auto merge of #9810 : huonw/rust/rand3, r=alexcrichton

- Adds the `Sample` and `IndependentSample` traits for generating numbers where there are parameters (e.g. a list of elements to draw from, or the mean/variance of a normal distribution). The former takes `&mut self` and the latter takes `&self` (this is the only difference).
- Adds proper `Normal` and `Exp`-onential distributions
- Adds `Range` which generates `[lo, hi)` generically & properly (via a new trait) replacing the incorrect behaviour of `Rng.gen_integer_range` (this has become `Rng.gen_range` for convenience, it's far more efficient to use `Range` itself)
- Move the `Weighted` struct from `std::rand` to `std::rand::distributions` & improve it
- optimisations and docs

7 files changed, 825 insertions, 234 deletions

diff --git a/src/libstd/rand/distributions.rs b/src/libstd/rand/distributions.rs
index 0902100dca6..e7bcf8ce5d3 100644
--- a/src/libstd/rand/distributions.rs
+++ b/src/libstd/rand/distributions.rs
@@ -8,38 +8,226 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Sampling from random distributions
+/*!
+Sampling from random distributions.
 
-// Some implementations use the Ziggurat method
-// https://en.wikipedia.org/wiki/Ziggurat_algorithm
-//
-// The version used here is ZIGNOR [Doornik 2005, "An Improved
-// Ziggurat Method to Generate Normal Random Samples"] which is slower
-// (about double, it generates an extra random number) than the
-// canonical version [Marsaglia & Tsang 2000, "The Ziggurat Method for
-// Generating Random Variables"], but more robust. If one wanted, one
-// could implement VIZIGNOR the ZIGNOR paper for more speed.
+This is a generalization of `Rand` to allow parameters to control the
+exact properties of the generated values, e.g. the mean and standard
+deviation of a normal distribution. The `Sample` trait is the most
+general, and allows for generating values that change some state
+internally. The `IndependentSample` trait is for generating values
+that do not need to record state.
+
+*/
 
+use iter::range;
+use option::{Some, None};
 use num;
 use rand::{Rng,Rand};
+use clone::Clone;
+
+pub use self::range::Range;
+
+pub mod range;
+
+/// Types that can be used to create a random instance of `Support`.
+pub trait Sample<Support> {
+    /// Generate a random value of `Support`, using `rng` as the
+    /// source of randomness.
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> Support;
+}
+
+/// `Sample`s that do not require keeping track of state.
+///
+/// Since no state is recored, each sample is (statistically)
+/// independent of all others, assuming the `Rng` used has this
+/// property.
+// XXX maybe having this separate is overkill (the only reason is to
+// take &self rather than &mut self)? or maybe this should be the
+// trait called `Sample` and the other should be `DependentSample`.
+pub trait IndependentSample<Support>: Sample<Support> {
+    /// Generate a random value.
+    fn ind_sample<R: Rng>(&self, &mut R) -> Support;
+}
+
+/// A wrapper for generating types that implement `Rand` via the
+/// `Sample` & `IndependentSample` traits.
+pub struct RandSample<Sup>;
+
+impl<Sup: Rand> Sample<Sup> for RandSample<Sup> {
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> Sup { self.ind_sample(rng) }
+}
+
+impl<Sup: Rand> IndependentSample<Sup> for RandSample<Sup> {
+    fn ind_sample<R: Rng>(&self, rng: &mut R) -> Sup {
+        rng.gen()
+    }
+}
+
+/// A value with a particular weight for use with `WeightedChoice`.
+pub struct Weighted<T> {
+    /// The numerical weight of this item
+    weight: uint,
+    /// The actual item which is being weighted
+    item: T,
+}
+
+/// A distribution that selects from a finite collection of weighted items.
+///
+/// Each item has an associated weight that influences how likely it
+/// is to be chosen: higher weight is more likely.
+///
+/// The `Clone` restriction is a limitation of the `Sample` and
+/// `IndepedentSample` traits. Note that `&T` is (cheaply) `Clone` for
+/// all `T`, as is `uint`, so one can store references or indices into
+/// another vector.
+///
+/// # Example
+///
+/// ```rust
+/// use std::rand;
+/// use std::rand::distributions::{Weighted, WeightedChoice, IndepedentSample};
+///
+/// fn main() {
+///     let wc = WeightedChoice::new(~[Weighted { weight: 2, item: 'a' },
+///                                    Weighted { weight: 4, item: 'b' },
+///                                    Weighted { weight: 1, item: 'c' }]);
+///     let rng = rand::task_rng();
+///     for _ in range(0, 16) {
+///          // on average prints 'a' 4 times, 'b' 8 and 'c' twice.
+///          println!("{}", wc.ind_sample(rng));
+///     }
+/// }
+/// ```
+pub struct WeightedChoice<T> {
+    priv items: ~[Weighted<T>],
+    priv weight_range: Range<uint>
+}
+
+impl<T: Clone> WeightedChoice<T> {
+    /// Create a new `WeightedChoice`.
+    ///
+    /// Fails if:
+    /// - `v` is empty
+    /// - the total weight is 0
+    /// - the total weight is larger than a `uint` can contain.
+    pub fn new(mut items: ~[Weighted<T>]) -> WeightedChoice<T> {
+        // strictly speaking, this is subsumed by the total weight == 0 case
+        assert!(!items.is_empty(), "WeightedChoice::new called with no items");
+
+        let mut running_total = 0u;
+
+        // we convert the list from individual weights to cumulative
+        // weights so we can binary search. This *could* drop elements
+        // with weight == 0 as an optimisation.
+        for item in items.mut_iter() {
+            running_total = running_total.checked_add(&item.weight)
+                .expect("WeightedChoice::new called with a total weight larger \
+                        than a uint can contain");
+
+            item.weight = running_total;
+        }
+        assert!(running_total != 0, "WeightedChoice::new called with a total weight of 0");
+
+        WeightedChoice {
+            items: items,
+            // we're likely to be generating numbers in this range
+            // relatively often, so might as well cache it
+            weight_range: Range::new(0, running_total)
+        }
+    }
+}
+
+impl<T: Clone> Sample<T> for WeightedChoice<T> {
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> T { self.ind_sample(rng) }
+}
+
+impl<T: Clone> IndependentSample<T> for WeightedChoice<T> {
+    fn ind_sample<R: Rng>(&self, rng: &mut R) -> T {
+        // we want to find the first element that has cumulative
+        // weight > sample_weight, which we do by binary since the
+        // cumulative weights of self.items are sorted.
+
+        // choose a weight in [0, total_weight)
+        let sample_weight = self.weight_range.ind_sample(rng);
+
+        // short circuit when it's the first item
+        if sample_weight < self.items[0].weight {
+            return self.items[0].item.clone();
+        }
+
+        let mut idx = 0;
+        let mut modifier = self.items.len();
+
+        // now we know that every possibility has an element to the
+        // left, so we can just search for the last element that has
+        // cumulative weight <= sample_weight, then the next one will
+        // be "it". (Note that this greatest element will never be the
+        // last element of the vector, since sample_weight is chosen
+        // in [0, total_weight) and the cumulative weight of the last
+        // one is exactly the total weight.)
+        while modifier > 1 {
+            let i = idx + modifier / 2;
+            if self.items[i].weight <= sample_weight {
+                // we're small, so look to the right, but allow this
+                // exact element still.
+                idx = i;
+                // we need the `/ 2` to round up otherwise we'll drop
+                // the trailing elements when `modifier` is odd.
+                modifier += 1;
+            } else {
+                // otherwise we're too big, so go left. (i.e. do
+                // nothing)
+            }
+            modifier /= 2;
+        }
+        return self.items[idx + 1].item.clone();
+    }
+}
 
