bigint::monty: use mac_digit

bigint/src/algorithms.rs

@@ -220,7 +220,7 @@ pub fn sub_sign(a: &[BigDigit], b: &[BigDigit]) -> (Sign, BigUint) {
 
 /// Three argument multiply accumulate:
 /// acc += b * c
-fn mac_digit(acc: &mut [BigDigit], b: &[BigDigit], c: BigDigit) {
+pub fn mac_digit(acc: &mut [BigDigit], b: &[BigDigit], c: BigDigit) {
     if c == 0 {
         return;
     }

bigint/src/monty.rs

@@ -73,27 +73,11 @@ fn monty_redc(a: BigUint, mr: &MontyReducer) -> BigUint {
 
     // 1: for i = 0 to (n-1)
     for i in 0..n_size {
-        // Carry storage
-        let mut carry = 0;
-
         // 2: q_i <- mu*c_i mod β
         let q_i = ((c[i] as u64) * mu) & beta_mask;
 
         // 3: C <- C + q_i * N * β^i
-        // When iterating over each word, this becomes:
+        super::algorithms::mac_digit(&mut c[i..], n, q_i as u32);
-        for j in 0..n_size {
-            // c_(i+j) <- c_(i+j) + q_i * n_j
-            let x = (c[i+j] as u64) + q_i * (n[j] as u64) + carry;
-            c[i+j] = (x & beta_mask) as u32;
-            carry = x >> 32;
-        }
-
-        // Apply the remaining carry to the rest of the work space
-        for j in n_size..2*n_size-i+2 {
-            let x = (c[i+j] as u64) + carry;
-            c[i+j] = (x & beta_mask) as u32;
-            carry = x >> 32;
-        }
     }
 
     // 4: R <- C * β^(-n)