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Pack plrand -- prolog/plrand.pl
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This module provides an interface to Pierre Ecuyer's pseudorandom generator library RngStream, which is a high-quality, long period pseudorandom generator that can do the neat trick of quickly skipping ahead an arbitrary number of samples in the stream without actually generating all the samples in between. This capability is used to implement a stream of random bits that can be split recursively into 'pseudoindependent' substreams.


The state of a random generator, as a blob atom.
A state represented as an ordinary Prolog term that can be written as text.
A blob representing an operator to advance a state by several steps
A splittable stream, containing a state and a jump.
A floating point number between 0 and 1.

Available but not exported

sample_Beta(+nonneg, +nonneg, -nonneg)//
sample_Gamma(+nonneg, -float)//
sample_Poisson(+nonneg, -natural)//
sample_Zeta(+nonneg, -natural)//
sample_Discrete(+nonneg, +list(prob), -natural)//
sample_DiscreteF(+nonneg, +array(prob), -natural)//
sample_Dirichlet(+nonneg, +list(nonneg), -list(prob))//
sample_DirichletF(+nonneg, +array(nonneg), -array(prob))//
sample_Stable(+float, +float, -float)//
sample_Binomial(+prob, +natural, -natural)//

prob_Uniform01(+float, -prob)
prob_Normal(+float, -prob)
prob_Exponential(+nonneg, -prob)
prob_Beta(+nonneg, +nonneg, +nonneg, -prob)
prob_Gamma(+nonneg, +float, -prob)
prob_Poisson(+nonneg, +natural, -prob)
prob_Zeta(+nonneg, +natural, -prob)
 with_brs(N:atom, G:dcg(rndstate)) is det
Runs goal G as a DCG goal with RNG state from a thread-local bactrackable gobal variable name N. The final state is stored in the global variable, but any subsequent failure will restore the previous value. If using in a thread other than main, you must call init_rnd_state/0 to initialise the global variable. NB, you will probably want to set the Prolog flag toplevel_mode to recursive.
 init_brs(+Name) is det
Initialise thread local backtrackable global variable with default RNG state.
 randomise_brs(+Name) is det
Randomise thread local backtrackable global variable using randomise/1.
 get_rnd_state(-State:state) is det
Unifies State with the current global RNG state. @see set_rnd_state/1
 set_rnd_state(+State:state) is det
Sets the globab RNG state to State. @see get_rnd_state/1
 init_rnd_state(-State:state) is det
Unifies State with the state that was set at load time.
 randomise(-State:state) is det
Returns a truly random state by reading from /dev/random.
 is_rnd_state(+State:state) is semidet
Succeeds if State is a BLOB atom representing a random generator state.
 with_rnd_state(Cmd:phrase(state)) is nondet
Runs DCG goal Cmd using the current global RNG state as the initial state the global RNG state afterwards.
 rnd_state_term(+State:state, -Term:state_term) is det
rnd_state_term(-State:state, +Term:state_term) is det
Convert between blob and term representation of random state.
 run_rnd_state(Cmd:phrase(state), +State1:state, -State2:state) is nondet
run_rnd_state(Cmd:phrase(state), +State1:state_term, -State2:state_term) is nondet
Runs DCG phrase Cmd as with call_dcg/3, except that, if State1 is in the term representation (type state), it is converted to the blob representation (type state) and the final state is converted back from term representation to get State2.
 init_jump(+E:integer, -Jump:jump) is det
Unifies Jump with a BLOB atom representing an operator to jump forwards 2^E steps in the stream of random numbers. The generator has a period of about 2^191. Operators for jumping ahead by 2^76 and 2^127 are precomputed and so can be returned faster. The resulting BLOB can be used with jump/3 to advance any given random generator state.
 double_jump(+Jump1:jump, -Jump2:jump) is det
Unifies Jump2 with an operator that jumps ahead twice as far Jump1, ie if Jump1 jumps by 2^E, then Jump2 jumps by 2^(E+1). Jump operators can be created by init_jump/2 and applied by jump/3.
 jump(+Jump:jump, +State1:state, -State2:state) is det
Advances random generator state represented in State1 by the number of steps represented by Jump, unifying State2 with the result.
See also
- double_jump/2
- init_jump/2
 spawn(-New:state, +Orig:state, -Next:state) is det
Samples enough bits from Orig to specify a new generator state New, leaving the original generator in Next. If generator states represent streams of random numbers, then you can think of it as sampling a whole stream of values instead of just one value. Note: New is likely to point to a new point in the original stream far away from Orig and Next, simply because the period of the generator is so large (about 2^191) but there is no guarantee of this. Therefore, it's possible (but unlikely) that New might produce a stream that overlaps significantly with samples drawn from Next. If you need to be sure that New is a long way from Orig and Next, then use jump/3 instead.
New- is the state of the newly created generator.
Orig- is the state of the source generator, the original stream.
Next- is the state of the source generator after extracting New.
 stream_init(+State:state, +E:integer, -Stream:stream) is det
stream ---> stream(state, integer).

[EXPERIMENTAL] Used to initialise a splittable stream. The idea is that a stream of random number can be recursively split into many substreams, each of which will not overlap unless very many samples are drawn.

For example init_rnd_state(S), stream_init(S,76,Z) produces a stream such that the first split jumps 2^76 steps along. Thus, 2^76 samples can be drawn from either before overlap occurs. Since the period of the generator is more than 2^190, there can be up to 114 levels of recursive splitting, resulting in up to 2^114 substreams.

State- is the initial state of the generator.
E- is a nonnegative integer such that, when the stream is first split, the new substream begins 2^E steps along the sequence. Subsequent splits will jump twice as far each time.
Stream- is the new stream (a term)
 stream_split(+Stream1, -Stream2, -Stream3) is det
[EXPERIMENTAL] Stream1 is split into independent streams Stream2 and Stream3. Stream2 is actually the same as Stream1. Stream3 is a window on the same strem, but starting a long way down the sequence.
 sample_Single_(-Float) is det
Samples a single precision floating point value in [0,1) using the internal (global) random generator state. It consumes one 32 bit value from the generator. This is the fasted way to generate a random value using this library.
 sample_Double_(-Float) is det
Samples a double precision floating point value in [0,1) using the internal (global) random generator state. It consumes two 32 bit values from the generator.
 mean_log_dirichlet(+Alphas:list(nonneg), -X:list(number)) is det
Compute the expectation of the component-wise log of a Dirichilet distributed vector represented as a list.
 log_partition_dirichlet(+Alphas:list(nonneg), -X:number) is det
Compute the log of the normalisation constant of the Dirichlet PDF with parameters Alphas, sum(map(gammaln,Alphas)) - gammaln(sum(Alphas)).
 kldiv_dirichlet(+A:list(nonneg), +B:list(nonneg), -D:number) is det

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 log_prob_dirichlet(Arg1, Arg2, Arg3)
 lngamma(Arg1, Arg2)
 digamma(Arg1, Arg2)