 mod ziggurat_tables;
 
-// inlining should mean there is no performance penalty for this
-#[inline]
+/// Sample a random number using the Ziggurat method (specifically the
+/// ZIGNOR variant from Doornik 2005). Most of the arguments are
+/// directly from the paper:
+///
+/// * `rng`: source of randomness
+/// * `symmetric`: whether this is a symmetric distribution, or one-sided with P(x < 0) = 0.
+/// * `X`: the $x_i$ abscissae.
+/// * `F`: precomputed values of the PDF at the $x_i$, (i.e. $f(x_i)$)
+/// * `F_DIFF`: precomputed values of $f(x_i) - f(x_{i+1})$
+/// * `pdf`: the probability density function
+/// * `zero_case`: manual sampling from the tail when we chose the
+///    bottom box (i.e. i == 0)
+
+// the perf improvement (25-50%) is definitely worth the extra code
+// size from force-inlining.
+#[inline(always)]
 fn ziggurat<R:Rng>(rng: &mut R,
-                   center_u: bool,
+                   symmetric: bool,
                    X: ziggurat_tables::ZigTable,
                    F: ziggurat_tables::ZigTable,
                    F_DIFF: ziggurat_tables::ZigTable,
-                   pdf: &'static fn(f64) -> f64, // probability density function
+                   pdf: &'static fn(f64) -> f64,
                    zero_case: &'static fn(&mut R, f64) -> f64) -> f64 {
+    static SCALE: f64 = (1u64 << 53) as f64;
     loop {
-        let u = if center_u {2.0 * rng.gen() - 1.0} else {rng.gen()};
-        let i: uint = rng.gen::<uint>() & 0xff;
+        // reimplement the f64 generation as an optimisation suggested
+        // by the Doornik paper: we have a lot of precision-space
+        // (i.e. there are 11 bits of the 64 of a u64 to use after
+        // creating a f64), so we might as well reuse some to save
+        // generating a whole extra random number. (Seems to be 15%
+        // faster.)
+        let bits: u64 = rng.gen();
+        let i = (bits & 0xff) as uint;
+        let f = (bits >> 11) as f64 / SCALE;
+
+        // u is either U(-1, 1) or U(0, 1) depending on if this is a
+        // symmetric distribution or not.
+        let u = if symmetric {2.0 * f - 1.0} else {f};
         let x = u * X[i];
 
-        let test_x = if center_u {num::abs(x)} else {x};
+        let test_x = if symmetric {num::abs(x)} else {x};
 
         // algebraically equivalent to |u| < X[i+1]/X[i] (or u < X[i+1]/X[i])
         if test_x < X[i + 1] {
@@ -49,30 +237,25 @@ fn ziggurat<R:Rng>(rng: &mut R,
             return zero_case(rng, u);
         }
         // algebraically equivalent to f1 + DRanU()*(f0 - f1) < 1
-        if F[i+1] + F_DIFF[i+1] * rng.gen() < pdf(x) {
+        if F[i + 1] + F_DIFF[i + 1] * rng.gen() < pdf(x) {
             return x;
         }
     }
 }
 
-/// A wrapper around an `f64` to generate N(0, 1) random numbers (a.k.a.  a
-/// standard normal, or Gaussian). Multiplying the generated values by the
-/// desired standard deviation `sigma` then adding the desired mean `mu` will
-/// give N(mu, sigma^2) distributed random numbers.
+/// A wrapper around an `f64` to generate N(0, 1) random numbers
+/// (a.k.a.  a standard normal, or Gaussian).
 ///
-/// Note that this has to be unwrapped before use as an `f64` (using either
-/// `*` or `cast::transmute` is safe).
+/// See `Normal` for the general normal distribution. That this has to
+/// be unwrapped before use as an `f64` (using either `*` or
+/// `cast::transmute` is safe).
 ///
-/// # Example
+/// Implemented via the ZIGNOR variant[1] of the Ziggurat method.
 ///
-/// ```
-/// use std::rand::distributions::StandardNormal;
-///
-/// fn main() {
-///     let normal = 2.0 + (*rand::random::<StandardNormal>()) * 3.0;
-///     println!("{} is from a N(2, 9) distribution", normal)
-/// }
-/// ```
+/// [1]: Jurgen A. Doornik (2005). [*An Improved Ziggurat Method to
+/// Generate Normal Random
+/// Samples*](http://www.doornik.com/research/ziggurat.pdf). Nuffield
+/// College, Oxford
 pub struct StandardNormal(f64);
 
 impl Rand for StandardNormal {
@@ -110,23 +293,62 @@ impl Rand for StandardNormal {
     }
 }
 
