Struct rand::isaac::IsaacRng
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pub struct IsaacRng { /* fields omitted */ }
A random number generator that uses the ISAAC algorithm.
ISAAC stands for "Indirection, Shift, Accumulate, Add, and Count" which are the principal bitwise operations employed. It is the most advanced of a series of array based random number generator designed by Robert Jenkins in 1996[1][2].
Although ISAAC is designed to be cryptographically secure, its design is not founded in cryptographic theory. Therefore it is not recommended for cryptographic purposes. It is however one of the strongest non-cryptograpic RNGs, and that while still being reasonably fast.
Where fast random numbers are needed which should still be secure, but where speed is more important than absolute (cryptographic) security (e.g. to initialise hashes in the std library), a generator like ISAAC may be a good choice.
In 2006 an improvement to ISAAC was suggested by Jean-Philippe Aumasson, named ISAAC+[3]. But because the specification is not complete, there is no good implementation, and because the suggested bias may not exist, it is not implemented here.
Overview of the ISAAC algorithm:
(in pseudo-code)
Input: a, b, c, s[256] // state
Output: r[256] // results
mix(a,i) = a ^ a << 13 if i = 0 mod 4
a ^ a >> 6 if i = 1 mod 4
a ^ a << 2 if i = 2 mod 4
a ^ a >> 16 if i = 3 mod 4
c = c + 1
b = b + c
for i in 0..256 {
x = s_[i]
a = f(a,i) + s[i+128 mod 256]
y = a + b + s[x>>2 mod 256]
s[i] = y
b = x + s[y>>10 mod 256]
r[i] = b
}
Numbers are generated in blocks of 256. This means the function above only runs once every 256 times you ask for a next random number. In all other circumstances the last element of the results array is returned.
ISAAC therefore needs a lot of memory, relative to other non-vrypto RNGs. 2 * 256 * 4 = 2 kb to hold the state and results.
References
[1]: Bob Jenkins, ISAAC: A fast cryptographic random number generator
[2]: Bob Jenkins, ISAAC and RC4
[3]: Jean-Philippe Aumasson, On the pseudo-random generator ISAAC
Methods
impl IsaacRng
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pub fn new_unseeded() -> IsaacRng
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: use the NewRng or SeedableRng trait
Create an ISAAC random number generator using the default fixed seed.
DEPRECATED. IsaacRng::new_from_u64(0)
will produce identical results.
pub fn new_from_u64(seed: u64) -> IsaacRng
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Creates an ISAAC random number generator using an u64 as seed.
If seed == 0
this will produce the same stream of random numbers as
the reference implementation when used unseeded.
Trait Implementations
impl Clone for IsaacRng
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fn clone(&self) -> IsaacRng
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Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
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Performs copy-assignment from source
. Read more
impl Debug for IsaacRng
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fn fmt(&self, f: &mut Formatter) -> Result
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Formats the value using the given formatter. Read more
impl RngCore for IsaacRng
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fn next_u32(&mut self) -> u32
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Return the next random u32
. Read more
fn next_u64(&mut self) -> u64
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Return the next random u64
. Read more
fn fill_bytes(&mut self, dest: &mut [u8])
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Fill dest
with random data. Read more
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>
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Fill dest
entirely with random data. Read more
impl SeedableRng for IsaacRng
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type Seed = [u8; 32]
Seed type, which is restricted to types mutably-dereferencable as u8
arrays (we recommend [u8; N]
for some N
). Read more
fn from_seed(seed: Self::Seed) -> Self
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Create a new PRNG using the given seed. Read more
fn from_rng<R: RngCore>(rng: R) -> Result<Self, Error>
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Create a new PRNG seeded from another Rng
. Read more