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I'm working on a component where I put in data and I get different data as a result. The input is always the same (3 Objects). From these 3 Objects up to 9 other Objects can be calculated. One calculation for each output Object is performed. Before the output data is calculated I want to set which of the 9 Objects shall be calculated.

  1. Set input data
  2. Set which output data shall be calculated
  3. Get Output data

I'm working in Visual C++ (C99). At the moment I'm facing a bit of a design problem here. After I set the input data, I am using a bit mask to configure which output data is supposed to be calculated. 

const int  foo1 = 1;   //000000001
const int  foo2 = 2;   //000000010
const int  foo3 = 4;   //000000100
const int  foo4 = 8;   //000001000
const int  foo5 = 16;  //000010000
const int  foo6 = 32;  //000100000
const int  foo7 = 64;  //001000000
const int  foo8 = 128; //010000000
const int  foo9 = 256; //100000000

SetOutput(foo2 | foo4 | foo5 );

The data is held in an array ARRAY[9] . For those that have been set (foo2, foo4 and foo5) the elements contain valid data after the calculation (ARRAY[2], ARRAY[4], ARRAY[5])

I then want to get the the resulting data.

GetData(foo2);
GetData(foo4);
GetData(foo5);

My problem is how do I get the element index from the mask values foo2 = 2, foo4 = 8 or for example foo5 = 16?
The only possibility would be taking log2(n) and get 2, 3 and 4, which would be the correct element indexes of ARRAY.

Is there a simpler way than this? I thought about using a map instead, with the mask value (1,2,4,8...256) as key field and the calculated data in the value field. But would prefer to keep the static array. Thanks in advance!

tzippy
  • 167

1 Answers1

4

An array with 257 entries will be fast, portable, and again fast. Many compilers provide a very fast built-in function to count leading or trailing zero bits in a number, for example __builtin_clz in gcc and Clang, which is likely about hundred times faster than a call to the log2 function.

gnasher729
  • 49,096