make_digits without allocation (#684)

ec/src/scalar_mul/variable_base/mod.rs

@@ -246,7 +246,7 @@ fn msm_bigint<V: VariableBaseMSM>(
 }
 
 // From: https://github.com/arkworks-rs/gemini/blob/main/src/kzg/msm/variable_base.rs#L20
-fn make_digits(a: &impl BigInteger, w: usize, num_bits: usize) -> Vec<i64> {
+fn make_digits(a: &impl BigInteger, w: usize, num_bits: usize) -> impl Iterator<Item = i64> + '_ {
     let scalar = a.as_ref();
     let radix: u64 = 1 << w;
     let window_mask: u64 = radix - 1;
@@ -258,8 +258,8 @@ fn make_digits(a: &impl BigInteger, w: usize, num_bits: usize) -> Vec<i64> {
         num_bits
     };
     let digits_count = (num_bits + w - 1) / w;
-    let mut digits = vec![0i64; digits_count];
-    for (i, digit) in digits.iter_mut().enumerate() {
+
+    (0..digits_count).into_iter().map(move |i| {
         // Construct a buffer of bits of the scalar, starting at `bit_offset`.
         let bit_offset = i * w;
         let u64_idx = bit_offset / 64;
@@ -279,10 +279,11 @@ fn make_digits(a: &impl BigInteger, w: usize, num_bits: usize) -> Vec<i64> {
 
         // Recenter coefficients from [0,2^w) to [-2^w/2, 2^w/2)
         carry = (coef + radix / 2) >> w;
-        *digit = (coef as i64) - (carry << w) as i64;
-    }
+        let mut digit = (coef as i64) - (carry << w) as i64;
 
-    digits[digits_count - 1] += (carry << w) as i64;
-
-    digits
+        if i == digits_count - 1 {
+            digit += (carry << w) as i64;
+        }
+        digit
+    })
 }