package make-random

Signatures for providers of bits.

Bits Providers

This module contains the signatures of the modules that can be given to the various functors in MakeRandom, specifying how to provide bits.

Basic

Degenerated, stateless bits provider. This is basically only a function bits taking nothing as argument and returning a random value, either 30-bits with Basic30 or 64-bits with Basic64. When using such an interface, all the initialisation, state manipulation, etc., must be done outside of the functor. It is therefore preferable to provide a full interface (see below).

State

Bits provider taking a state as input. This is basically the pair of an abstract type state and a function bits taking the state as argument and returning a random value, either 30-bits with State30 or 64-bits with State64.

Init

State-manipulation functions. If some of them are very similar to the signature of Stdlib.Random (and thus not bringing much improvement), there is one big change which lies within these init_size and init values: they are a convenience to build the Stdlib.Random.full_init function.

The full_init function takes a state and an array of any size containing any kind of integers (in particular, the array can be empty or it can contain only very-low-entropy integers). Instead, this functor requires a value init_size stating how many values you will need and a function init taking a state and an array of values of initialising the state from it. The difference is that the array of values (either 30-bits in Init30 or 64-bits in Init64) is always of size init_size and all the values have already "rather-good"-entropy (that is, as much as possible given the initial seed).

Finally, instead of giving a default state, we only ask for a default seed from which the default state will be initialised. Any value can be used here (we like to use 135801055; can you guess why?).

Full

Full providers. These are only the combination of a bits provider (eg. State30) and state-manipulation functions (eg. Init30). The common uses would only require Full30 or Full64 depending on whether the PRNG manipulates values of 30 or 64 bits (usually, the output value is of same precision as the state).

Some other functors are provided if the state and the bits provider differ. This is for instance useful if the bits provider returns 64-bits outputs and is written in C. In this case, it is more interesting to turn it into a 30-bits provider in C and bind this 30-bits function (because 30-bits integers are faster to convert to OCaml than 64-bits integers). In order to limit the code duplication, one can keep the state manipulation functions as 64 bits and, in such a situation, we end up providing Full30Init64.