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POCalculating parity bit with the preprocessor (parity functional style with call by ref)
 

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  1. POCalculating parity bit with the preprocessor (parity functional style with call by ref)
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    <p>Consider I want to generate parities at compile time. The parity calculation is given literal constants and with any decent optimizer it will boil down to a single constant itself. Now look at the following parity calculation with the <strong>C</strong> preprocessor:</p> <pre><code>#define PARITY16(u16) (PARITY8((u16)&amp;0xff) ^ PARITY8((u16)&gt;&gt;8)) #define PARITY8(u8) (PARITY4((u8)&amp;0x0f) ^ PARITY4((u8)&gt;&gt;4)) #define PARITY4(u4) (PARITY2((u4)&amp;0x03) ^ PARITY2((u4)&gt;&gt;2)) #define PARITY2(u2) (PARITY1((u2)&amp;0x01) ^ PARITY1((u2)&gt;&gt;1)) #define PARITY1(u1) (u1) int message[] = { 0x1234, 0x5678, PARITY16(0x1234^0x5678)); </code></pre> <p>This will calculate the parity at compile time, but it will produce an enormous amount of intermediate code, expanding to 16 instances of the expression <code>u16</code> which itself can be e.g. an arbitrary complex expression. The problem is that the <strong>C</strong> preprocessor can't evaluate intermediary expressions and in the general case only expands text (you can force it to do integer arithmetic in-situ but only for trivial cases, or with gigabytes of #defines). I have found that the parity for 3 bits can be generated at once by an arithmetic expression: <code>([0..7]*3+1)/4</code>. This reduces the 16-bit parity to the following macro:</p> <pre><code>#define PARITY16(u16) ((4 &amp; ((((u16)&amp;7)*3+1) ^ \ ((((u16)&gt;&gt;3)&amp;7)*3+1) ^ \ ((((u16)&gt;&gt;6)&amp;7)*3+1) ^ \ ((((u16)&gt;&gt;9)&amp;7)*3+1) ^ \ ((((u16)&gt;&gt;12)&amp;7)*3+1) ^ \ ((((u16)&gt;&gt;15)&amp;1)*3+1))) &gt;&gt; 2)) </code></pre> <p>which expands <code>u16</code>only 6 times. Is there an even cheaper (in terms of number of expansions) way, e.g. a direct formula for a 4,5,etc. bit parity? I couldn't find a solution for a linear expression of the form <code>(x*k+d)/m</code> for acceptable (non-overflowing) values k,d,m for a range > 3 bits. Anyone out there with a more clever shortcut for preprocessor parity calculation?</p>
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    1. USslartibartfast
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      1. VTUpMod
    1. CODon't use macros, use inline functions. That provides the intermediate expression evaluation you are looking for.
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      1. USmfontanini
    2. COI have to agree with mfontanini that it would be better to avoid macros here. If you are using c++11 you can use a `constexpr` method. If you don't use c++11 and need the result to be a compiletime constant (not just likely to be evaluated at compiletime, but usable as a constant) use template metaprogramming to evaluate that at compiletime, otherwise just use an inline function. If you use plain C, you might want to remove C++ from the tags.
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      1. USGrizzly
    3. COSorry that I tagged it with C++ also, didn't want to exclude them V8-autodrive-endless-horsepower guys ;). Alas, this should execute in a pure C89 environnment
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