doc fix

Author: oleganza

src/util.rs

@@ -146,13 +146,13 @@ pub fn scalar_exp_vartime(x: &Scalar, mut n: u64) -> Scalar {
 /// S(0)    &= 0 & \\\\
 /// S(1)    &= 1 & \\\\
 /// S(i)    &= S(i-1) + x^{n_i} s_i b_i\\\\
-///         &= S(i) + x^{n_{i-1}} (x^{2^{i-1}})^{b_{i-1}} s_i b_i
+///         &= S(i-1) + x^{n_{i-1}} (x^{2^{i-1}})^{b_{i-1}} s_i b_i
 /// \end{aligned}
 /// \\]
 pub fn sum_of_powers(x: &Scalar, mut n: usize) -> Scalar {
     let mut result = Scalar::zero();
-    let mut f = Scalar::one(); // power of x to offset interim polynomial
-    let mut s = Scalar::one();
+    let mut f = Scalar::one(); // power of x to offset subsequent polynomials based on lower bits of n.
+    let mut s = Scalar::one(); // power-of-two polynomial: 1, 1+x, 1+x+x^2+x^3, ...
     let mut p = *x; // x, x^2, x^4, ..., x^{2^i}
     while n > 0 {
         // take a bit from n