Add Uint::bit_ct

README.md

@@ -153,7 +153,7 @@ named feature flag.
 * [`bn-rs`](https://docs.rs/bn-rs/latest/bn_rs/): Implements conversion to/from the [`BN`](https://docs.rs/bn-rs/latest/bn_rs/struct.BN.html) and [`BigNumber`](https://docs.rs/bn-rs/latest/bn_rs/struct.BigNumber.html).
 * [`bytemuck`](https://docs.rs/bytemuck): Implements the [`Pod`](https://docs.rs/bytemuck/latest/bytemuck/trait.Pod.html) and [`Zeroable`](https://docs.rs/bytemuck/latest/bytemuck/trait.Zeroable.html) traits for [`Uint`] where the size is a multiple of 64, up to 1024. This allows `Uint` to be used where a `Pod` trait bound exists.
 * [`num-traits`](https://docs.rs/num-traits): Implements about forty applicable traits.
-* [`subtle`](https://docs.rs/subtle): Implements [`ConditionallySelectable`](https://docs.rs/subtle/latest/subtle/trait.ConditionallySelectable.html),[`ConditionallyNegatable`](https://docs.rs/subtle/latest/subtle/trait.ConditionallyNegatable.html), [`ConstantTimeEq`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeEq.html)/[`ConstantTimeGreater`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeGreater.html)/[`ConstantTimeLess`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeLess.html).
+* [`subtle`](https://docs.rs/subtle): Implements [`Uint::bit_ct`], [`ConditionallySelectable`](https://docs.rs/subtle/latest/subtle/trait.ConditionallySelectable.html),[`ConditionallyNegatable`](https://docs.rs/subtle/latest/subtle/trait.ConditionallyNegatable.html), [`ConstantTimeEq`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeEq.html)/[`ConstantTimeGreater`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeGreater.html)/[`ConstantTimeLess`](https://docs.rs/subtle/latest/subtle/trait.ConstantTimeLess.html).
 * [`der`](https://docs.rs/der): Implements [`Encode`](https://docs.rs/der/latest/der/trait.Encode.html)/[`Decode`](https://docs.rs/der/latest/der/trait.Decode.html) and [`TryFrom`]/[`From`] casting for [`Any`](https://docs.rs/der/latest/der/asn1/struct.Any.html), [`AnyRef`](https://docs.rs/der/latest/der/asn1/struct.AnyRef.html), [`Int`](https://docs.rs/der/latest/der/asn1/struct.Int.html), [`IntRef`](https://docs.rs/der/latest/der/asn1/struct.IntRef.html), [`Uint`](https://docs.rs/der/latest/der/asn1/struct.Uint.html), [`UintRef`](https://docs.rs/der/latest/der/asn1/struct.UintRef.html).
 
 ## Building and testing

src/support/subtle.rs

@@ -8,6 +8,22 @@ use subtle::{
     Choice, ConditionallySelectable, ConstantTimeEq, ConstantTimeGreater, ConstantTimeLess,
 };
 
+impl<const BITS: usize, const LIMBS: usize> Uint<BITS, LIMBS> {
+    /// Returns a [`Choice`] if the bit at index is set.
+    ///
+    /// Constant time version of [`bit`]
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index >= Self::BITS`.
+    #[must_use]
+    pub fn bit_ct(&self, index: usize) -> Choice {
+        assert!(index < BITS);
+        let (limbs, bits) = (index / 64, index % 64);
+        (self.limbs[limbs] & (1 << bits)).ct_eq(&(1 << bits))
+    }
+}
+
 impl<const BITS: usize, const LIMBS: usize> ConditionallySelectable for Uint<BITS, LIMBS> {
     fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
         let mut limbs = [0_u64; LIMBS];
@@ -74,6 +90,18 @@ mod tests {
     use proptest::proptest;
     use subtle::ConditionallyNegatable;
 
+    #[test]
+    fn test_bit() {
+        const_for!(BITS in SIZES {
+            const LIMBS: usize = nlimbs(BITS);
+            proptest!(|(n: Uint<BITS, LIMBS>, i in 0..BITS)| {
+                let r = n.bit_ct(i);
+                let e = n.bit(i);
+                assert_eq!(bool::from(r), e);
+            });
+        });
+    }
+
     #[test]
     fn test_select() {
         const_for!(BITS in SIZES {