Reconstruction/MdcPatRec/MdcTrkRecon/include/MdcTrkRecon/countBits.h

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// Count the number of bits set in a word; return that number.
// Use a small lookup table. Size of lookup table is tradeoff between
// # of iterations needed and cache space used.
 
inline int countbits( unsigned word ) {
  const int nbit[16] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };
  int       nset     = 0;
  for ( unsigned i = 0; i < 2 * sizeof( unsigned ); i++ )
  {                           // assume char = 8 bits, i.e. two
    nset += nbit[word & 0xf]; // iterations per 'char' size object
    word >>= 4;
  }
  return nset;
}