postgresql: calculates next subnet's address

#include "postgres.h"
#include "fmgr.h"
#include "catalog/pg_type.h"
#include "utils/builtins.h"
#include "utils/inet.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

#define ip_bits(inetptr) \
     (((inet_struct *) VARDATA_ANY(inetptr))->bits)

#define ip_addr(inetptr) \
     (((inet_struct *) VARDATA_ANY(inetptr))->ipaddr)

Datum cidr_inc( PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1( cidr_inc );
Datum
cidr_inc( PG_FUNCTION_ARGS )
{
    inet *src = PG_GETARG_INET_P(0);
    int bits = ip_bits(src); //netmask length

    inet *dst = (inet *) palloc(VARSIZE_ANY(src));
    memcpy(dst, src, VARSIZE_ANY(src));

    int i = bits / 8;
    ip_addr(dst)[i] += 1 << (8 - (bits % 8));
    while(i > 0 && ip_addr(dst)[i--] == 0){
        ++ip_addr(dst)[i];
    }

    PG_RETURN_INET_P( dst );
}