/* yet more bit hacks */
#include <stdio.h>
typedef int bool;
#define true 1
#define false 0
#define CHAR_BIT 8
int main(void) {
/* 1) count parity */
unsigned int v = 847362849;
bool parity = false;
while (v)
{
parity = !parity;
v = v & (v - 1);
}
printf("%d\n", parity);
static const bool ParityTable256[256] =
{
# define P2(n) n, n^1, n^1, n
# define P4(n) P2(n), P2(n^1), P2(n^1), P2(n)
# define P6(n) P4(n), P4(n^1), P4(n^1), P4(n)
P6(0), P6(1), P6(1), P6(0)
};
v = 847362849;
v ^= v >> 16;
v ^= v >> 8;
parity = ParityTable256[v & 0xff];
printf("%d\n", parity);
v = 847362849;
unsigned char * p = (unsigned char *) &v;
parity = ParityTable256[p[0] ^ p[1] ^ p[2] ^ p[3]];
printf("%d\n", parity);
unsigned char b = 73; // byte value to compute the parity of
parity =
(((b * 0x0101010101010101ULL) & 0x8040201008040201ULL) % 0x1FF) & 1;
printf("%d\n", parity);
printf("\n");
/* 2) reversing bits */
v = 847362849; // input bits to be reversed
unsigned int r = v; // r will be reversed bits of v; first get LSB of v
int s = sizeof(v) * CHAR_BIT - 1; // extra shift needed at end
for (v >>= 1; v; v >>= 1)
{
r <<= 1;
r |= v & 1;
s--;
}
r <<= s; // shift when v's highest bits are zero
printf("%d\n", r);
static const unsigned char BitReverseTable256[256] =
{
# define R2(n) n, n + 2*64, n + 1*64, n + 3*64
# define R4(n) R2(n), R2(n + 2*16), R2(n + 1*16), R2(n + 3*16)
# define R6(n) R4(n), R4(n + 2*4 ), R4(n + 1*4 ), R4(n + 3*4 )
R6(0), R6(2), R6(1), R6(3)
};
v = 847362849; // reverse 32-bit value, 8 bits at time
unsigned int c; // c will get v reversed
c = (BitReverseTable256[v & 0xff] << 24) |
(BitReverseTable256[(v >> 8) & 0xff] << 16) |
(BitReverseTable256[(v >> 16) & 0xff] << 8) |
(BitReverseTable256[(v >> 24) & 0xff]);
printf("%d\n", c);
p = (unsigned char *) &v;
unsigned char * q = (unsigned char *) &c;
q[3] = BitReverseTable256[p[0]];
q[2] = BitReverseTable256[p[1]];
q[1] = BitReverseTable256[p[2]];
q[0] = BitReverseTable256[p[3]];
printf("%d\n", c);
b = 73; // reverse this (8-bit) byte
b = (b * 0x0202020202ULL & 0x010884422010ULL) % 1023;
printf("%d\n", b);
printf("\n");
/* 3) integer logarithm base-2 */
v = 847362849; // 32-bit word to find the log base 2 of
r = 0; // r will be lg(v)
while (v >>= 1)
{
r++;
}
printf("%d\n", r);
static const char LogTable256[256] =
{
#define LT(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n
-1, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
LT(4), LT(5), LT(5), LT(6), LT(6), LT(6), LT(6),
LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7)
};
v = 847362849; // 32-bit word to find the log of
register unsigned int t, tt; // temporaries
if (tt = v >> 16)
{
r = (t = tt >> 8) ? 24 + LogTable256[t] : 16 + LogTable256[tt];
}
else
{
r = (t = v >> 8) ? 8 + LogTable256[t] : LogTable256[v];
}
printf("%d\n", r);
if (tt = v >> 24)
{
r = 24 + LogTable256[tt];
}
else if (tt = v >> 16)
{
r = 16 + LogTable256[tt];
}
else if (tt = v >> 8)
{
r = 8 + LogTable256[tt];
}
else
{
r = LogTable256[v];
}
printf("%d\n", r);
return 0; }