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POFastest Method to Split a 32 Bit number into Bytes in C++
 

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  1. POFastest Method to Split a 32 Bit number into Bytes in C++
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    <p>I am writing a piece of code designed to do some data compression on CLSID structures. I'm storing them as a compressed stream of 128 bit integers. However, the code in question has to be able to place invalid CLSIDs into the stream. In order to do this, I have left them as one big string. On disk, it would look something like this:</p> <pre><code>+--------------------------+-----------------+------------------------+ | | | | | Length of Invalid String | Invalid String | Compressed Data Stream | | | | | +--------------------------+-----------------+------------------------+ </code></pre> <p>To encode the length of the string, I need to output the 32 bit integer that is the length of the string one byte at a time. Here's my current code:</p> <pre><code>std::vector&lt;BYTE&gt; compressedBytes; DWORD invalidLength = (DWORD) invalidClsids.length(); compressedBytes.push_back((BYTE) invalidLength &amp; 0x000000FF); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8) &amp; 0x000000FF)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8) &amp; 0x000000FF)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8)); </code></pre> <p>This code won't be called often, but there will need to be a similar structure in the decoding stage called many thousands of times. I'm curious if this is the most efficient method or if someone can come up with one better?</p> <p>Thanks all!</p> <p>Billy3</p> <p>EDIT: After looking over some of the answers, I created this mini test program to see which was the fastest:</p> <pre><code>// temp.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include &lt;windows.h&gt; #include &lt;ctime&gt; #include &lt;iostream&gt; #include &lt;vector&gt; 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 &lt; 0x008FFFFF; forLoopTest++) { testAssignedShifts(); } std::clock_t assignedShiftsFinishedTime = std::clock(); for (register unsigned __int32 forLoopTest = 0; forLoopTest &lt; 0x008FFFFF; forLoopTest++) { testRawShifts(); } std::clock_t rawShiftsFinishedTime = std::clock(); for (register unsigned __int32 forLoopTest = 0; forLoopTest &lt; 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 &lt; 0x008FFFFF; forLoopTest++) { testAssignedShifts(); } assignedShiftsFinishedTime = std::clock(); for (register unsigned __int32 forLoopTest = 0; forLoopTest &lt; 0x008FFFFF; forLoopTest++) { testRawShifts(); } rawShiftsFinishedTime = std::clock(); for (register unsigned __int32 forLoopTest = 0; forLoopTest &lt; 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&lt;BYTE&gt; compressedBytes; DWORD invalidLength = (DWORD) invalidClsids.length(); compressedBytes.push_back((BYTE) invalidLength); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt;= 8)); } void testRawShifts() { std::string invalidClsids("This is a test string"); std::vector&lt;BYTE&gt; compressedBytes; DWORD invalidLength = (DWORD) invalidClsids.length(); compressedBytes.push_back((BYTE) invalidLength); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt; 8)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt; 16)); compressedBytes.push_back((BYTE) (invalidLength &gt;&gt; 24)); } typedef union _choice { DWORD dwordVal; BYTE bytes[4]; } choice; void testUnion() { std::string invalidClsids("This is a test string"); std::vector&lt;BYTE&gt; 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]); } </code></pre> <p>Running this a few times results in:</p> <pre><code>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 </code></pre> <p>Looks to be about a tie between assigned shifts and union. Since I'm going to need the value later, union it is! Thanks!</p> <p>Billy3</p>
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      1. POFastest Method to Split a 32 Bit number into Bytes in C++
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    1. COSigh. You threw out a perfectly good method for something much worse w.r.t. portability.
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    2. CO@BillyONeal: If you're *that* concerned about times, I would forget about union/shift differences -- you will get a much bigger win by just figuring out how many BYTEs the vector will grow to and calling reserve() once at the start to avoid numerous vector reallocations.
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    3. CO@starblue: Actually I tend to agree, especially given that unions provide no observable speedup. Even though in this case we established that this was Windows-specific code, that's today -- who knows where it will wind up running one day...
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