Trap weighted index overflow (#1353)

* WeightedIndex: add test overflow (expected to panic)

* WeightedIndex::new: trap overflow in release builds only

* Introduce trait Weight

* Update regarding nightly SIMD changes

src/distributions/float.rs

@@ -12,7 +12,7 @@ use crate::distributions::utils::{IntAsSIMD, FloatAsSIMD, FloatSIMDUtils};
 use crate::distributions::{Distribution, Standard};
 use crate::Rng;
 use core::mem;
-#[cfg(feature = "simd_support")] use core::simd::*;
+#[cfg(feature = "simd_support")] use core::simd::prelude::*;
 
 #[cfg(feature = "serde1")]
 use serde::{Serialize, Deserialize};

src/distributions/mod.rs

@@ -126,7 +126,7 @@ pub use self::slice::Slice;
 #[doc(inline)]
 pub use self::uniform::Uniform;
 #[cfg(feature = "alloc")]
-pub use self::weighted_index::{WeightedError, WeightedIndex};
+pub use self::weighted_index::{Weight, WeightedError, WeightedIndex};
 
 #[allow(unused)]
 use crate::Rng;

src/distributions/other.rs

@@ -22,7 +22,9 @@ use crate::Rng;
 use serde::{Serialize, Deserialize};
 use core::mem::{self, MaybeUninit};
 #[cfg(feature = "simd_support")]
-use core::simd::*;
+use core::simd::prelude::*;
+#[cfg(feature = "simd_support")]
+use core::simd::{LaneCount, MaskElement, SupportedLaneCount};
 
 
 // ----- Sampling distributions -----
@@ -163,7 +165,7 @@ impl Distribution<bool> for Standard {
 /// Since most bits are unused you could also generate only as many bits as you need, i.e.:
 /// ```
 /// #![feature(portable_simd)]
-/// use std::simd::*;
+/// use std::simd::prelude::*;
 /// use rand::prelude::*;
 /// let mut rng = thread_rng();
 ///

src/distributions/uniform.rs

@@ -119,7 +119,8 @@ use crate::{Rng, RngCore};
 #[allow(unused_imports)] // rustc doesn't detect that this is actually used
 use crate::distributions::utils::Float;
 
-#[cfg(feature = "simd_support")] use core::simd::*;
+#[cfg(feature = "simd_support")] use core::simd::prelude::*;
+#[cfg(feature = "simd_support")] use core::simd::{LaneCount, SupportedLaneCount};
 
 /// Error type returned from [`Uniform::new`] and `new_inclusive`.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -1433,7 +1434,7 @@ mod tests {
                     (-::core::$f_scalar::MAX * 0.2, ::core::$f_scalar::MAX * 0.7),
                 ];
                 for &(low_scalar, high_scalar) in v.iter() {
-                    for lane in 0..<$ty>::LANES {
+                    for lane in 0..<$ty>::LEN {
                         let low = <$ty>::splat(0.0 as $f_scalar).replace(lane, low_scalar);
                         let high = <$ty>::splat(1.0 as $f_scalar).replace(lane, high_scalar);
                         let my_uniform = Uniform::new(low, high).unwrap();
@@ -1565,7 +1566,7 @@ mod tests {
                     (::std::$f_scalar::NEG_INFINITY, ::std::$f_scalar::INFINITY),
                 ];
                 for &(low_scalar, high_scalar) in v.iter() {
-                    for lane in 0..<$ty>::LANES {
+                    for lane in 0..<$ty>::LEN {
                         let low = <$ty>::splat(0.0 as $f_scalar).replace(lane, low_scalar);
                         let high = <$ty>::splat(1.0 as $f_scalar).replace(lane, high_scalar);
                         assert!(catch_unwind(|| range(low, high)).is_err());

src/distributions/utils.rs

@@ -8,7 +8,8 @@
 
 //! Math helper functions
 
-#[cfg(feature = "simd_support")] use core::simd::*;
+#[cfg(feature = "simd_support")] use core::simd::prelude::*;
+#[cfg(feature = "simd_support")] use core::simd::{LaneCount, SimdElement, SupportedLaneCount};
 
 
 pub(crate) trait WideningMultiply<RHS = Self> {
@@ -245,7 +246,7 @@ pub(crate) trait Float: Sized {
 