-/// A wrapper around an `f64` to generate Exp(1) random numbers. Dividing by
-/// the desired rate `lambda` will give Exp(lambda) distributed random
-/// numbers.
+/// The normal distribution `N(mean, std_dev**2)`.
 ///
-/// Note that this has to be unwrapped before use as an `f64` (using either
-/// `*` or `cast::transmute` is safe).
+/// This uses the ZIGNOR variant of the Ziggurat method, see
+/// `StandardNormal` for more details.
 ///
 /// # Example
 ///
 /// ```
-/// use std::rand::distributions::Exp1;
+/// use std::rand;
+/// use std::rand::distributions::{Normal, IndependentSample};
 ///
 /// fn main() {
-///     let exp2 = (*rand::random::<Exp1>()) * 0.5;
-///     println!("{} is from a Exp(2) distribution", exp2);
+///     let normal = Normal::new(2.0, 3.0);
+///     let v = normal.ind_sample(rand::task_rng());
+///     println!("{} is from a N(2, 9) distribution", v)
 /// }
 /// ```
+pub struct Normal {
+    priv mean: f64,
+    priv std_dev: f64
+}
+
+impl Normal {
+    /// Construct a new `Normal` distribution with the given mean and
+    /// standard deviation. Fails if `std_dev < 0`.
+    pub fn new(mean: f64, std_dev: f64) -> Normal {
+        assert!(std_dev >= 0.0, "Normal::new called with `std_dev` < 0");
+        Normal {
+            mean: mean,
+            std_dev: std_dev
+        }
+    }
+}
+impl Sample<f64> for Normal {
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> f64 { self.ind_sample(rng) }
+}
+impl IndependentSample<f64> for Normal {
+    fn ind_sample<R: Rng>(&self, rng: &mut R) -> f64 {
+        self.mean + self.std_dev * (*rng.gen::<StandardNormal>())
+    }
+}
+
+/// A wrapper around an `f64` to generate Exp(1) random numbers.
+///
+/// See `Exp` for the general exponential distribution.Note that this
+ // has to be unwrapped before use as an `f64` (using either
+/// `*` or `cast::transmute` is safe).
+///
+/// Implemented via the ZIGNOR variant[1] of the Ziggurat method. The
+/// exact description in the paper was adjusted to use tables for the
+/// exponential distribution rather than normal.
+///
+/// [1]: Jurgen A. Doornik (2005). [*An Improved Ziggurat Method to
+/// Generate Normal Random
+/// Samples*](http://www.doornik.com/research/ziggurat.pdf). Nuffield
+/// College, Oxford
 pub struct Exp1(f64);
 
