rust/src/libstd/rand/isaac.rs

// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! The ISAAC random number generator.

use rand::{seed, Rng};
use iter::{Iterator, range, range_step};
use option::{None, Some};

use cast;
use cmp;
use sys;
use vec;

static RAND_SIZE_LEN: u32 = 8;
static RAND_SIZE: u32 = 1 << RAND_SIZE_LEN;

/// A random number generator that uses the [ISAAC
/// algorithm](http://en.wikipedia.org/wiki/ISAAC_%28cipher%29).
///
/// The ISAAC algorithm is suitable for cryptographic purposes.
pub struct IsaacRng {
    priv cnt: u32,
    priv rsl: [u32, .. RAND_SIZE],
    priv mem: [u32, .. RAND_SIZE],
    priv a: u32,
    priv b: u32,
    priv c: u32
}

impl IsaacRng {
    /// Create an ISAAC random number generator with a random seed.
    pub fn new() -> IsaacRng {
        IsaacRng::new_seeded(seed(RAND_SIZE as uint * 4))
    }

    /// Create an ISAAC random number generator with a seed. This can be any
    /// length, although the maximum number of bytes used is 1024 and any more
    /// will be silently ignored. A generator constructed with a given seed
    /// will generate the same sequence of values as all other generators
    /// constructed with the same seed.
    pub fn new_seeded(seed: &[u8]) -> IsaacRng {
        let mut rng = IsaacRng {
            cnt: 0,
            rsl: [0, .. RAND_SIZE],
            mem: [0, .. RAND_SIZE],
            a: 0, b: 0, c: 0
        };

        let array_size = sys::size_of_val(&rng.rsl);
        let copy_length = cmp::min(array_size, seed.len());

        // manually create a &mut [u8] slice of randrsl to copy into.
        let dest = unsafe { cast::transmute((&mut rng.rsl, array_size)) };
        vec::bytes::copy_memory(dest, seed, copy_length);
        rng.init(true);
        rng
    }

    /// Create an ISAAC random number generator using the default
    /// fixed seed.
    pub fn new_unseeded() -> IsaacRng {
        let mut rng = IsaacRng {
            cnt: 0,
            rsl: [0, .. RAND_SIZE],
            mem: [0, .. RAND_SIZE],
            a: 0, b: 0, c: 0
        };
        rng.init(false);
        rng
    }

    /// Initialises `self`. If `use_rsl` is true, then use the current value
    /// of `rsl` as a seed, otherwise construct one algorithmically (not
    /// randomly).
    fn init(&mut self, use_rsl: bool) {
        let mut a = 0x9e3779b9;
        let mut b = a;
        let mut c = a;
        let mut d = a;
        let mut e = a;
        let mut f = a;
        let mut g = a;
        let mut h = a;

        macro_rules! mix(
            () => {{
                a^=b<<11; d+=a; b+=c;
                b^=c>>2;  e+=b; c+=d;
                c^=d<<8;  f+=c; d+=e;
                d^=e>>16; g+=d; e+=f;
                e^=f<<10; h+=e; f+=g;
                f^=g>>4;  a+=f; g+=h;
                g^=h<<8;  b+=g; h+=a;
                h^=a>>9;  c+=h; a+=b;
            }}
        );

        do 4.times { mix!(); }

        if use_rsl {
            macro_rules! memloop (
                ($arr:expr) => {{
                    for i in range_step(0u32, RAND_SIZE, 8) {
                        a+=$arr[i  ]; b+=$arr[i+1];
                        c+=$arr[i+2]; d+=$arr[i+3];
                        e+=$arr[i+4]; f+=$arr[i+5];
                        g+=$arr[i+6]; h+=$arr[i+7];
                        mix!();
                        self.mem[i  ]=a; self.mem[i+1]=b;
                        self.mem[i+2]=c; self.mem[i+3]=d;
                        self.mem[i+4]=e; self.mem[i+5]=f;
                        self.mem[i+6]=g; self.mem[i+7]=h;
                    }
                }}
            );

            memloop!(self.rsl);
            memloop!(self.mem);
        } else {
            for i in range_step(0u32, RAND_SIZE, 8) {
                mix!();
                self.mem[i  ]=a; self.mem[i+1]=b;
                self.mem[i+2]=c; self.mem[i+3]=d;
                self.mem[i+4]=e; self.mem[i+5]=f;
                self.mem[i+6]=g; self.mem[i+7]=h;
            }
        }

        self.isaac();
    }

    /// Refills the output buffer (`self.rsl`)
    #[inline]
    fn isaac(&mut self) {
        self.c += 1;
        // abbreviations
        let mut a = self.a;
        let mut b = self.b + self.c;

        static MIDPOINT: uint = RAND_SIZE as uint / 2;

        macro_rules! ind (($x:expr) => {
            self.mem[($x >> 2) & (RAND_SIZE - 1)]
        });
        macro_rules! rngstep(
            ($j:expr, $shift:expr) => {{
                let base = $j;
                let mix = if $shift < 0 {
                    a >> -$shift as uint
                } else {
                    a << $shift as uint
                };

                let x = self.mem[base  + mr_offset];
                a = (a ^ mix) + self.mem[base + m2_offset];
                let y = ind!(x) + a + b;
                self.mem[base + mr_offset] = y;

                b = ind!(y >> RAND_SIZE_LEN) + x;
                self.rsl[base + mr_offset] = b;
            }}
        );

        let r = [(0, MIDPOINT), (MIDPOINT, 0)];
        for &(mr_offset, m2_offset) in r.iter() {
            for i in range_step(0u, MIDPOINT, 4) {
                rngstep!(i + 0, 13);
                rngstep!(i + 1, -6);
                rngstep!(i + 2, 2);
                rngstep!(i + 3, -16);
            }
        }

        self.a = a;
        self.b = b;
        self.cnt = RAND_SIZE;
    }
}

impl Rng for IsaacRng {
    #[inline]
    fn next(&mut self) -> u32 {
        if self.cnt == 0 {
            // make some more numbers
            self.isaac();
        }
        self.cnt -= 1;
        self.rsl[self.cnt]
    }
}