 /// Implement functions on f32/f64 to give them APIs similar to SIMD types
 pub(crate) trait FloatAsSIMD: Sized {
-    const LANES: usize = 1;
+    const LEN: usize = 1;
     #[inline(always)]
     fn splat(scalar: Self) -> Self {
         scalar

src/distributions/weighted_index.rs

@@ -99,12 +99,12 @@ impl<X: SampleUniform + PartialOrd> WeightedIndex<X> {
     where
         I: IntoIterator,
         I::Item: SampleBorrow<X>,
-        X: for<'a> ::core::ops::AddAssign<&'a X> + Clone + Default,
+        X: Weight,
     {
         let mut iter = weights.into_iter();
         let mut total_weight: X = iter.next().ok_or(WeightedError::NoItem)?.borrow().clone();
 
-        let zero = <X as Default>::default();
+        let zero = X::ZERO;
         if !(total_weight >= zero) {
             return Err(WeightedError::InvalidWeight);
         }
@@ -117,7 +117,10 @@ impl<X: SampleUniform + PartialOrd> WeightedIndex<X> {
                 return Err(WeightedError::InvalidWeight);
             }
             weights.push(total_weight.clone());
-            total_weight += w.borrow();
+
+            if let Err(()) = total_weight.checked_add_assign(w.borrow()) {
+                return Err(WeightedError::Overflow);
+            }
         }
 
         if total_weight == zero {
@@ -236,6 +239,60 @@ where X: SampleUniform + PartialOrd
     }
 }
 
+/// Bounds on a weight
+///
+/// See usage in [`WeightedIndex`].
+pub trait Weight: Clone {
+    /// Representation of 0
+    const ZERO: Self;
+
+    /// Checked addition
+    ///
+    /// -   `Result::Ok`: On success, `v` is added to `self`
+    /// -   `Result::Err`: Returns an error when `Self` cannot represent the
+    ///     result of `self + v` (i.e. overflow). The value of `self` should be
+    ///     discarded.
+    fn checked_add_assign(&mut self, v: &Self) -> Result<(), ()>;
+}
+
+macro_rules! impl_weight_int {
+    ($t:ty) => {
+        impl Weight for $t {
+            const ZERO: Self = 0;
+            fn checked_add_assign(&mut self, v: &Self) -> Result<(), ()> {
+                match self.checked_add(*v) {
+                    Some(sum) => {
+                        *self = sum;
+                        Ok(())
+                    }
+                    None => Err(()),
+                }
+            }
+        }
+    };
+    ($t:ty, $($tt:ty),*) => {
+        impl_weight_int!($t);
+        impl_weight_int!($($tt),*);
+    }
+}
+impl_weight_int!(i8, i16, i32, i64, i128, isize);
+impl_weight_int!(u8, u16, u32, u64, u128, usize);
+
+macro_rules! impl_weight_float {
+    ($t:ty) => {
+        impl Weight for $t {
+            const ZERO: Self = 0.0;
+            fn checked_add_assign(&mut self, v: &Self) -> Result<(), ()> {
+                // Floats have an explicit representation for overflow
+                *self += *v;
+                Ok(())
+            }
+        }
+    }
+}
+impl_weight_float!(f32);
+impl_weight_float!(f64);
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -388,12 +445,11 @@ mod test {
 
