replace double logic

zcash · Jan 28, 2023 · 3036891 · 3036891
1 parent 9578fc8
commit 3036891
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Showing 2 changed files with 16 additions and 6 deletions.
diff --git a/src/fields/fp.rs b/src/fields/fp.rs
@@ -11,7 +11,7 @@ use lazy_static::lazy_static;
 #[cfg(feature = "bits")]
 use ff::{FieldBits, PrimeFieldBits};
 
-use crate::arithmetic::{adc, mac, sbb, SqrtTableHelpers};
+use crate::arithmetic::{adc, mac, sbb, shl1, SqrtTableHelpers};
 
 #[cfg(feature = "sqrt-table")]
 use crate::arithmetic::SqrtTables;
@@ -275,8 +275,12 @@ impl Fp {
     /// Doubles this field element.
     #[inline]
     pub const fn double(&self) -> Fp {
-        // TODO: This can be achieved more efficiently with a bitshift.
-        self.add(self)
+        let (d0, c) = shl1(self.0[0], 0);
+        let (d1, c) = shl1(self.0[1], c);
+        let (d2, c) = shl1(self.0[2], c);
+        let (d3, _) = shl1(self.0[3], c);
+
+        (&Fp([d0, d1, d2, d3])).sub(&MODULUS)
     }
 
     fn from_u512(limbs: [u64; 8]) -> Fp {

diff --git a/src/fields/fq.rs b/src/fields/fq.rs
@@ -11,7 +11,7 @@ use lazy_static::lazy_static;
 #[cfg(feature = "bits")]
 use ff::{FieldBits, PrimeFieldBits};
 
-use crate::arithmetic::{adc, mac, sbb, SqrtTableHelpers};
+use crate::arithmetic::{adc, mac, sbb, shl1, SqrtTableHelpers};
 
 #[cfg(feature = "sqrt-table")]
 use crate::arithmetic::SqrtTables;
@@ -275,8 +275,14 @@ impl Fq {
     /// Doubles this field element.
     #[inline]
     pub const fn double(&self) -> Fq {
-        // TODO: This can be achieved more efficiently with a bitshift.
-        self.add(self)
+        let (d0, carry) = shl1(self.0[0], 0);
+        let (d1, carry) = shl1(self.0[1], carry);
+        let (d2, carry) = shl1(self.0[2], carry);
+        let (d3, _) = shl1(self.0[3], carry);
+
+        // Attempt to subtract the modulus, to ensure the value
+        // is smaller than the modulus.
+        (&Fq([d0, d1, d2, d3])).sub(&MODULUS)
     }
 
     fn from_u512(limbs: [u64; 8]) -> Fq {