Interpreter: add bitmask to extract bits from foreign field limbs

msm/src/serialization/interpreter.rs

@@ -13,6 +13,17 @@ pub trait InterpreterEnv {
         // TODO
     }
 
+    /// Extract the bits from the variable `x` between `highest_bit` and `lowest_bit`, and store
+    /// the result in `position`.
+    /// `lowest_bit` becomes the least-significant bit of the resulting value.
+    fn bitmask(
+        &mut self,
+        x: &Self::Variable,
+        highest_bit: u128,
+        lowest_bit: u128,
+        position: Self::Position,
+    ) -> Self::Variable;
+
     /// Deserialize the next field element given as input
     fn deserialize_field_element(&mut self);
 

msm/src/serialization/witness.rs

@@ -28,6 +28,21 @@ impl<const N: usize, Fp: Field> InterpreterEnv for Env<N, Fp> {
     fn deserialize_field_element(&mut self) {
         // TODO
     }
+
+    /// Returns the bits between [highest_bit, lowest_bit] of the variable `x`,
+    /// and copy the result in the column `position`.
+    fn bitmask(
+        &mut self,
+        x: &Self::Variable,
+        highest_bit: u128,
+        lowest_bit: u128,
+        position: Self::Position,
+    ) -> Self::Variable {
+        let x: u128 = *x;
+        let res = (x >> lowest_bit) & ((1 << (highest_bit - lowest_bit)) - 1);
+        self.write_column(position, res);
+        res
+    }
 }
 
 impl<const N: usize, Fp: Field> Env<N, Fp> {