perf(crypto): Small optimizations for Lagrange interpolation

rs/crypto/internal/crypto_lib/bls12_381/type/src/interpolation.rs

@@ -120,9 +120,10 @@ impl LagrangeCoefficients {
             // Compute the value at 0 of the i-th Lagrange polynomial that is `0` at the
             // other data points but `1` at `x_i`.
             let mut denom = Scalar::one();
-            let x_i = samples[i].clone();
-            for x_j in samples.iter().filter(|x_j| **x_j != x_i) {
-                denom *= x_j - &x_i;
+            for (j, x_j) in samples.iter().enumerate() {
+                if j != i {
+                    denom *= x_j - &samples[i];
+                }
             }
 
             denominator.push(denom);
@@ -152,6 +153,11 @@ impl LagrangeCoefficients {
         &self.coefficients
     }
 
+    /// Return the Lagrange coefficients, consuming self
+    pub fn into_coefficients(self) -> Vec<Scalar> {
+        self.coefficients
+    }
+
     /// Given a list of samples `(x, f(x) * g)` for a set of unique `x`, some
     /// polynomial `f`, and some integer `g`, returns `f(value) * g`.
     ///

rs/crypto/internal/crypto_lib/threshold_sig/bls12_381/src/ni_dkg/groth20_bls12_381/transcript.rs

@@ -206,9 +206,7 @@ fn compute_transcript(
             let indices = NodeIndices::from_slice(&reshare_x)
                 .expect("Cannot fail because all x are distinct.");
 
-            LagrangeCoefficients::at_zero(&indices)
-                .coefficients()
-                .to_vec()
+            LagrangeCoefficients::at_zero(&indices).into_coefficients()
         };
 
         let mut combined = vec![];

rs/crypto/internal/crypto_lib/threshold_sig/bls12_381/src/types/public_coefficients.rs

@@ -123,9 +123,7 @@ impl PublicCoefficients {
 
         let indices = NodeIndices::from_slice(samples).map_err(|_| ThresholdError::DuplicateX)?;
 
-        Ok(LagrangeCoefficients::at_zero(&indices)
-            .coefficients()
-            .to_vec())
+        Ok(LagrangeCoefficients::at_zero(&indices).into_coefficients())
     }
 }