 // This could be done via `-rng.gen::<f64>().ln()` but that is slower.
@@ -148,3 +370,221 @@ impl Rand for Exp1 {
                       pdf, zero_case))
     }
 }
+
+/// The exponential distribution `Exp(lambda)`.
+///
+/// This distribution has density function: `f(x) = lambda *
+/// exp(-lambda * x)` for `x > 0`.
+///
+/// # Example
+///
+/// ```
+/// use std::rand;
+/// use std::rand::distributions::{Exp, IndependentSample};
+///
+/// fn main() {
+///     let exp = Exp::new(2.0);
+///     let v = exp.ind_sample(rand::task_rng());
+///     println!("{} is from a Exp(2) distribution", v);
+/// }
+/// ```
+pub struct Exp {
+    /// `lambda` stored as `1/lambda`, since this is what we scale by.
+    priv lambda_inverse: f64
+}
+
+impl Exp {
+    /// Construct a new `Exp` with the given shape parameter
+    /// `lambda`. Fails if `lambda <= 0`.
+    pub fn new(lambda: f64) -> Exp {
+        assert!(lambda > 0.0, "Exp::new called with `lambda` <= 0");
+        Exp { lambda_inverse: 1.0 / lambda }
+    }
+}
+
+impl Sample<f64> for Exp {
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> f64 { self.ind_sample(rng) }
+}
+impl IndependentSample<f64> for Exp {
+    fn ind_sample<R: Rng>(&self, rng: &mut R) -> f64 {
+        (*rng.gen::<Exp1>()) * self.lambda_inverse
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use rand::*;
+    use super::*;
+    use iter::range;
+    use option::{Some, None};
+
+    struct ConstRand(uint);
+    impl Rand for ConstRand {
+        fn rand<R: Rng>(_: &mut R) -> ConstRand {
+            ConstRand(0)
+        }
+    }
+
+    // 0, 1, 2, 3, ...
+    struct CountingRng { i: u32 }
+    impl Rng for CountingRng {
+        fn next_u32(&mut self) -> u32 {
+            self.i += 1;
+            self.i - 1
+        }
+        fn next_u64(&mut self) -> u64 {
+            self.next_u32() as u64
+        }
+    }
+
+    #[test]
+    fn test_rand_sample() {
+        let mut rand_sample = RandSample::<ConstRand>;
+
+        assert_eq!(*rand_sample.sample(task_rng()), 0);
+        assert_eq!(*rand_sample.ind_sample(task_rng()), 0);
+    }
+
+    #[test]
+    fn test_normal() {
+        let mut norm = Normal::new(10.0, 10.0);
+        let rng = task_rng();
+        for _ in range(0, 1000) {
+            norm.sample(rng);
+            norm.ind_sample(rng);
+        }
+    }
+    #[test]
+    #[should_fail]
+    fn test_normal_invalid_sd() {
+        Normal::new(10.0, -1.0);
+    }
+
+    #[test]
+    fn test_exp() {
+        let mut exp = Exp::new(10.0);
+        let rng = task_rng();
+        for _ in range(0, 1000) {
+            assert!(exp.sample(rng) >= 0.0);
+            assert!(exp.ind_sample(rng) >= 0.0);
+        }
+    }
+    #[test]
+    #[should_fail]
+    fn test_exp_invalid_lambda_zero() {
+        Exp::new(0.0);
+    }
+    #[test]
+    #[should_fail]
+    fn test_exp_invalid_lambda_neg() {
+        Exp::new(-10.0);
+    }
+
+    #[test]
+    fn test_weighted_choice() {
+        // this makes assumptions about the internal implementation of
+        // WeightedChoice, specifically: it doesn't reorder the items,
+        // it doesn't do weird things to the RNG (so 0 maps to 0, 1 to
+        // 1, internally; modulo a modulo operation).
+
+        macro_rules! t (
+            ($items:expr, $expected:expr) => {{
+                let wc = WeightedChoice::new($items);
+                let expected = $expected;
+
+                let mut rng = CountingRng { i: 0 };
+
+                for &val in expected.iter() {
+                    assert_eq!(wc.ind_sample(&mut rng), val)
+                }
+            }}
+        );
+
+        t!(~[Weighted { weight: 1, item: 10}], ~[10]);
+
+        // skip some
+        t!(~[Weighted { weight: 0, item: 20},
+             Weighted { weight: 2, item: 21},
+             Weighted { weight: 0, item: 22},
+             Weighted { weight: 1, item: 23}],
+           ~[21,21, 23]);
+
+        // different weights
+        t!(~[Weighted { weight: 4, item: 30},
+             Weighted { weight: 3, item: 31}],
+           ~[30,30,30,30, 31,31,31]);
+
+        // check that we're binary searching
+        // correctly with some vectors of odd
+        // length.
+        t!(~[Weighted { weight: 1, item: 40},
+             Weighted { weight: 1, item: 41},
+             Weighted { weight: 1, item: 42},
+             Weighted { weight: 1, item: 43},
+             Weighted { weight: 1, item: 44}],
+           ~[40, 41, 42, 43, 44]);
+        t!(~[Weighted { weight: 1, item: 50},
+             Weighted { weight: 1, item: 51},
+             Weighted { weight: 1, item: 52},
+             Weighted { weight: 1, item: 53},
+             Weighted { weight: 1, item: 54},
+             Weighted { weight: 1, item: 55},
+             Weighted { weight: 1, item: 56}],
+           ~[50, 51, 52, 53, 54, 55, 56]);
+    }
+
+    #[test] #[should_fail]
+    fn test_weighted_choice_no_items() {
+        WeightedChoice::<int>::new(~[]);
+    }
+    #[test] #[should_fail]
+    fn test_weighted_choice_zero_weight() {
+        WeightedChoice::new(~[Weighted { weight: 0, item: 0},
+                              Weighted { weight: 0, item: 1}]);
+    }
+    #[test] #[should_fail]
+    fn test_weighted_choice_weight_overflows() {
+        let x = (-1) as uint / 2; // x + x + 2 is the overflow
+        WeightedChoice::new(~[Weighted { weight: x, item: 0 },
+                              Weighted { weight: 1, item: 1 },
+                              Weighted { weight: x, item: 2 },
+                              Weighted { weight: 1, item: 3 }]);
+    }
+}
+
+#[cfg(test)]
+mod bench {
+    use extra::test::BenchHarness;
+    use rand::*;
+    use super::*;
+    use iter::range;
+    use option::{Some, None};
+    use mem::size_of;
+
+    static N: u64 = 100;
+
+    #[bench]
+    fn rand_normal(bh: &mut BenchHarness) {
+        let mut rng = XorShiftRng::new();
+        let mut normal = Normal::new(-2.71828, 3.14159);
+
+        do bh.iter {
+            for _ in range(0, N) {
+                normal.sample(&mut rng);
+            }
+        }
+        bh.bytes = size_of::<f64>() as u64 * N;
+    }
+    #[bench]
+    fn rand_exp(bh: &mut BenchHarness) {
+        let mut rng = XorShiftRng::new();
+        let mut exp = Exp::new(2.71828 * 3.14159);
+
+        do bh.iter {
+            for _ in range(0, N) {
+                exp.sample(&mut rng);
+            }
+        }
+        bh.bytes = size_of::<f64>() as u64 * N;
+    }
+}
diff --git a/src/libstd/rand/isaac.rs b/src/libstd/rand/isaac.rs
index 7dc4e5b868b..42254b211a1 100644
--- a/src/libstd/rand/isaac.rs
+++ b/src/libstd/rand/isaac.rs
@@ -19,10 +19,15 @@ use mem;
 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).
+/// A random number generator that uses the ISAAC algorithm[1].
 ///
-/// The ISAAC algorithm is suitable for cryptographic purposes.
+/// The ISAAC algorithm is generally accepted as suitable for
+/// cryptographic purposes, but this implementation has not be
+/// verified as such. Prefer a generator like `OSRng` that defers to
+/// the operating system for cases that need high security.
+///
+/// [1]: Bob Jenkins, [*ISAAC: A fast cryptographic random number
+/// generator*](http://www.burtleburtle.net/bob/rand/isaacafa.html)
 pub struct IsaacRng {
     priv cnt: u32,
     priv rsl: [u32, .. RAND_SIZE],
@@ -216,11 +221,16 @@ impl<'self> SeedableRng<&'self [u32]> for IsaacRng {
 static RAND_SIZE_64_LEN: uint = 8;
 static RAND_SIZE_64: uint = 1 << RAND_SIZE_64_LEN;
 
-/// A random number generator that uses the 64-bit variant of the
-/// [ISAAC
-/// algorithm](http://en.wikipedia.org/wiki/ISAAC_%28cipher%29).
+/// A random number generator that uses ISAAC-64[1], the 64-bit
+/// variant of the ISAAC algorithm.
+///
+/// The ISAAC algorithm is generally accepted as suitable for
+/// cryptographic purposes, but this implementation has not be
+/// verified as such. Prefer a generator like `OSRng` that defers to
+/// the operating system for cases that need high security.
 ///
-/// The ISAAC algorithm is suitable for cryptographic purposes.
+/// [1]: Bob Jenkins, [*ISAAC: A fast cryptographic random number
+/// generator*](http://www.burtleburtle.net/bob/rand/isaacafa.html)
 pub struct Isaac64Rng {
     priv cnt: uint,
     priv rsl: [u64, .. RAND_SIZE_64],
diff --git a/src/libstd/rand/mod.rs b/src/libstd/rand/mod.rs
index 9f611578c6a..a6efdfc66db 100644
--- a/src/libstd/rand/mod.rs
+++ b/src/libstd/rand/mod.rs
@@ -28,6 +28,23 @@ from an operating-system source of randomness, e.g. `/dev/urandom` on
 Unix systems, and will automatically reseed itself from this source
 after generating 32 KiB of random data.
 
