 not found in 'Sec-Websocket-Version' header")
	}

	if _, ok := responseHeader["Sec-Websocket-Extensions"]; ok {
		return u.returnError(w, r, http.StatusInternalServerError, "websocket: application specific 'Sec-WebSocket-Extensions' headers are unsupported")
	}

	checkOrigin := u.CheckOrigin
	if checkOrigin == nil {
		checkOrigin = checkSameOrigin
	}
	if !checkOrigin(r) {
		return u.returnError(w, r, http.StatusForbidden, "websocket: request origin not allowed by Upgrader.CheckOrigin")
	}

	challengeKey := r.Header.Get("Sec-Websocket-Key")
	if !isValidChallengeKey(challengeKey) {
		return u.returnError(w, r, http.StatusBadRequest, "websocket: not a websocket handshake: 'Sec-WebSocket-Key' header must be Base64 encoded value of 16-byte in length")
	}

	subprotocol := u.selectSubprotocol(r, responseHeader)

	// Negotiate PMCE
	var compress bool
	if u.EnableCompression {
		for _, ext := range parseExtensions(r.Header) {
			if ext[""] != "permessage-deflate" {
				continue
			}
			compress = true
			break
		}
	}

	netConn, brw, err := http.NewResponseController(w).Hijack()
	if err != nil {
		return u.returnError(w, r, http.StatusInternalServerError, err.Error())
	}

	if brw.Reader.Buffered() > 0 {
		if err := netConn.Close(); err != nil {
			log.Printf("websocket: failed to close network connection: %v", err)
		}
		return nil, errors.New("websocket: client sent data before handshake is complete")
	}

	var br *bufio.Reader
	if u.ReadBufferSize == 0 && bufioReaderSize(netConn, brw.Reader) > 256 {
		// Reuse hijacked buffered reader as connection reader.
		br = brw.Reader
	}

	buf := bufioWriterBuffer(netConn, brw.Writer)

	var writeBuf []byte
	if u.WriteBufferPool == nil && u.WriteBufferSize == 0 && len(buf) >= maxFrameHeaderSize+256 {
		// Reuse hijacked write buffer as connection buffer.
		writeBuf = buf
	}

	c := newConn(netConn, true, u.ReadBufferSize, u.WriteBufferSize, u.WriteBufferPool, br, writeBuf)
	c.subprotocol = subprotocol

	if compress {
		c.newCompressionWriter = compressNoContextTakeover
		c.newDecompressionReader = decompressNoContextTakeover
	}

	// Use larger of hijacked buffer and connection write buffer for header.
	p := buf
	if len(c.writeBuf) > len(p) {
		p = c.writeBuf
	}
	p = p[: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"...)
	if c.subprotocol != "" {
		p = append(p, "Sec-WebSocket-Protocol: "...)
		p = append(p, c.subprotocol...)
		p = append(p, "\r\n"...)
	}
	if compress {
		p = append(p, "Sec-WebSocket-Extensions: permessage-deflate; server_no_context_takeover; client_no_context_takeover\r\n"...)
	}
	for k, vs := range responseHeader {
		if k == "Sec-Websocket-Protocol" {
			continue
		}
		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"...)

	// Clear deadlines set by HTTP server.
	if err := netConn.SetDeadline(time.Time{}); err != nil {
		if err := netConn.Close(); err != nil {
			log.Printf("websocket: failed to close network connection: %v", err)
		}
		return nil, err
	}

	if u.HandshakeTimeout > 0 {
		if err := netConn.SetWriteDeadline(time.Now().Add(u.HandshakeTimeout)); err != nil {
			if err := netConn.Close(); err != nil {
				log.Printf("websocket: failed to close network connection: %v", err)
			}
			return nil, err
		}
	}
	if _, err = netConn.Write(p); err != nil {
		if err := netConn.Close(); err != nil {
			log.Printf("websocket: failed to close network connection: %v", err)
		}
		return nil, err
	}
	if u.HandshakeTimeout > 0 {
		if err := netConn.SetWriteDeadline(time.Time{}); err != nil {
			if err := netConn.Close(); err != nil {
				log.Printf("websocket: failed to close network connection: %v", err)
			}
			return nil, err
		}
	}

	return c, nil
}