Implemented div_by_2 for BoxedResidue

src/modular/boxed_residue.rs

@@ -21,6 +21,18 @@ use std::sync::Arc;
 #[cfg(feature = "zeroize")]
 use zeroize::Zeroize;
 
+fn div_by_2(a: &BoxedUint, modulus: &BoxedUint) -> BoxedUint {
+    let (half, is_odd) = a.shr1_with_carry();
+    let half_modulus = modulus.shr1();
+
+    let if_even = half.clone();
+    let if_odd = half
+        .wrapping_add(&half_modulus)
+        .wrapping_add(&BoxedUint::one_with_precision(a.bits_precision()));
+
+    BoxedUint::conditional_select(&if_even, &if_odd, is_odd)
+}
+
 /// Parameters to efficiently go to/from the Montgomery form for an odd modulus whose size and value
 /// are both chosen at runtime.
 #[derive(Clone, Debug, Eq, PartialEq)]
@@ -159,6 +171,18 @@ impl BoxedResidue {
         }
     }
 
+    /// Performs the modular division by 2, that is for given `x` returns `y`
+    /// such that `y * 2 = x mod p`. This means:
+    /// - if `x` is even, returns `x / 2`,
+    /// - if `x` is odd, returns `(x + p) / 2`
+    ///   (since the modulus `p` in Montgomery form is always odd, this divides entirely).
+    pub fn div_by_2(&self) -> Self {
+        Self {
+            montgomery_form: div_by_2(&self.montgomery_form, &self.residue_params.modulus),
+            residue_params: self.residue_params.clone(),
+        }
+    }
+
     /// Instantiates a new [`BoxedResidue`] that represents an integer modulo the provided params.
     #[cfg(feature = "std")]
     pub fn new_with_arc(mut integer: BoxedUint, residue_params: Arc<BoxedResidueParams>) -> Self {