Use Euclid to compute the inverse faster

lambdaclass · Jan 9, 2023 · 30c5148 · 30c5148 · github-actions · Jan 9, 2023
1 parent 3d1e376
commit 30c5148
Showing 1 changed file with 21 additions and 9 deletions.
diff --git a/felt/src/bigint_felt.rs b/felt/src/bigint_felt.rs
@@ -423,27 +423,39 @@ impl<'a> Pow<u32> for &'a FeltBigInt {
 impl Div for FeltBigInt {
     type Output = Self;
     fn div(self, rhs: Self) -> Self::Output {
-        let mut x = rhs.0.modpow(&(&*CAIRO_PRIME - 2usize), &CAIRO_PRIME);
-        x *= self.0;
-        FeltBigInt::from(x)
+        let x = rhs
+            .0
+            .to_bigint()
+            .unwrap()
+            .extended_gcd(&CAIRO_SIGNED_PRIME)
+            .x;
+        self * &FeltBigInt::from(x)
     }
 }
 
 impl<'a> Div for &'a FeltBigInt {
     type Output = FeltBigInt;
     fn div(self, rhs: Self) -> Self::Output {
-        let mut x = rhs.0.modpow(&(&*CAIRO_PRIME - 2usize), &CAIRO_PRIME);
-        x *= &self.0;
-        FeltBigInt::from(x)
+        let x = rhs
+            .0
+            .to_bigint()
+            .unwrap()
+            .extended_gcd(&CAIRO_SIGNED_PRIME)
+            .x;
+        self * &FeltBigInt::from(x)
     }
 }
 
 impl<'a> Div<FeltBigInt> for &'a FeltBigInt {
     type Output = FeltBigInt;
     fn div(self, rhs: FeltBigInt) -> Self::Output {
-        let mut x = rhs.0.modpow(&(&*CAIRO_PRIME - 2usize), &CAIRO_PRIME);
-        x *= &self.0;
-        FeltBigInt::from(x)
+        let x = rhs
+            .0
+            .to_bigint()
+            .unwrap()
+            .extended_gcd(&CAIRO_SIGNED_PRIME)
+            .x;
+        self * &FeltBigInt::from(x)
     }
 }