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POWhat is the fastest way(s) to loop through a large data chunk on a per-bit basis
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  1. POWhat is the fastest way(s) to loop through a large data chunk on a per-bit basis
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    copied!<p>I am running through a memory block of binary data byte-wise.</p> <p>Currently I am doing something like this:</p> <pre><code>for (i = 0; i &lt; data-&gt;Count; i++) { byte = &amp;data-&gt;Data[i]; ((*byte &amp; Masks[0]) == Masks[0]) ? Stats.FreqOf1++; // syntax incorrect but you get the point. ((*byte &amp; Masks[1]) == Masks[1]) ? Stats.FreqOf1++; ((*byte &amp; Masks[2]) == Masks[2]) ? Stats.FreqOf1++; ((*byte &amp; Masks[3]) == Masks[3]) ? Stats.FreqOf1++; ((*byte &amp; Masks[4]) == Masks[4]) ? Stats.FreqOf1++; ((*byte &amp; Masks[5]) == Masks[5]) ? Stats.FreqOf1++; ((*byte &amp; Masks[6]) == Masks[6]) ? Stats.FreqOf1++; ((*byte &amp; Masks[7]) == Masks[7]) ? Stats.FreqOf1++; } </code></pre> <p>Where Masks is:</p> <pre><code>for (i = 0; i &lt; 8; i++) { Masks[i] = 1 &lt;&lt; i; } </code></pre> <p>(I somehow did not manage to do it as fast in a loop or in an inlined function, so I wrote it out.)</p> <p>Does anyone have any suggestions on how to to improve this first loop? I am rather inexperienced with getting down to bits.</p> <p>This may seem like a stupid thing to do. But I am in the process of implementing a compression algorithm. I just want to have the bit accessing part down right.</p> <p>Thanks!</p> <p>PS: This is in on the Visual Studio 2008 compiler. So it would be nice if the suggestions applied to that compiler.</p> <p>PPS: I just realized, that I don't need to increment two counts. One would be enough. Then compute the difference to the total bits at the end. But that would be specific to just counting. What I really want done fast is the bit extraction.</p> <p>EDIT: The lookup table idea that was brought forward is nice. I realize though that I posed the question wrong in the title. Because in the end what I want to do is not count the bits, but access each bit as fast as possible.</p> <p>ANOTHER EDIT: Is it possible to advance a pointer by just one bit in the data?</p> <p>ANOTHER EDIT: Thank you for all your answers so far.</p> <p>What I want to implement in the next steps is a nonsophisticated binary arithmetic coder that does not analyze the context. So I am only interested in single bits for now. Eventually it will become a Context-adaptive BAC but I will leave that for later.</p> <p>Processing 4 bytes instead of 1 byte could be an option. But a loop over 32 bits is costly as well, isn't it?</p>
 

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