压缩速率追踪
Flash Player 11.3添加了一个压缩和解压ByteArray的新方法：LZMA算法。这个算法很有用，因为和现存的zlib和deflate算法相比，LZMA通常能压缩到更小的程度。但这会导致压缩速度减少多少呢？今天这篇文章就是为了找出这个问题的答案。
首先，如果你感兴趣的话，可以很容易切换到LZMA。所有你需要做的就是将CompressionAlgorithm.LZMA传递给ByteArray.compress或ByteArray.uncompress。当然你还需要建立好Flash Player 11.3，并且保证你的用户至少在使用这个版本。
至于你将得到的额外压缩，这取决于你要压缩的数据，但大约比gzip要好50%，可以有效地减少下载时间！因此，在压缩速率这篇文章的测试应用程序的基础上，我们通过下面的测试程序来看看LZMA与zlib和Deflate性能上的区别：
package
{
import flash.display.*;
import flash.utils.*;
import flash.text.*;
public class CompressionSpeedFollowup extends Sprite
{
private var logger:TextField = new TextField();
private function row(...cols): void
{
logger.appendText(cols.join(",") + "");
}
public function CompressionSpeedFollowup()
{
logger.autoSize = TextFieldAutoSize.LEFT;
addChild(logger);
row(
"Size",
"deflate (compress)",
"zlib (compress)",
"LZMA (compress)",
"deflate (uncompress)",
"zlib (uncompress)",
"LZMA (compress)"
);
runTests("1 KB", 1024, 1024);
runTests("1 MB", 1024*1024, 1);
}
private function runTests(label:String, size:int, reps:int): void
{
var beforeTime:int;
var afterTime:int;
var emptyTime:int;
var deflateTimeCompress:int;
var zlibTimeCompress:int;
var lzmaTimeCompress:int;
var deflateTimeUncompress:int;
var zlibTimeUncompress:int;
var lzmaTimeUncompress:int;
var bytes:ByteArray = new ByteArray();
var originalBytes:ByteArray = new ByteArray();
var compressedBytes:ByteArray = new ByteArray();
var i:int;
var zlib:String = CompressionAlgorithm.ZLIB;
var deflate:String = CompressionAlgorithm.DEFLATE;
var lzma:String = CompressionAlgorithm.LZMA;
fillBytes(originalBytes, size);
// Empty
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(originalBytes, bytes);
}
afterTime = getTimer();
emptyTime = afterTime - beforeTime;
// Compress
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(originalBytes, bytes);
bytes.compress(deflate);
}
afterTime = getTimer();
deflateTimeCompress = afterTime - beforeTime - emptyTime;
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(originalBytes, bytes);
bytes.compress(zlib);
}
afterTime = getTimer();
zlibTimeCompress = afterTime - beforeTime - emptyTime;
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(originalBytes, bytes);
bytes.compress(lzma);
}
afterTime = getTimer();
lzmaTimeCompress = afterTime - beforeTime - emptyTime;
// Uncompress
copyBytes(originalBytes, compressedBytes);
compressedBytes.compress(deflate);
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(compressedBytes, bytes);
bytes.uncompress(deflate);
}
afterTime = getTimer();
deflateTimeUncompress = afterTime - beforeTime - emptyTime;
copyBytes(originalBytes, compressedBytes);
compressedBytes.compress(zlib);
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(compressedBytes, bytes);
bytes.uncompress(zlib);
}
afterTime = getTimer();
zlibTimeUncompress = afterTime - beforeTime - emptyTime;
copyBytes(originalBytes, compressedBytes);
compressedBytes.compress(lzma);
beforeTime = getTimer();
for (i = 0; i < reps; ++i)
{
copyBytes(compressedBytes, bytes);
bytes.uncompress(lzma);
}
afterTime = getTimer();
lzmaTimeUncompress = afterTime - beforeTime - emptyTime;
row(
label,
deflateTimeCompress,
zlibTimeCompress,
lzmaTimeCompress,
deflateTimeUncompress,
zlibTimeUncompress,
lzmaTimeUncompress
);
}
private function fillBytes(bytes:ByteArray, size:int): void
{
bytes.length = 0;
bytes.position = 0;
for (var i:int; i < size; ++i)
{
bytes.writeByte(Math.random()*256);
}
bytes.position = 0;
}
private function copyBytes(bytes:ByteArray, into:ByteArray): void
{
bytes.position = 0;
into.position = 0;
into.length = 0;
into.writeBytes(bytes);
bytes.position = 0;
into.position = 0;
}
}
}
我是在以下环境中运行的测试应用程序：
· Flex SDK（MXMLC）4.6.0.23201，在发布模式下编译（没有调试或冗长的堆栈跟踪）
· Flash Player 11.4.402.287
· 2.3 GHz的英特尔酷睿i7处理器
· Mac OS X 10.8.2
下面是我得到的结果：（最下面的LZMA标签改为LZMA（uncompress））
正如图中所描述的，LZMA的压缩和解压速率要比zlib和Deflate慢很多。大约压缩慢了28倍，解压慢了16倍，至少我使用的随机测试数据是这样。然而，与大多数编程任务一样，这里还是有个权衡的。如果你需要提高下载速度的话，你现在就可以使用LZMA压缩，但下载之后就要花费额外的处理能力来解压。这可能是你愿意做的一个权衡了，但你一定要注意上面的结果：你的CPU将遇到麻烦了！