TIL: Neat way to convert hex to BCD (decimal)


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Often for debugging purposes in a live system I need to display internal states on a seven segment display. This is trivial when I only need to display hex numbers but recently I needed the value represented in decimal because it made more sense for my application. I had a need for a hex to dec converter and as I usually do, I google it first, in case it had been done already. Sure enough, someone has, and the solution involves only shifting and adding 3. The method involved was new to me as I hadn't heard of this way of converting hex to dec, but it works, see his blog. Hope I taught some of you something today :)


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The last bit of http://people.ee.duke.edu/~dwyer/courses/ece52/Binary_to_BCD_Converter.pdf has how you can chain them in a a very cunning way that is even more confusing!


For some other bit twiddling magic have a look at http://graphics.stanford.edu/~seander/bithacks.html. Mostly of interest to software use, but interesting if you like that stuff... eg:


min = y ^ ((x ^ y) & -(x < y)); // min(x, y)

Or for the ultimate of bit twiddling hack (based on the quake III arena source):


float fast_inverse_square_root( float number ){        long i;        float x2, y;        const float threehalfs = 1.5F;         x2 = number * 0.5F;        y  = number;        i  = * ( long * ) &y;                       // evil floating point bit level hacking        i  = 0x5f3759df - ( i >> 1 );               // what the f#@k?        y  = * ( float * ) &i;        y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration               y = y * ( threehalfs - ( x2 * y * y ) );        return y;}
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