+# Cryptographic security
+
+An application that requires random numbers for cryptographic purposes
+should prefer `OSRng`, which reads randomness from one of the source
+that the operating system provides (e.g. `/dev/urandom` on
+Unixes). The other random number generators provided by this module
+are either known to be insecure (`XorShiftRng`), or are not verified
+to be secure (`IsaacRng`, `Isaac64Rng` and `StdRng`).
+
+*Note*: on Linux, `/dev/random` is more secure than `/dev/urandom`,
+but it is a blocking RNG, and will wait until it has determined that
+it has collected enough entropy to fulfill a request for random
+data. It can be used with the `Rng` trait provided by this module by
+opening the file and passing it to `reader::ReaderRng`. Since it
+blocks, `/dev/random` should only be used to retrieve small amounts of
+randomness.
+
 # Examples
 
 ```rust
@@ -53,17 +70,20 @@ fn main () {
 */
 
 use cast;
+use cmp::Ord;
 use container::Container;
 use iter::{Iterator, range};
 use local_data;
 use prelude::*;
 use str;
-use u64;
 use vec;
 
 pub use self::isaac::{IsaacRng, Isaac64Rng};
 pub use self::os::OSRng;
 
+use self::distributions::{Range, IndependentSample};
+use self::distributions::range::SampleRange;
+
 pub mod distributions;
 pub mod isaac;
 pub mod os;
@@ -78,14 +98,6 @@ pub trait Rand {
     fn rand<R: Rng>(rng: &mut R) -> Self;
 }
 
-/// A value with a particular weight compared to other values
-pub struct Weighted<T> {
-    /// The numerical weight of this item
-    weight: uint,
-    /// The actual item which is being weighted
-    item: T,
-}
-
 /// A random number generator
 pub trait Rng {
     /// Return the next random u32. This rarely needs to be called
@@ -196,14 +208,14 @@ pub trait Rng {
         vec::from_fn(len, |_| self.gen())
     }
 
-    /// Generate a random primitive integer in the range [`low`,
-    /// `high`). Fails if `low >= high`.
+    /// Generate a random value in the range [`low`, `high`). Fails if
+    /// `low >= high`.
     ///
-    /// This gives a uniform distribution (assuming this RNG is itself
-    /// uniform), even for edge cases like `gen_integer_range(0u8,
-    /// 170)`, which a naive modulo operation would return numbers
-    /// less than 85 with double the probability to those greater than
-    /// 85.
+    /// This is a convenience wrapper around
+    /// `distributions::Range`. If this function will be called
+    /// repeatedly with the same arguments, one should use `Range`, as
+    /// that will amortize the computations that allow for perfect
+    /// uniformity, as they only happen on initialization.
     ///
     /// # Example
     ///
@@ -213,22 +225,15 @@ pub trait Rng {
     ///
     /// fn main() {
     ///    let mut rng = rand::task_rng();
-    ///    let n: uint = rng.gen_integer_range(0u, 10);
+    ///    let n: uint = rng.gen_range(0u, 10);
     ///    println!("{}", n);
-    ///    let m: int = rng.gen_integer_range(-40, 400);
+    ///    let m: float = rng.gen_range(-40.0, 1.3e5);
     ///    println!("{}", m);
     /// }
     /// ```
-    fn gen_integer_range<T: Rand + Int>(&mut self, low: T, high: T) -> T {
-        assert!(low < high, "RNG.gen_integer_range called with low >= high");
-        let range = (high - low).to_u64().unwrap();
-        let accept_zone = u64::max_value - u64::max_value % range;
-        loop {
-            let rand = self.gen::<u64>();
-            if rand < accept_zone {
-                return low + NumCast::from(rand % range).unwrap();
-            }
-        }
+    fn gen_range<T: Ord + SampleRange>(&mut self, low: T, high: T) -> T {
+        assert!(low < high, "Rng.gen_range called with low >= high");
+        Range::new(low, high).ind_sample(self)
     }
 
     /// Return a bool with a 1 in n chance of true
@@ -245,7 +250,7 @@ pub trait Rng {
     /// }
     /// ```
     fn gen_weighted_bool(&mut self, n: uint) -> bool {
-        n == 0 || self.gen_integer_range(0, n) == 0
+        n == 0 || self.gen_range(0, n) == 0
     }
 
     /// Return a random string of the specified length composed of
@@ -295,93 +300,8 @@ pub trait Rng {
         if values.is_empty() {
             None
         } else {
-            Some(&values[self.gen_integer_range(0u, values.len())])
-        }
-    }
-
-    /// Choose an item respecting the relative weights, failing if the sum of
-    /// the weights is 0
-    ///
-    /// # Example
-    ///
-    /// ```rust
-    /// use std::rand;
-    /// use std::rand::Rng;
-    ///
-    /// fn main() {
-    ///     let mut rng = rand::rng();
-    ///     let x = [rand::Weighted {weight: 4, item: 'a'},
-    ///              rand::Weighted {weight: 2, item: 'b'},
-    ///              rand::Weighted {weight: 2, item: 'c'}];
-    ///     println!("{}", rng.choose_weighted(x));
-    /// }
-    /// ```
-    fn choose_weighted<T:Clone>(&mut self, v: &[Weighted<T>]) -> T {
-        self.choose_weighted_option(v).expect("Rng.choose_weighted: total weight is 0")
-    }
-
-    /// Choose Some(item) respecting the relative weights, returning none if
-    /// the sum of the weights is 0
-    ///
-    /// # Example
-    ///
-    /// ```rust
-    /// use std::rand;
-    /// use std::rand::Rng;
-    ///
-    /// fn main() {
-    ///     let mut rng = rand::rng();
-    ///     let x = [rand::Weighted {weight: 4, item: 'a'},
-    ///              rand::Weighted {weight: 2, item: 'b'},
-    ///              rand::Weighted {weight: 2, item: 'c'}];
-    ///     println!("{:?}", rng.choose_weighted_option(x));
-    /// }
-    /// ```
-    fn choose_weighted_option<T:Clone>(&mut self, v: &[Weighted<T>])
-                                       -> Option<T> {
-        let mut total = 0u;
-        for item in v.iter() {
-            total += item.weight;
-        }
-        if total == 0u {
-            return None;
-        }
-        let chosen = self.gen_integer_range(0u, total);
-        let mut so_far = 0u;
-        for item in v.iter() {
