I'm trying to do a bit reversal, I understand the straight forward implementation, but for performance purposes I need to reverse bits by building a lookup table and using that table to find the bit reversals, So my program will build a look up table for bitsize N, and then it will use the table to find the bit reversal for given bits.
for example, if bitSize = 9 and num = 6 (00000110)
reverse(num,bitSize) will return 192 which is 11000000
int lookup(int num, int bitSize)
{
return table[bitSize][num]; // == reverse(num,bitSize);
}
I think this is how the lookup function should look like, I've been told that it's possible to build the table but I don't have any idea how to, can someone explain how to build this table?
I just want to clarify that I'm looking for a way to build this table for given bitSize, not just for 32-bit, otherwise I would have used this method: http://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
Thank you for your help,
Edit: Both solutions work but kmkaplan's solution is more efficient, thanks to j_random_hacker's memory optimisation.
For 12-bit table:
int table[1<<12]; // or 4096
int i;
for (i=0;i<4096;i++) table[i] = my_straight_forward_bitreverse(12,i);
Then you have to solve the issue for other bit lengths.
Either have an array int table[12][4096]; which is about 90% unoccupied.
Or have
int table12[4096], table11[2048], table10[1024] /* , ...*/ ;
int *table[12]={ table1, table2, /* ... */ table12 };
int i, j;
for (j=0;j<12;j++)
for (i=0;i<1<<j;i++) table[j][i]=my_straight_forward_bitreverse(j+1,i);
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
const int bitSize = atoi(argv[1]);
printf("static unsigned int table[] = {");
unsigned int i;
for (i = 0; i < 1 << bitSize; i++) {
if ((i & 7) == 0)
printf("\n");
unsigned int v = i;
unsigned int r = 0;
int s = bitSize;
while (s--) {
r <<= 1;
r |= v & 1;
v >>= 1;
}
printf(" 0x%x,", r);
}
printf("\n};\n"
"unsigned int lookup(int num, int bitSize)\n"
"{\n"
" return table[num] >> (%d - bitSize);\n"
"}\n",
bitSize
);
return 0;
}
edit: implement j_random_hacker memory optimisation.
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