get uint32_t length

int_length_bit_twiddle.c

#include <stdlib.h>
#include <stdio.h>

void print_bits(uint32_t x);

uint32_t uint32_smear_right(uint32_t x){
  x = x | x>>1;
  x = x | x>>2;
  x = x | x>>4;
  x = x | x>>8;
  x = x | x>>16;
  return x;
}

int uint32_pop(uint32_t x){
  x = ( x & 0x55555555 ) + ( (x >> 1 ) & 0x55555555 );
  x = ( x & 0x33333333 ) + ( (x >> 2 ) & 0x33333333 );
  x = ( x & 0x0F0F0F0F ) + ( (x >> 4 ) & 0x0F0F0F0F );
  x = ( x & 0x00FF00FF ) + ( (x >> 8 ) & 0x00FF00FF );
  x = ( x & 0x0000FFFF ) + ( (x >> 16) & 0x0000FFFF );
  return x;
}

int uint32_length(uint32_t x){
  x = uint32_smear_right(x);
  return uint32_pop(x);
}

uint32_t uint32_smear_right_print(uint32_t x){
  x = x | x>>1;
  printf("Set every bit that is 1 place to the right of a 1\n");
  print_bits(x);
  printf("\n");
  x = x | x>>2;
  printf("Set every bit that is 2 places to the right of a 1\n");
  print_bits(x);
  printf("\n");
  x = x | x>>4;
  printf("Set every bit that is 4 places to the right of a 1\n");
  print_bits(x);
  printf("\n");
  x = x | x>>8;
  printf("Set every bit that is 8 places to the right of a 1\n");
  print_bits(x);
  printf("\n");
  x = x | x>>16;
  printf("Set every bit that is 16 places to the right of a 1\n");
  print_bits(x);
  printf("\n");
  return x;
}

int uint32_pop_print(uint32_t x){
  x = ( x & 0x55555555 ) + ( (x >> 1 ) & 0x55555555 );
  printf("Accumulate pop counts of bit buckets size 2\n");
  print_bits(x);
  printf("\n");
  x = ( x & 0x33333333 ) + ( (x >> 2 ) & 0x33333333 );
  printf("Accumulate pop counts of bit buckets size 4\n");
  print_bits(x);
  printf("\n");
  x = ( x & 0x0F0F0F0F ) + ( (x >> 4 ) & 0x0F0F0F0F );
  printf("Accumulate pop counts of bit buckets size 8\n");
  print_bits(x);
  printf("\n");
  x = ( x & 0x00FF00FF ) + ( (x >> 8 ) & 0x00FF00FF );
  printf("Accumulate pop counts of bit buckets size 16\n");
  print_bits(x);
  printf("\n");
  x = ( x & 0x0000FFFF ) + ( (x >> 16) & 0x0000FFFF );
  printf("Accumulate pop counts of bit buckets size 32\n");
  print_bits(x);
  printf("\n");
  return x;
}

uint32_t print_4_bits(uint32_t x){
  int i,res;
  for(i=0;i<4;i++){
    printf("%u",(x&0x80000000)>>31);
    x=x<<1;
  }
  return x;
}

void print_bits(uint32_t x){
  int i;
  for(i=0;i<8;i++){
    x=print_4_bits(x);
    printf(" ");
  }
}

void print_length_info(uint32_t X){
  int res;
  uint32_t X2;
  print_bits(X);
  printf("\n");
  X2=uint32_smear_right_print(X);
  res=uint32_pop_print(X2);
  printf("\n");
  printf("The length of %u is %d bits\n",X,res);
}

int main(){
  uint32_t X=0x007F8A24;
  uint32_t Y=0x000030AF;

  printf("X=");
  print_length_info(X);
  printf("\n");
  printf("Y=");
  print_length_info(Y);

  uint32_t x=X;
  x = x | x>>1;
  x = x | x>>2;
  x = x | x>>4;
  x = x | x>>8;
  x = x | x>>16;

  x = ( x & 0x55555555 ) + ( (x >> 1 ) & 0x55555555 );
  x = ( x & 0x33333333 ) + ( (x >> 2 ) & 0x33333333 );
  x = ( x & 0x0F0F0F0F ) + ( (x >> 4 ) & 0x0F0F0F0F );
  x = ( x & 0x00FF00FF ) + ( (x >> 8 ) & 0x00FF00FF );
  x = ( x & 0x0000FFFF ) + ( (x >> 16) & 0x0000FFFF );

  printf("Alternatively, use built-function\n");
  int resX=32-__builtin_clz(X);
  int resY=32-__builtin_clz(Y);
  printf("Length X: %d\nLength Y: %d\n",resX,resY);

  return 0;
}