     #[test]
     fn value_stability() {
-        fn test_samples<X: SampleUniform + PartialOrd, I>(
+        fn test_samples<X: Weight + SampleUniform + PartialOrd, I>(
             weights: I, buf: &mut [usize], expected: &[usize],
         ) where
             I: IntoIterator,
             I::Item: SampleBorrow<X>,
-            X: for<'a> ::core::ops::AddAssign<&'a X> + Clone + Default,
         {
             assert_eq!(buf.len(), expected.len());
             let distr = WeightedIndex::new(weights).unwrap();
@@ -420,6 +476,11 @@ mod test {
     fn weighted_index_distributions_can_be_compared() {
         assert_eq!(WeightedIndex::new(&[1, 2]), WeightedIndex::new(&[1, 2]));
     }
+
+    #[test]
+    fn overflow() {
+        assert_eq!(WeightedIndex::new([2, usize::MAX]), Err(WeightedError::Overflow));
+    }
 }
 
 /// Error type returned from `WeightedIndex::new`.
@@ -438,6 +499,9 @@ pub enum WeightedError {
 
     /// Too many weights are provided (length greater than `u32::MAX`)
     TooMany,
+
+    /// The sum of weights overflows
+    Overflow,
 }
 
 #[cfg(feature = "std")]
@@ -450,6 +514,7 @@ impl fmt::Display for WeightedError {
             WeightedError::InvalidWeight => "A weight is invalid in distribution",
             WeightedError::AllWeightsZero => "All weights are zero in distribution",
             WeightedError::TooMany => "Too many weights (hit u32::MAX) in distribution",
+            WeightedError::Overflow => "The sum of weights overflowed",
         })
     }
 }

src/seq/mod.rs

@@ -40,7 +40,7 @@ use alloc::vec::Vec;
 #[cfg(feature = "alloc")]
 use crate::distributions::uniform::{SampleBorrow, SampleUniform};
 #[cfg(feature = "alloc")]
-use crate::distributions::WeightedError;
+use crate::distributions::{Weight, WeightedError};
 use crate::Rng;
 
 use self::coin_flipper::CoinFlipper;
@@ -170,11 +170,7 @@ pub trait SliceRandom {
         R: Rng + ?Sized,
         F: Fn(&Self::Item) -> B,
         B: SampleBorrow<X>,
-        X: SampleUniform
-            + for<'a> ::core::ops::AddAssign<&'a X>
-            + ::core::cmp::PartialOrd<X>
-            + Clone
-            + Default;
+        X: SampleUniform + Weight + ::core::cmp::PartialOrd<X>;
 
     /// Biased sampling for one element (mut)
     ///
@@ -203,11 +199,7 @@ pub trait SliceRandom {
         R: Rng + ?Sized,
         F: Fn(&Self::Item) -> B,
         B: SampleBorrow<X>,
-        X: SampleUniform
-            + for<'a> ::core::ops::AddAssign<&'a X>
-            + ::core::cmp::PartialOrd<X>
-            + Clone
-            + Default;
+        X: SampleUniform + Weight + ::core::cmp::PartialOrd<X>;
 
     /// Biased sampling of `amount` distinct elements
     ///
@@ -585,11 +577,7 @@ impl<T> SliceRandom for [T] {
         R: Rng + ?Sized,
         F: Fn(&Self::Item) -> B,
         B: SampleBorrow<X>,
-        X: SampleUniform
-            + for<'a> ::core::ops::AddAssign<&'a X>
-            + ::core::cmp::PartialOrd<X>
-            + Clone
-            + Default,
+        X: SampleUniform + Weight + ::core::cmp::PartialOrd<X>,
     {
         use crate::distributions::{Distribution, WeightedIndex};
         let distr = WeightedIndex::new(self.iter().map(weight))?;
@@ -604,11 +592,7 @@ impl<T> SliceRandom for [T] {
         R: Rng + ?Sized,
         F: Fn(&Self::Item) -> B,
         B: SampleBorrow<X>,
-        X: SampleUniform
-            + for<'a> ::core::ops::AddAssign<&'a X>
-            + ::core::cmp::PartialOrd<X>
-            + Clone
-            + Default,
+        X: SampleUniform + Weight + ::core::cmp::PartialOrd<X>,
     {
         use crate::distributions::{Distribution, WeightedIndex};
         let distr = WeightedIndex::new(self.iter().map(weight))?;