//! This module defines traits that a step function must implement
use bellperson::{gadgets::num::AllocatedNum, ConstraintSystem, SynthesisError};
use core::marker::PhantomData;
use ff::PrimeField;

/// A helper trait for a step of the incremental computation (i.e., circuit for F)
pub trait StepCircuit<F: PrimeField>: Send + Sync + Clone {
  /// Sythesize the circuit for a computation step and return variable
  /// that corresponds to the output of the step z_{i+1}
  fn synthesize<CS: ConstraintSystem<F>>(
    &self,
    cs: &mut CS,
    z: AllocatedNum<F>,
  ) -> Result<AllocatedNum<F>, SynthesisError>;

  /// Execute the circuit for a computation step and return output
  fn compute(&self, z: &F) -> F;
}

/// A trivial step circuit that simply returns the input
#[derive(Clone, Debug, Default)]
pub struct TrivialTestCircuit<F: PrimeField> {
  _p: PhantomData<F>,
}

impl<F> StepCircuit<F> for TrivialTestCircuit<F>
where
  F: PrimeField,
{
  fn synthesize<CS: ConstraintSystem<F>>(
    &self,
    _cs: &mut CS,
    z: AllocatedNum<F>,
  ) -> Result<AllocatedNum<F>, SynthesisError> {
    Ok(z)
  }

  fn compute(&self, z: &F) -> F {
    *z
  }
}