Black-magic alternative to Range supporting custom parameters.

Props to @sgrif or this probably wouldn't have happened.
rust-lang/rust#20041 (comment)

src/distributions/mod.rs

@@ -30,6 +30,7 @@ pub use self::exponential::Exp;
 mod default;
 mod uniform;
 pub mod range;
+pub mod range2;
 pub mod gamma;
 pub mod normal;
 pub mod exponential;

src/distributions/range2.rs

@@ -0,0 +1,360 @@
+// Copyright 2017 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.
+
+//! Alternative design for `Range`.
+//! 
+//! TODO: decide whether to replace the old `range` module with this.
+//! Advantage: float ranges don't have to store a "zone" parameter.
+//! Advantage: custom implementations can store extra parameters.
+//! Possible advantage: easier implementations for custom types written in
+//! terms of implementations of other types.
+//! Disadvantage: complex?
+//! 
+//! This is *almost* like having separate `RangeInt<T>`, `RangeFloat<T>`,
+//! etc. (or just `RangeI32`, etc.) types, each implementing `Distribution`,
+//! but it adds some magic to support generic `range` and `new_range` methods.
+
+use std::num::Wrapping as w;
+
+use Rng;
+use distributions::{Distribution, Uniform01, Rand};
+
+/// Generate a random value in the range [`low`, `high`).
+///
+/// This is a convenience wrapper around `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.
+///
+/// # Panics
+///
+/// Panics if `low >= high`.
+///
+/// # Example
+///
+/// ```rust
+/// use rand::distributions::range2::range;
+///
+/// let mut rng = rand::thread_rng();
+/// let n: u32 = range(0, 10, &mut rng);
+/// println!("{}", n);
+/// let m: f64 = range(-40.0f64, 1.3e5f64, &mut rng);
+/// println!("{}", m);
+/// ```
+pub fn range<X: SampleRange, R: Rng+?Sized>(low: X, high: X, rng: &mut R) -> X {
+    assert!(low < high, "distributions::range called with low >= high");
+    Range { inner: X::T::new(low, high) }.sample(rng)
+}
+
+/// Convenience function to construct a `Range`
+pub fn new_range<X: SampleRange>(low: X, high: X) -> Range<X::T> {
+    assert!(low < high, "new_range called with `low >= high`");
+    Range { inner: RangeImpl::new(low, high) }
+}
+
+/// Sample values uniformly between two bounds.
+///
+/// This gives a uniform distribution (assuming the RNG used to sample
+/// it is itself uniform & the `RangeImpl` implementation 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 rand::distributions::Distribution;
+/// use rand::distributions::range2::new_range;
+///
+/// fn main() {
+///     let between = new_range(10, 10000);
+///     let mut rng = rand::thread_rng();
+///     let mut sum = 0;
+///     for _ in 0..1000 {
+///         sum += between.sample(&mut rng);
+///     }
+///     println!("{}", sum);
+/// }
+/// ```
+#[derive(Clone, Copy, Debug)]
+pub struct Range<T: RangeImpl> {
+    inner: T,
+}
+
+impl<T: RangeImpl> Range<T> {
+    /// Create a new `Range` instance that samples uniformly from
+    /// `[low, high)`. Panics if `low >= high`.
+    pub fn new(low: T::X, high: T::X) -> Range<T> {
+        assert!(low < high, "Range::new called with `low >= high`");
+        Range { inner: RangeImpl::new(low, high) }
+    }
+}
+
+impl<T: RangeImpl> Distribution<T::X> for Range<T> {
+    fn sample<R: Rng+?Sized>(&self, rng: &mut R) -> T::X {
+        self.inner.sample(rng)
+    }
+}
+
+/// Helper trait for creating implementations of `RangeImpl`.
+pub trait SampleRange: PartialOrd+Sized {
+    type T: RangeImpl<X = Self>;
+}
+
+/// Helper trait handling actual range sampling.
+pub trait RangeImpl {
+    /// The type sampled by this implementation.
+    type X: PartialOrd;
+
+    /// Construct self.
+    /// 
+    /// This should not be called directly. `Range::new` asserts that
+    /// `low < high` before calling this.
+    fn new(low: Self::X, high: Self::X) -> Self;
+
+    /// Sample a value.
+    fn sample<R: Rng+?Sized>(&self, rng: &mut R) -> Self::X;
+}
+
+/// Implementation of `RangeImpl` for integer types.
+#[derive(Clone, Copy, Debug)]
+pub struct RangeInt<X> {
+    low: X,
+    range: X,
+    zone: X,
+}
+
+macro_rules! range_int_impl {
+    ($ty:ty, $unsigned:ident) => {
+        impl SampleRange for $ty {
+            type T = RangeInt<$ty>;
+        }
+
+        impl RangeImpl for RangeInt<$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.
+
+            type X = $ty;
+
+            fn new(low: Self::X, high: Self::X) -> Self {
+                let range = (w(high as $unsigned) - w(low as $unsigned)).0;
+                let unsigned_max: $unsigned = ::std::$unsigned::MAX;
+
+                // 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;
+
+                RangeInt {
+                    low: low,
+                    range: range as $ty,
+                    zone: zone as $ty
+                }
+            }
+
+            fn sample<R: Rng+?Sized>(&self, rng: &mut R) -> Self::X {
+                use $crate::distributions::uniform;
+                loop {
+                    // rejection sample
+                    let v: $unsigned = uniform(rng);
+                    // until we find something that fits into the