-            so_far += item.weight;
-            if so_far > chosen {
-                return Some(item.item.clone());
-            }
-        }
-        unreachable!();
-    }
-
-    /// Return a vec containing copies of the items, in order, where
-    /// the weight of the item determines how many copies there are
-    ///
-    /// # Example
-    ///
-    /// ```rust
-    /// use std::rand;
-    /// use std::rand::Rng;
-    ///
-    /// fn main() {
-    ///     let mut rng = rand::rng();
-    ///     let x = [rand::Weighted {weight: 4, item: 'a'},
-    ///              rand::Weighted {weight: 2, item: 'b'},
-    ///              rand::Weighted {weight: 2, item: 'c'}];
-    ///     println!("{}", rng.weighted_vec(x));
-    /// }
-    /// ```
-    fn weighted_vec<T:Clone>(&mut self, v: &[Weighted<T>]) -> ~[T] {
-        let mut r = ~[];
-        for item in v.iter() {
-            for _ in range(0u, item.weight) {
-                r.push(item.item.clone());
-            }
+            Some(&values[self.gen_range(0u, values.len())])
         }
-        r
     }
 
     /// Shuffle a vec
@@ -425,7 +345,7 @@ pub trait Rng {
             // invariant: elements with index >= i have been locked in place.
             i -= 1u;
             // lock element i in place.
-            values.swap(i, self.gen_integer_range(0u, i + 1u));
+            values.swap(i, self.gen_range(0u, i + 1u));
         }
     }
 
@@ -451,7 +371,7 @@ pub trait Rng {
                 continue
             }
 
-            let k = self.gen_integer_range(0, i + 1);
+            let k = self.gen_range(0, i + 1);
             if k < reservoir.len() {
                 reservoir[k] = elem
             }
@@ -498,8 +418,8 @@ pub trait SeedableRng<Seed>: Rng {
 
 /// Create a random number generator with a default algorithm and seed.
 ///
-/// It returns the cryptographically-safest `Rng` algorithm currently
-/// available in Rust. If you require a specifically seeded `Rng` for
+/// It returns the strongest `Rng` algorithm currently implemented in
+/// pure Rust. If you require a specifically seeded `Rng` for
 /// consistency over time you should pick one algorithm and create the
 /// `Rng` yourself.
 ///
@@ -574,12 +494,16 @@ pub fn weak_rng() -> XorShiftRng {
     XorShiftRng::new()
 }
 
-/// An [Xorshift random number
-/// generator](http://en.wikipedia.org/wiki/Xorshift).
+/// An Xorshift[1] random number
+/// generator.
 ///
 /// The Xorshift algorithm is not suitable for cryptographic purposes
 /// but is very fast. If you do not know for sure that it fits your
-/// requirements, use a more secure one such as `IsaacRng`.
+/// requirements, use a more secure one such as `IsaacRng` or `OSRng`.
+///
+/// [1]: Marsaglia, George (July 2003). ["Xorshift
+/// RNGs"](http://www.jstatsoft.org/v08/i14/paper). *Journal of
+/// Statistical Software*. Vol. 8 (Issue 14).
 pub struct XorShiftRng {
     priv x: u32,
     priv y: u32,
@@ -759,36 +683,36 @@ mod test {
     }
 
     #[test]
-    fn test_gen_integer_range() {
+    fn test_gen_range() {
         let mut r = rng();
         for _ in range(0, 1000) {
-            let a = r.gen_integer_range(-3i, 42);
+            let a = r.gen_range(-3i, 42);
             assert!(a >= -3 && a < 42);
-            assert_eq!(r.gen_integer_range(0, 1), 0);
-            assert_eq!(r.gen_integer_range(-12, -11), -12);
+            assert_eq!(r.gen_range(0, 1), 0);
+            assert_eq!(r.gen_range(-12, -11), -12);
         }
 
         for _ in range(0, 1000) {
-            let a = r.gen_integer_range(10, 42);
+            let a = r.gen_range(10, 42);
             assert!(a >= 10 && a < 42);
-            assert_eq!(r.gen_integer_range(0, 1), 0);
-            assert_eq!(r.gen_integer_range(3_000_000u, 3_000_001), 3_000_000);
+            assert_eq!(r.gen_range(0, 1), 0);
+            assert_eq!(r.gen_range(3_000_000u, 3_000_001), 3_000_000);
         }
 
     }
 
     #[test]
     #[should_fail]
-    fn test_gen_integer_range_fail_int() {
+    fn test_gen_range_fail_int() {
         let mut r = rng();
-        r.gen_integer_range(5i, -2);
+        r.gen_range(5i, -2);
     }
 
     #[test]
     #[should_fail]
-    fn test_gen_integer_range_fail_uint() {
+    fn test_gen_range_fail_uint() {
         let mut r = rng();
-        r.gen_integer_range(5u, 2u);
+        r.gen_range(5u, 2u);
     }
 
     #[test]
@@ -843,44 +767,6 @@ mod test {
     }
 
     #[test]
-    fn test_choose_weighted() {
-        let mut r = rng();
-        assert!(r.choose_weighted([
-            Weighted { weight: 1u, item: 42 },
-        ]) == 42);
-        assert!(r.choose_weighted([
-            Weighted { weight: 0u, item: 42 },
-            Weighted { weight: 1u, item: 43 },
-        ]) == 43);
-    }
-
-    #[test]
-    fn test_choose_weighted_option() {
-        let mut r = rng();
-        assert!(r.choose_weighted_option([
-            Weighted { weight: 1u, item: 42 },
-        ]) == Some(42));
-        assert!(r.choose_weighted_option([
-            Weighted { weight: 0u, item: 42 },
-            Weighted { weight: 1u, item: 43 },
-        ]) == Some(43));
-        let v: Option<int> = r.choose_weighted_option([]);
-        assert!(v.is_none());
-    }
-
-    #[test]
-    fn test_weighted_vec() {
-        let mut r = rng();
-        let empty: ~[int] = ~[];
-        assert_eq!(r.weighted_vec([]), empty);
-        assert!(r.weighted_vec([
-            Weighted { weight: 0u, item: 3u },
-            Weighted { weight: 1u, item: 2u },
-            Weighted { weight: 2u, item: 1u },
-        ]) == ~[2u, 1u, 1u]);
-    }
-
-    #[test]
     fn test_shuffle() {
         let mut r = rng();
         let empty: ~[int] = ~[];
@@ -893,7 +779,7 @@ mod test {
         let mut r = task_rng();
         r.gen::<int>();
         assert_eq!(r.shuffle(~[1, 1, 1]), ~[1, 1, 1]);
-        assert_eq!(r.gen_integer_range(0u, 1u), 0u);
+        assert_eq!(r.gen_range(0u, 1u), 0u);
     }
 
     #[test]
@@ -952,41 +838,53 @@ mod bench {
