On Thu, Apr 20, 2006, christophpfister@xxxxxxxxxxx wrote:
> Now I have my functions ready [although it isn't a patch, there are two files
> attached]. For now log2 and log10 are implemented. The maximal error for
> log10 is 0.000199406% (for log2 it's even smaller...).
>
> Any comments are welcome, also requesting for additional functions.
It would be easier to comment your code if you wouldn't
sent it as application/octet-stream attachments.
Anyway...
The code is in no way DVB specific. IMHO it should
end up in linux/include/linux/log32.h and linux/lib/log32.c.
> #include <stdint.h>
Don't use stdint.h in the kernel. Use linux/types.h.
> const unsigned char msbtable[256] = {
...
> 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7};
put closing brace like so:
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
};
> const unsigned short logtable[256] = {
...
> 0xfa2f, 0xfaeb, 0xfba6, 0xfc60, 0xfd1b, 0xfdd5, 0xfe8e, 0xff47};
same
> unsigned int intlog2(unsigned int value) {
but for functions the opening brace must be in the next line:
unsigned int intlog2(unsigned int value)
{
Hm, your code depends on the fact that value and result
are 32bit entities. IMHO it's better to use u32 to
make this explicit (although I believe ints are 32bit
on all Linux platforms).
> /* returns: log2(value) * 2^16
> wrong result if value = 0 [ 0 instead of -inf ] */
log2(0) is undefined (NaN), not -inf. Add a WANR_ON().
> /* first detect the msb (count begins at 0)
> for that we check in which quarter the msb is
> e.g. for 0x00231f56 the four quarters are |00|, |23|, |1f| and |56|
> then we determine the msb of this quarter with a table lookup
> msbtable[23] = 5
> then we set the quarter in the context - i.e. adding (quarter - 1) * 8
> 5 + (3 - 1) * 8 = 21
> in binary form: 0x00231f56 = |00000000|00100011|00011111|01010110|
> ^ msb of the quarter at position 5
> ^ msb of the value at position 21 */
Please use block comment markers like so:
/* bla bla bla
* bla
*/
> if (value >= (1 << 16)) /* value >= 2^16 */
> if (value >= (1 << 24)) /* value >= 2^24 */
> msb = 24 + msbtable[value >> 24];
> else /* 2^16 <= value < 2^24 */
> msb = 16 + msbtable[value >> 16];
> else /* value < 2^16 */
> if (value >= (1 << 8)) /* 2^8 <= value < 2^16 */
> msb = 8 + msbtable[value >> 8];
> else /* value < 2^8 */
> msb = msbtable[value];
Isn't that what ffs() does?
> * @param value The value [ 0 gives a wrong result ]
* @param value The value (must be != 0)
Johannes
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