Quick introduction to reverse engineering for beginners

1.17 Unions
1.17.1 Pseudo-random number generator example
If we need float random numbers from 0 to 1, the most simplest thing is to use random numbers generator
like Mersenne twister producing random 32-bit values in DWORD form, transform this value to float and
then divide it by RAND_MAX (0xffffffff in our case) — value we got will be in 0..1 interval.
But as we know, division operation is almost always very slow. Will it be possible to get rid of it, as in
case of division by multiplication? 1.11
Let’s remember what float number consisted of: sign bit, significand bits and exponent bits. We need
just to store random bits to significand bits for getting float number!
Exponent cannot be zero (number will be denormalized in this case), so we will store 01111111 to
exponent — this mean exponent will be 1. Then fill significand with random bits, set sign bit to 0 (which
mean positive number) and voilà. Generated numbers will be in 1 to 2 interval, so we also should subtract
1 from it.
Very simple linear congruential random numbers generator is used in my example 61 , producing 32-bit
numbers. The PRNG initializing by current time in UNIX-style.
Then, float type represented as union — that is the C/C++ construction allowing us to interpret piece
of memory differently typed. In our case, we are able to create a variable of union type and then access to
it as it’s float or as it’s uint32_t. It can be said, it’s just a hack. A dirty one.
#include
#include
#include
union uint32_t_float
{
uint32_t i;
float f;
};
// from the Numerical Recipes book
const uint32_t RNG_a=1664525;
const uint32_t RNG_c=1013904223;
int main()
{
uint32_t_float tmp;
};
uint32_t RNG_state=time(NULL); // initial seed
for (int i=0; i