     use extra::test::BenchHarness;
     use rand::*;
     use mem::size_of;
+    use iter::range;
+    use option::{Some, None};
+
+    static N: u64 = 100;
 
     #[bench]
     fn rand_xorshift(bh: &mut BenchHarness) {
         let mut rng = XorShiftRng::new();
         do bh.iter {
-            rng.gen::<uint>();
+            for _ in range(0, N) {
+                rng.gen::<uint>();
+            }
         }
-        bh.bytes = size_of::<uint>() as u64;
+        bh.bytes = size_of::<uint>() as u64 * N;
     }
 
     #[bench]
     fn rand_isaac(bh: &mut BenchHarness) {
         let mut rng = IsaacRng::new();
         do bh.iter {
-            rng.gen::<uint>();
+            for _ in range(0, N) {
+                rng.gen::<uint>();
+            }
         }
-        bh.bytes = size_of::<uint>() as u64;
+        bh.bytes = size_of::<uint>() as u64 * N;
     }
 
     #[bench]
     fn rand_isaac64(bh: &mut BenchHarness) {
         let mut rng = Isaac64Rng::new();
         do bh.iter {
-            rng.gen::<uint>();
+            for _ in range(0, N) {
+                rng.gen::<uint>();
+            }
         }
-        bh.bytes = size_of::<uint>() as u64;
+        bh.bytes = size_of::<uint>() as u64 * N;
     }
 
     #[bench]
     fn rand_std(bh: &mut BenchHarness) {
         let mut rng = StdRng::new();
         do bh.iter {
-            rng.gen::<uint>();
+            for _ in range(0, N) {
+                rng.gen::<uint>();
+            }
         }
-        bh.bytes = size_of::<uint>() as u64;
+        bh.bytes = size_of::<uint>() as u64 * N;
     }
 
     #[bench]
diff --git a/src/libstd/rand/os.rs b/src/libstd/rand/os.rs
index 4c8cf06c55e..5ed8d6b10d1 100644
--- a/src/libstd/rand/os.rs
+++ b/src/libstd/rand/os.rs
@@ -30,8 +30,12 @@ type HCRYPTPROV = c_long;
 // assume they work when we call them.
 
 /// A random number generator that retrieves randomness straight from
-/// the operating system. On Unix-like systems this reads from
-/// `/dev/urandom`, on Windows this uses `CryptGenRandom`.
+/// the operating system. Platform sources:
+///
+/// - Unix-like systems (Linux, Android, Mac OSX): read directly from
+///   `/dev/urandom`.
+/// - Windows: calls `CryptGenRandom`, using the default cryptographic
+///   service provider with the `PROV_RSA_FULL` type.
 ///
 /// This does not block.
 #[cfg(unix)]
@@ -39,8 +43,12 @@ pub struct OSRng {
     priv inner: ReaderRng<file::FileStream>
 }
 /// A random number generator that retrieves randomness straight from
-/// the operating system. On Unix-like systems this reads from
-/// `/dev/urandom`, on Windows this uses `CryptGenRandom`.
+/// the operating system. Platform sources:
+///
+/// - Unix-like systems (Linux, Android, Mac OSX): read directly from
+///   `/dev/urandom`.
+/// - Windows: calls `CryptGenRandom`, using the default cryptographic
+///   service provider with the `PROV_RSA_FULL` type.
 ///
 /// This does not block.
 #[cfg(windows)]
diff --git a/src/libstd/rand/range.rs b/src/libstd/rand/range.rs
new file mode 100644
index 00000000000..1b805a0b8f7
--- /dev/null
+++ b/src/libstd/rand/range.rs
@@ -0,0 +1,235 @@
+// 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.
+
+//! Generating numbers between two others.
+
+// this is surprisingly complicated to be both generic & correct
+
+use cmp::Ord;
+use num::Bounded;
+use rand::Rng;
+use rand::distributions::{Sample, IndependentSample};
+
+/// Sample values uniformly between two bounds.
+///
+/// This gives a uniform distribution (assuming the RNG used to sample
+/// it is itself uniform & the `SampleRange` implementation for the
+/// given type is correct), even for edge cases like `low = 0u8`,
+/// `high = 170u8`, for which a naive modulo operation would return
+/// numbers less than 85 with double the probability to those greater
+/// than 85.
+///
+/// Types should attempt to sample in `[low, high)`, i.e., not
+/// including `high`, but this may be very difficult. All the
+/// primitive integer types satisfy this property, and the float types
+/// normally satisfy it, but rounding may mean `high` can occur.
+///
+/// # Example
+///
+/// ```rust
+/// use std::rand;
+/// use std::rand::distributions::{IndependentSample, Range};
+///
+/// fn main() {
+///     let between = Range::new(10u, 10000u);
+///     let rng = rand::task_rng();
+///     let mut sum = 0;
+///     for _ in range(0, 1000) {
+///         sum += between.ind_sample(rng);
+///     }
+///     println!("{}", sum);
+/// }
+/// ```
+pub struct Range<X> {
+    priv low: X,
+    priv range: X,
+    priv accept_zone: X
+}
+
+impl<X: SampleRange + Ord> Range<X> {
+    /// Create a new `Range` instance that samples uniformly from
+    /// `[low, high)`. Fails if `low >= high`.
+    pub fn new(low: X, high: X) -> Range<X> {
+        assert!(low < high, "Range::new called with `low >= high`");
+        SampleRange::construct_range(low, high)
+    }
+}
+
+impl<Sup: SampleRange> Sample<Sup> for Range<Sup> {
+    #[inline]
+    fn sample<R: Rng>(&mut self, rng: &mut R) -> Sup { self.ind_sample(rng) }
+}
+impl<Sup: SampleRange> IndependentSample<Sup> for Range<Sup> {
+    fn ind_sample<R: Rng>(&self, rng: &mut R) -> Sup {
+        SampleRange::sample_range(self, rng)
+    }
+}
+
+/// The helper trait for types that have a sensible way to sample
+/// uniformly between two values. This should not be used directly,
+/// and is only to facilitate `Range`.
+pub trait SampleRange {
+    /// Construct the `Range` object that `sample_range`
+    /// requires. This should not ever be called directly, only via
+    /// `Range::new`, which will check that `low < high`, so this
+    /// function doesn't have to repeat the check.
+    fn construct_range(low: Self, high: Self) -> Range<Self>;
+
+    /// Sample a value from the given `Range` with the given `Rng` as
+    /// a source of randomness.
+    fn sample_range<R: Rng>(r: &Range<Self>, rng: &mut R) -> Self;
+}
+
+macro_rules! integer_impl {
+    ($ty:ty, $unsigned:ty) => {
