Add extract() to SinsemillaInstructions

src/circuit/gadget/sinsemilla.rs

@@ -16,7 +16,9 @@ pub trait SinsemillaInstructions<C: CurveAffine> {
     /// A cell in the circuit
     type CellValue;
 
-    /// A point output of [ hash_to_point`].
+    /// The x-coordinate of a point output of [`hash_to_point`].
+    type X;
+    /// A point output of [`hash_to_point`].
     type Point: Clone + Debug;
     /// The fixed points used in `CommitDomains`.
     type FixedPoints: Clone + Debug;
@@ -61,6 +63,9 @@ pub trait SinsemillaInstructions<C: CurveAffine> {
         Q: C,
         message: Vec<Self::MessagePiece>,
     ) -> Result<Self::Point, Error>;
+
+    /// Extracts the x-coordinate of the output of a Sinsemilla hash.
+    fn extract(point: &Self::Point) -> &Self::X;
 }
 
 /// A message to be hashed.

src/circuit/gadget/sinsemilla/chip.rs

@@ -190,6 +190,7 @@ impl<C: CurveAffine> SinsemillaInstructions<C> for SinsemillaChip<C> {
     type MessagePiece = MessagePiece<C::Base>;
     type CellValue = CellValue<C::Base>;
 
+    type X = CellValue<C::Base>;
     type Point = EccPoint<C>;
     type FixedPoints = OrchardFixedBasesFull<C>;
 
@@ -345,6 +346,10 @@ impl<C: CurveAffine> SinsemillaInstructions<C> for SinsemillaChip<C> {
             |mut region| hash_to_point::hash_message::<C>(&mut region, config.clone(), Q, &message),
         )
     }
+
+    fn extract(point: &Self::Point) -> &Self::X {
+        &point.x
+    }
 }
 
 #[derive(Clone, Debug)]