Provided by: libdata-entropy-perl_0.007-2_all 
NAME
Data::Entropy::Algorithms - basic entropy-using algorithms
SYNOPSIS
use Data::Entropy::Algorithms
qw(rand_bits rand_int rand_prob);
$str = rand_bits(17);
$i = rand_int(12345);
$i = rand_int(Math::BigInt->new("1000000000000"));
$j = rand_prob(1, 2, 3);
$j = rand_prob([ 1, 2, 3 ]);
use Data::Entropy::Algorithms qw(rand_fix rand rand_flt);
$x = rand_fix(48);
$x = rand(7);
$x = rand_flt(0.0, 7.0);
use Data::Entropy::Algorithms
qw(pick pick_r choose choose_r shuffle shuffle_r);
$item = pick($item0, $item1, $item2);
$item = pick_r(\@items);
@chosen = choose(3, $item0, $item1, $item2, $item3, $item4);
$chosen = choose_r(3, \@items);
@shuffled = shuffle($item0, $item1, $item2, $item3, $item4);
$shuffled = shuffle_r(\@items);
DESCRIPTION
This module contains a collection of fundamental algorithms that use entropy. They all
use the entropy source mechanism described in Data::Entropy.
FUNCTIONS
All of these functions use entropy. The entropy source is not an explicit input in any
case. All functions use the current entropy source maintained by the "Data::Entropy"
module. To select an entropy source use the "with_entropy_source" function in that
module, or alternatively do nothing to use the default source.
Fundamental entropy extraction
rand_bits(NBITS)
Returns NBITS bits of entropy, as a string of octets. If NBITS is not a multiple of
eight then the last octet in the string has its most significant bits set to zero.
rand_int(LIMIT)
LIMIT must be a positive integer. Returns a uniformly-distributed random integer in
the range [0, LIMIT). LIMIT may be either a native integer, a "Math::BigInt" object,
or an integer-valued "Math::BigRat" object; the returned number is of the same type.
rand_prob(PROB ...)
rand_prob(PROBS)
Returns a random integer selected with non-uniform probability. The relative
probabilities are supplied as a list of non-negative integers (multiple PROB
arguments) or a reference to an array of integers (the PROBS argument). The relative
probabilities may be native integers, "Math::BigInt" objects, or integer-valued
"Math::BigRat" objects; they must all be of the same type. At least one probability
value must be positive.
The first relative probability value (the first PROB or the first element of PROBS) is
the relative probability of returning 0. The absolute probability of returning 0 is
this value divided by the total of all the relative probability values. Similarly the
second value controls the probability of returning 1, and so on.
Numbers
rand_fix(NBITS)
Returns a uniformly-distributed random NBITS-bit fixed-point fraction in the range [0,
1). That is, the result is a randomly-chosen multiple of 2^-NBITS, the multiplier
being a random integer in the range [0, 2^NBITS). The value is returned in the form
of a native floating point number, so NBITS can be at most one greater than the number
of bits of significand in the floating point format.
With NBITS = 48 the range of output values is the same as that of the Unix "drand48"
function.
rand([LIMIT])
Generates a random fixed-point fraction by "rand_fix" and then multiplies it by LIMIT,
returning the result. LIMIT defaults to 1, and if it is 0 then that is also treated
as 1. The length of the fixed-point fraction is 48 bits, unless that can't be
represented in the native floating point type, in which case the longest possible
fraction will be generated instead.
This is a drop-in replacement for "CORE::rand": it produces exactly the same range of
output values, but using the current entropy source instead of a sucky PRNG with
linear relationships between successive outputs. ("CORE::rand" does the type of
calculation described, but using the PRNG "drand48" to generate the fixed-point
fraction.) The details of behaviour may change in the future if the behaviour of
"CORE::rand" changes, to maintain the match.
Where the source of a module can't be readily modified, it can be made to use this
"rand" by an incantation such as
*Foreign::Module::rand = \&Data::Entropy::Algorithms::rand;
This must be done before the module is loaded, most likely in a "BEGIN" block. It is
also possible to override "CORE::rand" for all modules, by performing this similarly
early:
*CORE::GLOBAL::rand = \&Data::Entropy::Algorithms::rand;
This function should not be used in any new code, because the kind of output supplied
by "rand" is hardly ever the right thing to use. The "int(rand($n))" idiom to
generate a random integer has non-uniform probabilities of generating each possible
value, except when $n is a power of two. For floating point numbers, "rand" can't
generate most representable numbers in its output range, and the output is biased
towards zero. In new code use "rand_int" to generate integers and "rand_flt" to
generate floating point numbers.
rand_flt(MIN, MAX)
Selects a uniformly-distributed real number (with infinite precision) in the range
[MIN, MAX] and then rounds this number to the nearest representable floating point
value, which it returns. (Actually it is only as if the function worked this way: in
fact it never generates the number with infinite precision. It selects between the
representable floating point values with the probabilities implied by this process.)
This can return absolutely any floating point value in the range [MIN, MAX]; both MIN
and MAX themselves are possible return values. All bits of the floating point type
are filled randomly, so the range of values that can be returned depends on the
details of the floating point format. (See Data::Float for low-level floating point
utilities.)
The function "die"s if MIN and MAX are not both finite. If MIN is greater than MAX
then their roles are swapped: the order of the limit parameters actually doesn't
matter. If the limits are identical then that value is always returned. As a special
case, if the limits are positive zero and negative zero then a zero will be returned
with a randomly-chosen sign.
Combinatorics
pick(ITEM ...)
Randomly selects and returns one of the ITEMs. Each ITEM has equal probability of
being selected.
pick_r(ITEMS)
ITEMS must be a reference to an array. Randomly selects and returns one of the
elements of the array. Each element has equal probability of being selected.
This is the same operation as that performed by "pick", but using references to avoid
expensive copying of arrays.
choose(NCHOOSE, ITEM ...)
Randomly selects NCHOOSE of the ITEMs. Each ITEM has equal probability of being
selected. The chosen items are returned in a list in the same order in which they
appeared in the argument list.
choose_r(NCHOOSE, ITEMS)
ITEMS must be a reference to an array. Randomly selects NCHOOSE of the elements in
the array. Each element has equal probability of being selected. Returns a reference
to an array containing the chosen items in the same order in which they appeared in
the input array.
This is the same operation as that performed by "choose", but using references to
avoid expensive copying of arrays.
shuffle(ITEM ...)
Reorders the ITEMs randomly, and returns them in a list in random order. Each
possible order has equal probability.
shuffle_r(ITEMS)
ITEMS must be a reference to an array. Reorders the elements of the array randomly.
Each possible order has equal probability. Returns a reference to an array containing
the elements in random order.
This is the same operation as that performed by "shuffle", but using references to
avoid expensive copying of arrays.
SEE ALSO
Data::Entropy, Data::Entropy::Source
AUTHOR
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT
Copyright (C) 2006, 2007, 2009, 2011 Andrew Main (Zefram) <zefram@fysh.org>
LICENSE
This module is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.