1pub mod circom;
  2
  3use crate::Error;
  4use ark_ff::PrimeField;
  5use ark_r1cs_std::fields::fp::FpVar;
  6use ark_relations::r1cs::{ConstraintSystemRef, SynthesisError};
  7use ark_std::fmt::Debug;
  8
  9/// FCircuit defines the trait of the circuit of the F function, which is the one being folded (ie.
 10/// inside the agmented F' function).
 11/// The parameter z_i denotes the current state, and z_{i+1} denotes the next state after applying
 12/// the step.
 13pub trait FCircuit<F: PrimeField>: Clone + Copy + Debug {
 14    type Params: Debug;
 15
 16    /// returns a new FCircuit instance
 17    fn new(params: Self::Params) -> Self;
 18
 19    /// returns the number of elements in the state of the FCircuit, which corresponds to the
 20    /// FCircuit inputs.
 21    fn state_len(self) -> usize;
 22
 23    /// computes the next state values in place, assigning z_{i+1} into z_i, and computing the new
 24    /// z_{i+1}
 25    fn step_native(
 26        // this method uses self, so that each FCircuit implementation (and different frontends)
 27        // can hold a state if needed to store data to compute the next state.
 28        self,
 29        z_i: Vec<F>,
 30    ) -> Result<Vec<F>, Error>;
 31
 32    /// generates the constraints for the step of F for the given z_i
 33    fn generate_step_constraints(
 34        // this method uses self, so that each FCircuit implementation (and different frontends)
 35        // can hold a state if needed to store data to generate the constraints.
 36        self,
 37        cs: ConstraintSystemRef<F>,
 38        z_i: Vec<FpVar<F>>,
 39    ) -> Result<Vec<FpVar<F>>, SynthesisError>;
 40}
 41
 42#[cfg(test)]
 43pub mod tests {
 44    use super::*;
 45    use ark_pallas::Fr;
 46    use ark_r1cs_std::{alloc::AllocVar, eq::EqGadget};
 47    use ark_relations::r1cs::{
 48        ConstraintSynthesizer, ConstraintSystem, ConstraintSystemRef, SynthesisError,
 49    };
 50    use core::marker::PhantomData;
 51
 52    /// CubicFCircuit is a struct that implements the FCircuit trait, for the R1CS example circuit
 53    /// from https://www.vitalik.ca/general/2016/12/10/qap.html, which checks `x^3 + x + 5 = y`. It
 54    /// has 2 public inputs which are used as the state. `z_i` is used as `x`, and `z_{i+1}` is
 55    /// used as `y`, and at the next step, `z_{i+1}` will be assigned to `z_i`, and a new `z+{i+1}`
 56    /// will be computted.
 57    #[derive(Clone, Copy, Debug)]
 58    pub struct CubicFCircuit<F: PrimeField> {
 59        _f: PhantomData<F>,
 60    }
 61    impl<F: PrimeField> FCircuit<F> for CubicFCircuit<F> {
 62        type Params = ();
 63        fn new(_params: Self::Params) -> Self {
 64            Self { _f: PhantomData }
 65        }
 66        fn state_len(self) -> usize {
 67            1
 68        }
 69        fn step_native(self, z_i: Vec<F>) -> Result<Vec<F>, Error> {
 70            Ok(vec![z_i[0] * z_i[0] * z_i[0] + z_i[0] + F::from(5_u32)])
 71        }
 72        fn generate_step_constraints(
 73            self,
 74            cs: ConstraintSystemRef<F>,
 75            z_i: Vec<FpVar<F>>,
 76        ) -> Result<Vec<FpVar<F>>, SynthesisError> {
 77            let five = FpVar::<F>::new_constant(cs.clone(), F::from(5u32))?;
 78            let z_i = z_i[0].clone();
 79
 80            Ok(vec![&z_i * &z_i * &z_i + &z_i + &five])
 81        }
 82    }
 83
 84    /// CustomFCircuit is a circuit that has the number of constraints specified in the
 85    /// `n_constraints` parameter. Note that the generated circuit will have very sparse matrices.
