我正在编写一段代码,旨在对CLSID结构进行一些数据压缩.我将它们存储为128位整数的压缩流.但是,有问题的代码必须能够将无效的CLSID放入流中.为了做到这一点,我把它们留作一个大字符串.在磁盘上,它看起来像这样:
+--------------------------+-----------------+------------------------+
| | | |
| Length of Invalid String | Invalid String | Compressed Data Stream |
| | | |
+--------------------------+-----------------+------------------------+
要编码字符串的长度,我需要输出32位整数,该整数是一次一个字节的字符串长度.这是我目前的代码:
std::vector<BYTE> compressedBytes;
DWORD invalidLength = (DWORD) invalidClsids.length();
compressedBytes.push_back((BYTE) invalidLength & 0x000000FF);
compressedBytes.push_back((BYTE) (invalidLength >>= 8) & 0x000000FF));
compressedBytes.push_back((BYTE) (invalidLength >>= 8) & 0x000000FF));
compressedBytes.push_back((BYTE) (invalidLength >>= 8));
这段代码不会经常调用,但在解码阶段需要有类似的结构,称为数千次.我很好奇这是否是最有效的方法,或者有人能想出更好的方法吗？
谢谢大家!
Billy3
编辑:在查看了一些答案后,我创建了这个迷你测试程序,看看哪个是最快的:
// temp.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <windows.h>
#include <ctime>
#include <iostream>
#include <vector>
void testAssignedShifts();
void testRawShifts();
void testUnion();
int _tmain(int argc, _TCHAR* argv[])
{
std::clock_t startTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testAssignedShifts();
}
std::clock_t assignedShiftsFinishedTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testRawShifts();
}
std::clock_t rawShiftsFinishedTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testUnion();
}
std::clock_t unionFinishedTime = std::clock();
std::printf(
"Execution time for assigned shifts: %08u clocks\n"
"Execution time for raw shifts: %08u clocks\n"
"Execution time for union: %08u clocks\n\n",
assignedShiftsFinishedTime - startTime,
rawShiftsFinishedTime - assignedShiftsFinishedTime,
unionFinishedTime - rawShiftsFinishedTime);
startTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testAssignedShifts();
}
assignedShiftsFinishedTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testRawShifts();
}
rawShiftsFinishedTime = std::clock();
for (register unsigned __int32 forLoopTest = 0; forLoopTest < 0x008FFFFF; forLoopTest++)
{
testUnion();
}
unionFinishedTime = std::clock();
std::printf(
"Execution time for assigned shifts: %08u clocks\n"
"Execution time for raw shifts: %08u clocks\n"
"Execution time for union: %08u clocks\n\n"
"Finished. Terminate!\n\n",
assignedShiftsFinishedTime - startTime,
rawShiftsFinishedTime - assignedShiftsFinishedTime,
unionFinishedTime - rawShiftsFinishedTime);
system("pause");
return 0;
}
void testAssignedShifts()
{
std::string invalidClsids("This is a test string");
std::vector<BYTE> compressedBytes;
DWORD invalidLength = (DWORD) invalidClsids.length();
compressedBytes.push_back((BYTE) invalidLength);
compressedBytes.push_back((BYTE) (invalidLength >>= 8));
compressedBytes.push_back((BYTE) (invalidLength >>= 8));
compressedBytes.push_back((BYTE) (invalidLength >>= 8));
}
void testRawShifts()
{
std::string invalidClsids("This is a test string");
std::vector<BYTE> compressedBytes;
DWORD invalidLength = (DWORD) invalidClsids.length();
compressedBytes.push_back((BYTE) invalidLength);
compressedBytes.push_back((BYTE) (invalidLength >> 8));
compressedBytes.push_back((BYTE) (invalidLength >> 16));
compressedBytes.push_back((BYTE) (invalidLength >> 24));
}
typedef union _choice
{
DWORD dwordVal;
BYTE bytes[4];
} choice;
void testUnion()
{
std::string invalidClsids("This is a test string");
std::vector<BYTE> compressedBytes;
choice invalidLength;
invalidLength.dwordVal = (DWORD) invalidClsids.length();
compressedBytes.push_back(invalidLength.bytes[0]);
compressedBytes.push_back(invalidLength.bytes[1]);
compressedBytes.push_back(invalidLength.bytes[2]);
compressedBytes.push_back(invalidLength.bytes[3]);
}
运行几次会导致:
Execution time for assigned shifts: 00012484 clocks
Execution time for raw shifts: 00012578 clocks
Execution time for union: 00013172 clocks
Execution time for assigned shifts: 00012594 clocks
Execution time for raw shifts: 00013140 clocks
Execution time for union: 00012782 clocks
Execution time for assigned shifts: 00012500 clocks
Execution time for raw shifts: 00012515 clocks
Execution time for union: 00012531 clocks
Execution time for assigned shifts: 00012391 clocks
Execution time for raw shifts: 00012469 clocks
Execution time for union: 00012500 clocks
Execution time for assigned shifts: 00012500 clocks
Execution time for raw shifts: 00012562 clocks
Execution time for union: 00012422 clocks
Execution time for assigned shifts: 00012484 clocks
Execution time for raw shifts: 00012407 clocks
Execution time for union: 00012468 clocks
看起来是指定班次和工会之间的关系.因为我以后需要这个值,所以联合它!谢谢!
Billy3
这可能会像您一样优化.Bit-twiddling操作是处理器上最快的一些操作.
它可能更快>> 16,>> 24而不是>> = 8 >> = 8 - 你减少了一项任务.
此外,我认为你不需要& - 因为你要转换为BYTE(应该是一个8位字符),它将被适当地截断.(如果我错了,请纠正我)
总而言之,这些都是微小的变化.对其进行剖析以确定它是否确实有所不同:P
只需使用联盟:
assert(sizeof (DWORD) == sizeof (BYTE[4])); // Sanity check
union either {
DWORD dw;
struct {
BYTE b[4];
} bytes;
};
either invalidLength;
invalidLength.dw = (DWORD) invalidClsids.length();
compressedBytes.push_back(either.bytes.b[0]);
compressedBytes.push_back(either.bytes.b[1]);
compressedBytes.push_back(either.bytes.b[2]);
compressedBytes.push_back(either.bytes.b[3]);
注意:与原始问题中的位移方法不同,此代码产生依赖于字节序的输出. 这只有在一台计算机上运行的程序的输出将在具有不同字节顺序的计算机上读取时才重要 - 但由于使用此方法似乎没有可测量的速度增加,您可以使用更便携的位移方法, 以防万一.