+        impl SampleRange for $ty {
+            // we play free and fast with unsigned vs signed here
+            // (when $ty is signed), but that's fine, since the
+            // contract of this macro is for $ty and $unsigned to be
+            // "bit-equal", so casting between them is a no-op & a
+            // bijection.
+
+            fn construct_range(low: $ty, high: $ty) -> Range<$ty> {
+                let range = high as $unsigned - low as $unsigned;
+                let unsigned_max: $unsigned = Bounded::max_value();
+
+                // this is the largest number that fits into $unsigned
+                // that `range` divides evenly, so, if we've sampled
+                // `n` uniformly from this region, then `n % range` is
+                // uniform in [0, range)
+                let zone = unsigned_max - unsigned_max % range;
+
+                Range {
+                    low: low,
+                    range: range as $ty,
+                    accept_zone: zone as $ty
+                }
+            }
+            #[inline]
+            fn sample_range<R: Rng>(r: &Range<$ty>, rng: &mut R) -> $ty {
+                loop {
+                    // rejection sample
+                    let v = rng.gen::<$unsigned>();
+                    // until we find something that fits into the
+                    // region which r.range evenly divides (this will
+                    // be uniformly distributed)
+                    if v < r.accept_zone as $unsigned {
+                        // and return it, with some adjustments
+                        return r.low + (v % r.range as $unsigned) as $ty;
+                    }
+                }
+            }
+        }
+    }
+}
+
+integer_impl! { i8, u8 }
+integer_impl! { i16, u16 }
+integer_impl! { i32, u32 }
+integer_impl! { i64, u64 }
+integer_impl! { int, uint }
+integer_impl! { u8, u8 }
+integer_impl! { u16, u16 }
+integer_impl! { u32, u32 }
+integer_impl! { u64, u64 }
+integer_impl! { uint, uint }
+
+macro_rules! float_impl {
+    ($ty:ty) => {
+        impl SampleRange for $ty {
+            fn construct_range(low: $ty, high: $ty) -> Range<$ty> {
+                Range {
+                    low: low,
+                    range: high - low,
+                    accept_zone: 0.0 // unused
+                }
+            }
+            fn sample_range<R: Rng>(r: &Range<$ty>, rng: &mut R) -> $ty {
+                r.low + r.range * rng.gen()
+            }
+        }
+    }
+}
+
+float_impl! { f32 }
+float_impl! { f64 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rand::*;
+    use num::Bounded;
+    use iter::range;
+    use option::{Some, None};
+    use vec::ImmutableVector;
+
+    #[should_fail]
+    #[test]
+    fn test_range_bad_limits_equal() {
+        Range::new(10, 10);
+    }
+    #[should_fail]
+    #[test]
+    fn test_range_bad_limits_flipped() {
+        Range::new(10, 5);
+    }
+
+    #[test]
+    fn test_integers() {
+        let rng = task_rng();
+        macro_rules! t (
+            ($($ty:ty),*) => {{
+                $(
+                   let v: &[($ty, $ty)] = [(0, 10),
+                                           (10, 127),
+                                           (Bounded::min_value(), Bounded::max_value())];
+                   for &(low, high) in v.iter() {
+                        let mut sampler: Range<$ty> = Range::new(low, high);
+                        for _ in range(0, 1000) {
+                            let v = sampler.sample(rng);
+                            assert!(low <= v && v < high);
+                            let v = sampler.ind_sample(rng);
+                            assert!(low <= v && v < high);
+                        }
+                    }
+                 )*
+            }}
+        );
+        t!(i8, i16, i32, i64, int,
+           u8, u16, u32, u64, uint)
+    }
+
+    #[test]
+    fn test_floats() {
+        let rng = task_rng();
+        macro_rules! t (
+            ($($ty:ty),*) => {{
+                $(
+                   let v: &[($ty, $ty)] = [(0.0, 100.0),
+                                           (-1e35, -1e25),
+                                           (1e-35, 1e-25),
+                                           (-1e35, 1e35)];
+                   for &(low, high) in v.iter() {
+                        let mut sampler: Range<$ty> = Range::new(low, high);
+                        for _ in range(0, 1000) {
+                            let v = sampler.sample(rng);
+                            assert!(low <= v && v < high);
+                            let v = sampler.ind_sample(rng);
+                            assert!(low <= v && v < high);
+                        }
+                    }
+                 )*
+            }}
+        );
+
+        t!(f32, f64)
+    }
+
+}
diff --git a/src/libstd/rt/comm.rs b/src/libstd/rt/comm.rs
index 0d4271a33c2..6319fdead17 100644
--- a/src/libstd/rt/comm.rs
+++ b/src/libstd/rt/comm.rs
@@ -1117,7 +1117,7 @@ mod test {
             let total = stress_factor() + 10;
             let mut rng = rand::rng();
             do total.times {
-                let msgs = rng.gen_integer_range(0u, 10);
+                let msgs = rng.gen_range(0u, 10);
                 let pipe_clone = pipe.clone();
                 let end_chan_clone = end_chan.clone();
                 do spawntask_random {
diff --git a/src/libstd/rt/sched.rs b/src/libstd/rt/sched.rs
index 48cd7987507..ee163bab3c0 100644
--- a/src/libstd/rt/sched.rs
+++ b/src/libstd/rt/sched.rs
@@ -431,7 +431,7 @@ impl Scheduler {
     fn try_steals(&mut self) -> Option<~Task> {
         let work_queues = &mut self.work_queues;
         let len = work_queues.len();
-        let start_index = self.rng.gen_integer_range(0, len);
+        let start_index = self.rng.gen_range(0, len);
         for index in range(0, len).map(|i| (i + start_index) % len) {
             match work_queues[index].steal() {
                 Some(task) => {