Convert i128s to SignedWideWord

Author: tarcieri

src/limb.rs

@@ -45,6 +45,10 @@ pub type WideWord = u64;
 #[cfg(target_pointer_width = "32")]
 pub(crate) type SignedWord = i32;
 
+/// Signed equivalent of a wide word.
+#[cfg(target_pointer_width = "32")]
+pub(crate) type SignedWideWord = i64;
+
 //
 // 64-bit definitions
 //
@@ -61,6 +65,10 @@ pub type WideWord = u128;
 #[cfg(target_pointer_width = "64")]
 pub(crate) type SignedWord = i64;
 
+/// Signed equivalent of a wide word.
+#[cfg(target_pointer_width = "64")]
+pub(crate) type SignedWideWord = i128;
+
 /// Highest bit in a [`Limb`].
 pub(crate) const HI_BIT: usize = Limb::BITS - 1;
 

src/modular/bernstein_yang.rs

@@ -6,9 +6,9 @@
 //!
 //! Copyright (c) 2023 Privacy Scaling Explorations Team
 
-#![allow(clippy::needless_range_loop)]
+#![allow(clippy::needless_range_loop, trivial_numeric_casts)]
 
-use crate::limb::{Limb, SignedWord, WideWord, Word};
+use crate::limb::{Limb, SignedWideWord, SignedWord, WideWord, Word};
 use core::{
     cmp::PartialEq,
     ops::{Add, Mul, Neg, Sub},
@@ -95,7 +95,7 @@ impl<const L: usize> BernsteinYangInverter<L> {
         let (mut steps, mut f, mut g) = (
             BITS as SignedWord,
             f.lowest() as SignedWord,
-            g.lowest() as i128,
+            g.lowest() as SignedWideWord,
         );
         let mut t: Matrix = [[1, 0], [0, 1]];
 
@@ -108,7 +108,7 @@ impl<const L: usize> BernsteinYangInverter<L> {
                 break;
             }
             if delta > 0 {
-                (delta, f, g) = (-delta, g as SignedWord, -f as i128);
+                (delta, f, g) = (-delta, g as SignedWord, -f as SignedWideWord);
                 (t[0], t[1]) = (t[1], [-t[0][0], -t[0][1]]);
             }
 
@@ -119,7 +119,7 @@ impl<const L: usize> BernsteinYangInverter<L> {
             let w = (g as SignedWord).wrapping_mul(f.wrapping_mul(3) ^ 28) & mask;
 
             t[1] = [t[0][0] * w + t[1][0], t[0][1] * w + t[1][1]];
-            g += w as i128 * f as i128;
+            g += w as SignedWideWord * f as SignedWideWord;
         }
 
         (delta, t)