feat (circom): allow to define the step_native in Rust (#105)

* create function pointer

* custom logic via function pointer

* fmt

* clippy

* rust-version

* update review code

* fmt

folding-schemes/src/frontend/circom/mod.rs

@@ -10,17 +10,85 @@ use ark_relations::r1cs::{ConstraintSynthesizer, ConstraintSystemRef, SynthesisE
 use ark_std::fmt::Debug;
 use num_bigint::BigInt;
 use std::path::PathBuf;
+use std::rc::Rc;
+use std::{fmt, usize};
 
 pub mod utils;
 use utils::CircomWrapper;
 
+type ClosurePointer<F> = Rc<dyn Fn(usize, Vec<F>, Vec<F>) -> Result<Vec<F>, Error>>;
+
+#[derive(Clone)]
+struct CustomStepNative<F: PrimeField> {
+    func: ClosurePointer<F>,
+}
+
+impl<F: PrimeField> fmt::Debug for CustomStepNative<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Function pointer: {:?}",
+            std::any::type_name::<fn(usize, Vec<F>, Vec<F>) -> Result<Vec<F>, Error>>()
+        )
+    }
+}
+
 /// Define CircomFCircuit
 #[derive(Clone, Debug)]
 pub struct CircomFCircuit<F: PrimeField> {
     circom_wrapper: CircomWrapper<F>,
-    state_len: usize,
-    external_inputs_len: usize,
+    pub state_len: usize,
+    pub external_inputs_len: usize,
     r1cs: CircomR1CS<F>,
+    custom_step_native_code: Option<CustomStepNative<F>>,
+}
+
+impl<F: PrimeField> CircomFCircuit<F> {
+    pub fn set_custom_step_native(&mut self, func: ClosurePointer<F>) {
+        self.custom_step_native_code = Some(CustomStepNative::<F> { func });
+    }
+
+    pub fn execute_custom_step_native(
+        &self,
+        _i: usize,
+        z_i: Vec<F>,
+        external_inputs: Vec<F>,
+    ) -> Result<Vec<F>, Error> {
+        if let Some(code) = &self.custom_step_native_code {
+            (code.func)(_i, z_i, external_inputs)
+        } else {
+            #[cfg(test)]
+            assert_eq!(z_i.len(), self.state_len());
+            #[cfg(test)]
+            assert_eq!(external_inputs.len(), self.external_inputs_len());
+
+            let inputs_bi = z_i
+                .iter()
+                .map(|val| self.circom_wrapper.ark_primefield_to_num_bigint(*val))
+                .collect::<Vec<BigInt>>();
+            let mut inputs_map = vec![("ivc_input".to_string(), inputs_bi)];
+
+            if self.external_inputs_len() > 0 {
+                let external_inputs_bi = external_inputs
+                    .iter()
+                    .map(|val| self.circom_wrapper.ark_primefield_to_num_bigint(*val))
+                    .collect::<Vec<BigInt>>();
+                inputs_map.push(("external_inputs".to_string(), external_inputs_bi));
+            }
+
+            // Computes witness
+            let witness = self
+                .circom_wrapper
+                .extract_witness(&inputs_map)
+                .map_err(|e| {
+                    Error::WitnessCalculationError(format!("Failed to calculate witness: {}", e))
+                })?;
+
+            // Extracts the z_i1(next state) from the witness vector.
+            let z_i1 = witness[1..1 + self.state_len()].to_vec();
+            Ok(z_i1)
+        }
+    }
 }
 
 impl<F: PrimeField> FCircuit<F> for CircomFCircuit<F> {
@@ -37,6 +105,7 @@ impl<F: PrimeField> FCircuit<F> for CircomFCircuit<F> {
             state_len,
             external_inputs_len,
             r1cs,
+            custom_step_native_code: None,
         })
     }
 
@@ -53,36 +122,7 @@ impl<F: PrimeField> FCircuit<F> for CircomFCircuit<F> {
         z_i: Vec<F>,
         external_inputs: Vec<F>,
     ) -> Result<Vec<F>, Error> {
-        #[cfg(test)]
-        assert_eq!(z_i.len(), self.state_len());
-        #[cfg(test)]
-        assert_eq!(external_inputs.len(), self.external_inputs_len());
-
-        let inputs_bi = z_i
-            .iter()
-            .map(|val| self.circom_wrapper.ark_primefield_to_num_bigint(*val))
-            .collect::<Vec<BigInt>>();
-        let mut inputs_map = vec![("ivc_input".to_string(), inputs_bi)];
-
-        if self.external_inputs_len() > 0 {
-            let external_inputs_bi = external_inputs
-                .iter()
-                .map(|val| self.circom_wrapper.ark_primefield_to_num_bigint(*val))
-                .collect::<Vec<BigInt>>();
-            inputs_map.push(("external_inputs".to_string(), external_inputs_bi));
-        }
-
-        // Computes witness
-        let witness = self
-            .circom_wrapper
-            .extract_witness(&inputs_map)
-            .map_err(|e| {
-                Error::WitnessCalculationError(format!("Failed to calculate witness: {}", e))
-            })?;
-
-        // Extracts the z_i1(next state) from the witness vector.
-        let z_i1 = witness[1..1 + self.state_len()].to_vec();
-        Ok(z_i1)
+        self.execute_custom_step_native(_i, z_i, external_inputs)
     }
 
     fn generate_step_constraints(
@@ -305,4 +345,34 @@ pub mod tests {
         // Disable check for now
         // assert!(z_i1_var.is_err())
     }
+
+    #[test]
+    fn test_custom_code() {
+        let r1cs_path = PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit.r1cs");
+        let wasm_path =
+            PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit_js/cubic_circuit.wasm");
+
+        let mut circom_fcircuit = CircomFCircuit::<Fr>::new((r1cs_path, wasm_path, 1, 0)).unwrap(); // state_len:1, external_inputs_len:0
+
+        circom_fcircuit.set_custom_step_native(Rc::new(|_i, z_i, _external| {
+            let z = z_i[0];
+            Ok(vec![z * z * z + z + Fr::from(5)])
+        }));
+
+        // Allocates z_i1 by using step_native function.
+        let z_i = vec![Fr::from(3_u32)];
+        let wrapper_circuit = crate::frontend::tests::WrapperCircuit {
+            FC: circom_fcircuit.clone(),
+            z_i: Some(z_i.clone()),
+            z_i1: Some(circom_fcircuit.step_native(0, z_i.clone(), vec![]).unwrap()),
+        };
+
+        let cs = ConstraintSystem::<Fr>::new_ref();
+
+        wrapper_circuit.generate_constraints(cs.clone()).unwrap();
+        assert!(
+            cs.is_satisfied().unwrap(),
+            "Constraint system is not satisfied"
+        );
+    }
 }