# Random number generator¶

Often subgrid models require random numbers, for this purpose SWIFT has a random number generator. The random number generator of SWIFT is based on a combination of the standard rand_r and erand48 random number generators. Since for some applications in cosmology we want to be able to sample random numbers with a probability lower than $$10^{-8}$$, we could not simply use the 32-bit rand_r due to its cut-off and spacing of around $$2^{-32} \approx 2 \times 10^{-10}$$. For the erand48 algorithm with 48 bits the spacing and cutoff are significantly smaller and around $$2^{-48} \approx 3.5 \times 10^{-15}$$, so this is very suitable for our applications.

## Reproducible random numbers¶

In our simulations we want to be able to reproduce the exact same random numbers if we do exactly the same simulation twice. Because of this we want to seed the random number generator in a reproducible way. In order to do this we seed with the particle ID of the current particle and the current integer simulation time.

To produce several different types of random numbers we have an additional argument called the type of random number which is basically the nth random number for the specified seed, which is added to the particle ID, thus providing a distinct state per random number type.

If the user wishes to run a simulation with a different set of random number, an option during the configuration (--with-random-seed=INT) is available. This option simply flip some bits in the initial number composed of the ID and the current simulation time through the binary operator XOR.

## Implementation¶

Our random number generator packs the particle ID (plus the random number type) and the current simulation time as two 64-bit values, plus a constant 16-bit value, into a 144-bit buffer. This buffer is treated as an array of 9 uint16 values.

In a first pass we initialize the seed to 0 and run through the 9 uint16 values, xor-ing them with the seed and calling rand_r on the seed to advance it. Using rand_r with the thus-generated seed, we generate a sequence of 9 16-bit values and xor them with the original 144-bit buffer.

The 9 bytes of this modified buffer are then used for three passes of erand48, xor-ing the state in the same way as before. erand48 is then called one final time with the last state, producing a random double-precision value with a 48-bit mantissa.

## What to do if we break the random number generator?¶

The most likely case is that the RNG is not strong enough for our application, in this case we could simply do multiple passes of both shuffling the state and generating the final value from the state. This increases the computational cost but will make the random number generator stronger.

An other case is that we need probabilities that are lower than $$1 \times 10^{-17}$$, in this case we simply cannot use our random number generator and for example need to generate two random numbers in order to probe these low probabilities.