vendor/github.com/gorilla/websocket/mask.go


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// Copyright 2016 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.

// +build !appengine

package websocket

import "unsafe"

const wordSize = int(unsafe.Sizeof(uintptr(0)))

func maskBytes(key [4]byte, pos int, b []byte) int {
    // Mask one byte at a time for small buffers.
    if len(b) < 2*wordSize {
        for i := range b {
            b[i] ^= key[pos&3]
            pos++
        }
        return pos & 3
    }

    // Mask one byte at a time to word boundary.
    if n := int(uintptr(unsafe.Pointer(&b[0]))) % wordSize; n != 0 {
        n = wordSize - n
        for i := range b[:n] {
            b[i] ^= key[pos&3]
            pos++
        }
        b = b[n:]
    }

    // Create aligned word size key.
    var k [wordSize]byte
    for i := range k {
        k[i] = key[(pos+i)&3]
    }
    kw := *(*uintptr)(unsafe.Pointer(&k))

    // Mask one word at a time.
    n := (len(b) / wordSize) * wordSize
    for i := 0; i < n; i += wordSize {
        *(*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(i))) ^= kw
    }

    // Mask one byte at a time for remaining bytes.
    b = b[n:]
    for i := range b {
        b[i] ^= key[pos&3]
        pos++
    }

    return pos & 3
}