Add FixedInteger trait

src/traits.rs

@@ -32,7 +32,7 @@ pub trait Integer:
     + for<'a> CheckedSub<&'a Self, Output = Self>
     + for<'a> CheckedMul<&'a Self, Output = Self>
     + Copy
-    + ConditionallySelectable
+    // + ConditionallySelectable (see dalek-cryptography/subtle#94)
     + ConstantTimeEq
     + ConstantTimeGreater
     + ConstantTimeLess
@@ -71,7 +71,12 @@ pub trait Integer:
     /// # Returns
     ///
     /// If odd, returns `Choice(1)`. Otherwise, returns `Choice(0)`.
-    fn is_odd(&self) -> Choice;
+    fn is_odd(&self) -> Choice {
+        self.as_ref()
+            .first()
+            .map(|limb| limb.is_odd())
+            .unwrap_or_else(|| Choice::from(0))
+    }
 
     /// Is this integer value an even number?
     ///
@@ -83,6 +88,30 @@ pub trait Integer:
     }
 }
 
+/// Fixed-width integers.
+pub trait FixedInteger: Bounded + Constants + ConditionallySelectable + Integer {
+    /// The number of limbs used on this platform.
+    const LIMBS: usize;
+}
+
+impl<T: FixedInteger> Integer for T {
+    fn one() -> Self {
+        T::ONE
+    }
+
+    fn bits_precision(&self) -> usize {
+        T::BITS
+    }
+
+    fn bytes_precision(&self) -> usize {
+        T::BYTES
+    }
+
+    fn nlimbs(&self) -> usize {
+        T::LIMBS
+    }
+}
+
 /// Zero values.
 pub trait Zero: ConstantTimeEq + Sized {
     /// The value `0`.
@@ -121,12 +150,6 @@ pub trait Constants: ZeroConstant {
     const MAX: Self;
 }
 
-/// Constant representing the number of limbs used to represent a type.
-pub trait LimbsConstant {
-    /// The number of limbs used on this platform.
-    const LIMBS: usize;
-}
-
 /// Random number generation support.
 #[cfg(feature = "rand_core")]
 pub trait Random: Sized {

src/uint.rs

@@ -38,7 +38,7 @@ pub(crate) mod boxed;
 #[cfg(feature = "rand_core")]
 mod rand;
 
-use crate::{Bounded, Constants, Encoding, Integer, Limb, LimbsConstant, Word, ZeroConstant};
+use crate::{Bounded, Constants, Encoding, FixedInteger, Limb, Word, ZeroConstant};
 use core::fmt;
 use subtle::{Choice, ConditionallySelectable};
 
@@ -209,29 +209,8 @@ impl<const LIMBS: usize> Default for Uint<LIMBS> {
     }
 }
 
-impl<const LIMBS: usize> Integer for Uint<LIMBS> {
-    fn one() -> Self {
-        Self::ONE
-    }
-
-    fn bits_precision(&self) -> usize {
-        Self::BITS
-    }
-
-    fn bytes_precision(&self) -> usize {
-        Self::BYTES
-    }
-
-    fn nlimbs(&self) -> usize {
-        Self::LIMBS
-    }
-
-    fn is_odd(&self) -> Choice {
-        self.limbs
-            .first()
-            .map(|limb| limb.is_odd())
-            .unwrap_or_else(|| Choice::from(0))
-    }
+impl<const LIMBS: usize> FixedInteger for Uint<LIMBS> {
+    const LIMBS: usize = LIMBS;
 }
 
 impl<const LIMBS: usize> Bounded for Uint<LIMBS> {
@@ -244,10 +223,6 @@ impl<const LIMBS: usize> Constants for Uint<LIMBS> {
     const MAX: Self = Self::MAX;
 }
 
-impl<const LIMBS: usize> LimbsConstant for Uint<LIMBS> {
-    const LIMBS: usize = Self::LIMBS;
-}
-
 impl<const LIMBS: usize> ZeroConstant for Uint<LIMBS> {
     const ZERO: Self = Self::ZERO;
 }