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# random

### MODULE

random

### MODULE SUMMARY

Pseudo random number generation

### DESCRIPTION

Random number generator. The method is attributed to B.A. Wichmann and I.D.Hill, in 'An efficient and portable pseudo-random number generator', Journal of Applied Statistics. AS183. 1982. Also Byte March 1987.

The current algorithm is a modification of the version attributed to Richard A O'Keefe in the standard Prolog library.

Every time a random number is requested, a state is used to calculate it, and a new state produced. The state can either be implicit (kept in the process dictionary) or be an explicit argument and return value. In this implementation, the state (the type ran()) consists of a tuple of three integers.

It should be noted that this random number generator is not cryptographically strong. If a strong cryptographic random number generator is needed for example crypto:rand_bytes/1 could be used instead.

### DATA TYPES

ran() = {integer(), integer(), integer()}

The state.

### EXPORTS

seed() -> ran()

Seeds random number generation with default (fixed) values in the process dictionary, and returns the old state.

seed(A1, A2, A3) -> undefined | ran()

Types:

A1 = A2 = A3 = integer()

Seeds random number generation with integer values in the process dictionary, and returns the old state.

One way of obtaining a seed is to use the BIF now/0:

              ...
              {A1,A2,A3} = now(),
              random:seed(A1, A2, A3),
              ...

seed(X1 :: {A1, A2, A3}) -> undefined | ran()

Types:

A1 = A2 = A3 = integer()

seed({A1, A2, A3}) is equivalent to seed(A1, A2, A3).

seed0() -> ran()

Returns the default state.

uniform() -> float()

Returns a random float uniformly distributed between 0.0 and 1.0, updating the state in the process dictionary.

uniform(N) -> integer() >= 1

Types:

N = integer() >= 1

Given an integer N >= 1, uniform/1 returns a random integer uniformly distributed between 1 and N, updating the state in the process dictionary.

uniform_s(State0) -> {float(), State1}

Types:

State0 = State1 = ran()

Given a state, uniform_s/1returns a random float uniformly distributed between 0.0 and 1.0, and a new state.

uniform_s(N, State0) -> {integer(), State1}

Types:

N = integer() >= 1
State0 = State1 = ran()

Given an integer N >= 1 and a state, uniform_s/2 returns a random integer uniformly distributed between 1 and N, and a new state.

### Note

Some of the functions use the process dictionary variable random_seed to remember the current seed.

If a process calls uniform/0 or uniform/1 without setting a seed first, seed/0 is called automatically.

The implementation changed in R15. Upgrading to R15 will break applications that expect a specific output for a given seed. The output is still deterministic number series, but different compared to releases older than R15. The seed {0,0,0} will for example no longer produce a flawed series of only zeros.