Reversing bit sequences
Contents
- 1 Reverse bits the obvious way
- 2 Reverse bits in word by lookup table
- 3 Reverse the bits in a byte with 3 operations (64-bit multiply and modulus division)
- 4 Reverse the bits in a byte with 4 operations (64-bit multiply, no division)
- 5 Reverse the bits in a byte with 7 operations (no 64-bit)
- 6 Reverse an N-bit quantity in parallel in 5 * lg(N) operations
- 7 References
Reverse bits the obvious way
unsigned int v; // 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
Reverse bits in word by lookup table
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)
};
unsigned int v; // reverse 32-bit value, 8 bits at time
unsigned int c; // c will get v reversed
// Option 1:
c = (BitReverseTable256[v & 0xff] << 24) |
(BitReverseTable256[(v >> 8) & 0xff] << 16) |
(BitReverseTable256[(v >> 16) & 0xff] << 8) |
(BitReverseTable256[(v >> 24) & 0xff]);
// Option 2:
unsigned char * 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]];
The first method takes about 17 operations, and the second takes about 12, assuming your CPU can load and store bytes easily.
Reverse the bits in a byte with 3 operations (64-bit multiply and modulus division)
unsigned char b; // reverse this (8-bit) byte
b = (b * 0x0202020202ULL & 0x010884422010ULL) % 1023;
The multiply operation creates five separate copies of the 8-bit byte pattern to fan-out into a 64-bit value. The AND operation selects the bits that are in the correct (reversed) positions, relative to each 10-bit groups of bits. The multiply and the AND operations copy the bits from the original byte so they each appear in only one of the 10-bit sets. The reversed positions of the bits from the original byte coincide with their relative positions within any 10-bit set. The last step, which involves modulus division by 2^10 - 1, has the effect of merging together each set of 10 bits (from positions 0-9, 10-19, 20-29, ...) in the 64-bit value. They do not overlap, so the addition steps underlying the modulus division behave like or operations.
Reverse the bits in a byte with 4 operations (64-bit multiply, no division)
unsigned char b; // reverse this byte
b = ((b * 0x80200802ULL) & 0x0884422110ULL) * 0x0101010101ULL >> 32;
The following shows the flow of the bit values with the boolean variables a, b, c, d, e, f, g, and h, which comprise an 8-bit byte. Notice how the first multiply fans out the bit pattern to multiple copies, while the last multiply combines them in the fifth byte from the right.
abcd efgh (-> hgfe dcba)
* 1000 0000 0010 0000 0000 1000 0000 0010 (0x80200802)
-------------------------------------------------------------------------------------------------
0abc defg h00a bcde fgh0 0abc defg h00a bcde fgh0
& 0000 1000 1000 0100 0100 0010 0010 0001 0001 0000 (0x0884422110)
-------------------------------------------------------------------------------------------------
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
* 0000 0001 0000 0001 0000 0001 0000 0001 0000 0001 (0x0101010101)
-------------------------------------------------------------------------------------------------
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
0000 d000 h000 0c00 0g00 00b0 00f0 000a 000e 0000
-------------------------------------------------------------------------------------------------
0000 d000 h000 dc00 hg00 dcb0 hgf0 dcba hgfe dcba hgfe 0cba 0gfe 00ba 00fe 000a 000e 0000
>> 32
-------------------------------------------------------------------------------------------------
0000 d000 h000 dc00 hg00 dcb0 hgf0 dcba hgfe dcba
& 1111 1111
-------------------------------------------------------------------------------------------------
hgfe dcba
Note that the last two steps can be combined on some processors because the registers can be accessed as bytes; just multiply so that a register stores the upper 32 bits of the result and the take the low byte. Thus, it may take only 6 operations.
Reverse the bits in a byte with 7 operations (no 64-bit)
b = ((b * 0x0802LU & 0x22110LU) | (b * 0x8020LU & 0x88440LU)) * 0x10101LU >> 16;
Make sure you assign or cast the result to an unsigned char to remove garbage in the higher bits.
Reverse an N-bit quantity in parallel in 5 * lg(N) operations
unsigned int v; // 32-bit word to reverse bit order
// swap odd and even bits
v = ((v >> 1) & 0x55555555) | ((v & 0x55555555) << 1);
// swap consecutive pairs
v = ((v >> 2) & 0x33333333) | ((v & 0x33333333) << 2);
// swap nibbles ...
v = ((v >> 4) & 0x0F0F0F0F) | ((v & 0x0F0F0F0F) << 4);
// swap bytes
v = ((v >> 8) & 0x00FF00FF) | ((v & 0x00FF00FF) << 8);
// swap 2-byte long pairs
v = ( v >> 16 ) | ( v << 16);
The following variation is also O(lg(N)), however it requires more operations to reverse v. Its virtue is in taking slightly less memory by computing the constants on the fly.
unsigned int s = sizeof(v) * CHAR_BIT; // bit size; must be power of 2
unsigned int mask = ~0;
while ((s >>= 1) > 0)
{
mask ^= (mask << s);
v = ((v >> s) & mask) | ((v << s) & ~mask);
}
These methods above are best suited to situations where N is large. If you use the above with 64-bit ints (or larger), then you need to add more lines (following the pattern); otherwise only the lower 32 bits will be reversed and the result will be in the lower 32 bits.