+                    // region which self.range evenly divides (this will
+                    // be uniformly distributed)
+                    if v < self.zone as $unsigned {
+                        // and return it, with some adjustments
+                        return (w(self.low) + w((v % self.range as $unsigned) as $ty)).0;
+                    }
+                }
+            }
+        }
+    }
+}
+
+range_int_impl! { i8, u8 }
+range_int_impl! { i16, u16 }
+range_int_impl! { i32, u32 }
+range_int_impl! { i64, u64 }
+range_int_impl! { isize, usize }
+range_int_impl! { u8, u8 }
+range_int_impl! { u16, u16 }
+range_int_impl! { u32, u32 }
+range_int_impl! { u64, u64 }
+range_int_impl! { usize, usize }
+
+/// Implementation of `RangeImpl` for float types.
+#[derive(Clone, Copy, Debug)]
+pub struct RangeFloat<X> {
+    low: X,
+    range: X,
+}
+
+macro_rules! range_float_impl {
+    ($ty:ty) => {
+        impl SampleRange for $ty {
+            type T = RangeFloat<$ty>;
+        }
+
+        impl RangeImpl for RangeFloat<$ty> {
+            type X = $ty;
+
+            fn new(low: Self::X, high: Self::X) -> Self {
+                RangeFloat {
+                    low: low,
+                    range: high - low,
+                }
+            }
+
+            fn sample<R: Rng+?Sized>(&self, rng: &mut R) -> Self::X {
+                let x: $ty = Rand::rand(rng, Uniform01);
+                self.low + self.range * x
+            }
+        }
+    }
+}
+
+range_float_impl! { f32 }
+range_float_impl! { f64 }
+
+#[cfg(test)]
+mod tests {
+    use {Rng, thread_rng};
+    use distributions::Rand;
+    use distributions::range2::{Range, range, new_range, RangeImpl, RangeFloat};
+
+    #[test]
+    fn test_fn_range() {
+        let mut r = thread_rng();
+        for _ in 0..1000 {
+            let a = range(-3, 42, &mut r);
+            assert!(a >= -3 && a < 42);
+            assert_eq!(range(0, 1, &mut r), 0);
+            assert_eq!(range(-12, -11, &mut r), -12);
+        }
+
+        for _ in 0..1000 {
+            let a = range(10, 42, &mut r);
+            assert!(a >= 10 && a < 42);
+            assert_eq!(range(0, 1, &mut r), 0);
+            assert_eq!(range(3_000_000, 3_000_001, &mut r), 3_000_000);
+        }
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_fn_range_panic_int() {
+        let mut r = thread_rng();
+        range(5, -2, &mut r);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_fn_range_panic_usize() {
+        let mut r = thread_rng();
+        range(5, 2, &mut r);
+    }
+
+    #[should_panic]
+    #[test]
+    fn test_range_bad_limits_equal() {
+        new_range(10, 10);
+    }
+    #[should_panic]
+    #[test]
+    fn test_range_bad_limits_flipped() {
+        new_range(10, 5);
+    }
+
+    #[test]
+    fn test_integers() {
+        let mut rng = ::test::rng();
+        macro_rules! t {
+            ($($ty:ident),*) => {{
+                $(
+                   let v: &[($ty, $ty)] = &[(0, 10),
+                                            (10, 127),
+                                            (::std::$ty::MIN, ::std::$ty::MAX)];
+                   for &(low, high) in v.iter() {
+                        let range = new_range(low, high);
+                        for _ in 0..1000 {
+                            let v: $ty = Rand::rand(&mut rng, range);
+                            assert!(low <= v && v < high);
+                        }
+                    }
+                 )*
+            }}
+        }
+        t!(i8, i16, i32, i64, isize,
+           u8, u16, u32, u64, usize)
+    }
+
+    #[test]
+    fn test_floats() {
+        let mut rng = ::test::rng();
+        macro_rules! t {
+            ($($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 range = new_range(low, high);
+                        for _ in 0..1000 {
+                            let v: $ty = Rand::rand(&mut rng, range);
+                            assert!(low <= v && v < high);
+                        }
+                    }
+                 )*
+            }}
+        }
+
+        t!(f32, f64)
+    }
+
+    #[test]
+    fn test_custom_range() {
+        #[derive(Clone, Copy, PartialEq, PartialOrd)]
+        struct MyF32 {
+            x: f32,
+        }
+        #[derive(Clone, Copy, Debug)]
+        struct RangeMyF32 {
+            inner: RangeFloat<f32>,
+        }
+        impl RangeImpl for RangeMyF32 {
+            type X = MyF32;
+            fn new(low: Self::X, high: Self::X) -> Self {
+                RangeMyF32 {
+                    inner: RangeFloat::<f32>::new(low.x, high.x),
+                }
+            }
+            fn sample<R: Rng+?Sized>(&self, rng: &mut R) -> Self::X {
+                MyF32 { x: self.inner.sample(rng) }
+            }
+        }
+
+        let (low, high) = (MyF32{ x: 17.0f32 }, MyF32{ x: 22.0f32 });
+        let range = Range::<RangeMyF32>::new(low, high);
+        let mut rng = ::test::rng();
+        for _ in 0..100 {
+            let x = MyF32::rand(&mut rng, range);
+            assert!(low <= x && x < high);
+        }
+    }
+}