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avutil/common: add FFDIFFSIGN macro 

This is of use for defining comparator callbacks. Common approaches like
return x-y are not safe due to the risks of overflow.
Furthermore, the (x > y) - (x < y) trick is optimized to branchless
code.
This also documents this macro accordingly.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
This commit is contained in:
Ganesh Ajjanagadde 2015-10-30 14:21:15 -04:00
parent 03f5bcd921
commit 265f83fd35
1 changed files with 11 additions and 0 deletions
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11
libavutil/common.h
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@ -76,6 +76,17 @@
*/
#define FFNABS(a) ((a) <= 0 ? (a) : (-(a)))
/**
* Comparator.
* For two numerical expressions x and y, gives 1 if x > y, -1 if x < y, and 0
* if x == y. This is useful for instance in a qsort comparator callback.
* Furthermore, compilers are able to optimize this to branchless code, and
* there is no risk of overflow with signed types.
* As with many macros, this evaluates its argument multiple times, it thus
* must not have a side-effect.
*/
#define FFDIFFSIGN(x,y) (((x)>(y)) - ((x)<(y)))
#define FFMAX(a,b) ((a) > (b) ? (a) : (b))
#define FFMAX3(a,b,c) FFMAX(FFMAX(a,b),c)
#define FFMIN(a,b) ((a) > (b) ? (b) : (a))