 86    #[derive(Clone, Copy, Debug)]
 87    pub struct CustomFCircuit<F: PrimeField> {
 88        _f: PhantomData<F>,
 89        pub n_constraints: usize,
 90    }
 91    impl<F: PrimeField> FCircuit<F> for CustomFCircuit<F> {
 92        type Params = usize;
 93
 94        fn new(params: Self::Params) -> Self {
 95            Self {
 96                _f: PhantomData,
 97                n_constraints: params,
 98            }
 99        }
100        fn state_len(self) -> usize {
101            1
102        }
103        fn step_native(self, z_i: Vec<F>) -> Result<Vec<F>, Error> {
104            let mut z_i1 = F::one();
105            for _ in 0..self.n_constraints - 1 {
106                z_i1 *= z_i[0];
107            }
108            Ok(vec![z_i1])
109        }
110        fn generate_step_constraints(
111            self,
112            cs: ConstraintSystemRef<F>,
113            z_i: Vec<FpVar<F>>,
114        ) -> Result<Vec<FpVar<F>>, SynthesisError> {
115            let mut z_i1 = FpVar::<F>::new_witness(cs.clone(), || Ok(F::one()))?;
116            for _ in 0..self.n_constraints - 1 {
117                z_i1 *= z_i[0].clone();
118            }
119
120            Ok(vec![z_i1])
121        }
122    }
123
124    /// WrapperCircuit is a circuit that wraps any circuit that implements the FCircuit trait. This
125    /// is used to test the `FCircuit.generate_step_constraints` method. This is a similar wrapping
126    /// than the one done in the `AugmentedFCircuit`, but without adding all the extra constraints
127    /// of the AugmentedF circuit logic, in order to run lighter tests when we're not interested in
128    /// the the AugmentedF logic but in the wrapping of the circuits.
129    pub struct WrapperCircuit<F: PrimeField, FC: FCircuit<F>> {
130        pub FC: FC, // F circuit
131        pub z_i: Option<Vec<F>>,
132        pub z_i1: Option<Vec<F>>,
133    }
134    impl<F, FC> ConstraintSynthesizer<F> for WrapperCircuit<F, FC>
135    where
136        F: PrimeField,
137        FC: FCircuit<F>,
138    {
139        fn generate_constraints(self, cs: ConstraintSystemRef<F>) -> Result<(), SynthesisError> {
140            let z_i = Vec::<FpVar<F>>::new_witness(cs.clone(), || {
141                Ok(self.z_i.unwrap_or(vec![F::zero()]))
142            })?;
143            let z_i1 = Vec::<FpVar<F>>::new_input(cs.clone(), || {
144                Ok(self.z_i1.unwrap_or(vec![F::zero()]))
145            })?;
146            let computed_z_i1 = self.FC.generate_step_constraints(cs.clone(), z_i.clone())?;
147
148            computed_z_i1.enforce_equal(&z_i1)?;
149            Ok(())
150        }
151    }
152
153    #[test]
154    fn test_testfcircuit() {
155        let cs = ConstraintSystem::<Fr>::new_ref();
156        let F_circuit = CubicFCircuit::<Fr>::new(());
157
158        let wrapper_circuit = WrapperCircuit::<Fr, CubicFCircuit<Fr>> {
159            FC: F_circuit,
160            z_i: Some(vec![Fr::from(3_u32)]),
161            z_i1: Some(vec![Fr::from(35_u32)]),
162        };
163        wrapper_circuit.generate_constraints(cs.clone()).unwrap();
164        assert_eq!(cs.num_constraints(), 3);
165    }
166
167    #[test]
168    fn test_customtestfcircuit() {
169        let cs = ConstraintSystem::<Fr>::new_ref();
170        let n_constraints = 1000;
171        let custom_circuit = CustomFCircuit::<Fr>::new(n_constraints);
172        let z_i = vec![Fr::from(5_u32)];
173        let wrapper_circuit = WrapperCircuit::<Fr, CustomFCircuit<Fr>> {
174            FC: custom_circuit,
175            z_i: Some(z_i.clone()),
176            z_i1: Some(custom_circuit.step_native(z_i).unwrap()),
177        };
178        wrapper_circuit.generate_constraints(cs.clone()).unwrap();
179        assert_eq!(cs.num_constraints(), n_constraints);
180    }
181}