Re: [B.A.T.M.A.N.] [PATCH] batman-adv: Fixing wrap-around bug in vis

[Sven Eckelmann <sven.eckelmann@gmx.de>]

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Sven Eckelmann wrote:
> Now it gets real cruel, but who cares... C++ wasn't accept in the kernel,
>  so we have to deal with it using some GNUish stuff.
> 
> #define seq_before(x,y) ({typeof(x) _dummy = (x - y); \
>                           _dummy > 1u << (7u + 8u * (sizeof(_dummy) -
>  1u));}) #define seq_after(x,y) ({typeof(x) _dummy = (y - x); \
>                          _dummy >= 1u << (7u + 8u * (sizeof(_dummy) -
>  1u));})

After a small discussion with Marek I would like to suggest a easier readable 
and symmetric definition

/* Returns the smallest signed integer in two's complement with the sizeof x 
*/
#define smallest_signed_int(x) (1u << (7u + 8u * (sizeof(x) - 1u)))

/* Checks if a sequence number x is a predecessor/successor of y.
   they handle overflows/underflows and can correctly check for a
   predecessor/successor unless the variable sequence number has grown by
   more then 2**(bitwidth(x)-1)-1.
   This means that for an uint8_t with the maximum value 255, it would think:
    * when adding nothing - it is neither a predecessor nor a successor
    * before adding more than 127 to the starting value - it is a predecessor,
    * when adding 128 - it is neither a predecessor nor a successor,
    * after adding more than 127 to the starting value - it is a successor */
#define seq_before(x,y) ({typeof(x) _dummy = (x - y); \
                         _dummy > smallest_signed_int(_dummy); })
#define seq_after(x,y) seq_before(y,x)

Best regards,
	Sven

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