调整ArrayBuffer大小

Question

如果我想创建一个数组缓冲区，我编写：var buff = new ArrayBuffer(size)

但是，如何调整现有缓冲区的大小呢？我的意思是，在缓冲区的末尾再加一些字节。

Answer 1

var buff = new ArrayBuffer(32);
buff[31] = 43;
var newBuff = new ArrayBuffer(buff.byteLength*2);

for (var i=0;i<buff.byteLength;i++){
    newBuff[i] = buff[i];
}

buff = newBuff;

我在C++做过这样的事。只需创建一个较大的数组并将内容复制到上面，然后返回较大的数组并将其设置为原始数组。

Answer 2

ArrayBuffer本身没有设置。不过，set在TypedArray上。像这样使用：

var oldBuffer = new ArrayBuffer(20);

var newBuffer = new ArrayBuffer(40);
new Uint8Array(newBuffer).set(oldBuffer);

Answer 3

这样做的方法是ArrayBuffer.transfer(oldBuffer, newByteLength)，如下所示：

var buffA = new ArrayBuffer(30);
var buffB = ArrayBuffer.transfer(buffA, 40);

// buffA is now an empty array buffer (`buffA.byteLength === 0`)
// whereas buffB is a new buffer that starts with a's contents
// and ends with 10 bytes that are 0s.

// Much faster than manually copying. You could even just do:

var buffA = new ArrayBuffer(30);
buffA = ArrayBuffer.transfer(buffA, 40);

// Or as a prototype method

ArrayBuffer.prototype.resize = function(newByteLength) {
    return ArrayBuffer.transfer(this, newByteLength);
}

var buffA = new ArrayBuffer(30);
buffA = buffA.resize(40);

然而(截至2017年10月)，浏览器支持率为0% (甚至连Node.js支持都没有)，甚至还没有起草。

在可用之前，您可以使用页面上给定的多边形填充，它一次复制8个字节，因此对于大型数组(虽然它没有空给定的数组，这是不可能的)，速度相对较快。

下面是一个使用.prototype.set而不是for循环的修改版本：

if (!ArrayBuffer.transfer) {
  ArrayBuffer.transfer = function(oldBuffer, newByteLength) {
    var srcBuffer = Object(oldBuffer);
    var destBuffer = new ArrayBuffer(newByteLength);
    if (!(srcBuffer instanceof ArrayBuffer) || !(destBuffer instanceof ArrayBuffer)) {
      throw new TypeError('Source and destination must be ArrayBuffer instances');
    }
    var copylen = Math.min(srcBuffer.byteLength, destBuffer.byteLength);

    /* Copy 8 bytes at a time */
    var length = Math.trunc(copylen / 64);
    (new Float64Array(destBuffer, 0, length))
      .set(new Float64Array(srcBuffer, 0, length));

    /* Copy the remaining 0 to 7 bytes, 1 byte at a time */
    var offset = length * 64;
    length = copylen - offset;
    (new Uint8Array(srcBuffer, offset, length))
      .set(new Uint8Array(destBuffer, offset, length));

    return destBuffer;
  };
}

编辑

不使用ArrayBuffer命名空间的上述替代版本，并采取稍微不同的方法

          function arrayBufferTransfer(oldBuffer, newByteLength) {
             const 
             srcArray  = new Uint8Array(oldBuffer),
             destArray = new Uint8Array(newByteLength),
             copylen = Math.min(srcArray.buffer.byteLength, destArray.buffer.byteLength),
             floatArrayLength   = Math.trunc(copylen / 8),
             floatArraySource   = new Float64Array(srcArray.buffer,0,floatArrayLength),
             floarArrayDest     = new Float64Array(destArray.buffer,0,floatArrayLength);
             
             floarArrayDest.set(floatArraySource);
                 
             let bytesCopied = floatArrayLength * 8;
             
         
             // slowpoke copy up to 7 bytes.
             while (bytesCopied < copylen ) {
                 destArray[bytesCopied]=srcArray[bytesCopied];
                 bytesCopied++;
             }
             
           
             return destArray.buffer;
         }