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use ark_ff::Field;
use ark_relations::gr1cs::{Namespace, SynthesisError};
use ark_std::vec::Vec;
use core::borrow::Borrow;
/// Describes the mode that a variable should be allocated in within
/// a `ConstraintSystem`.
#[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Copy, Clone)]
pub enum AllocationMode {
/// Indicate to the `ConstraintSystem` that the high-level variable should
/// be allocated as a constant. That is, no `Variable`s should be
/// generated.
Constant = 0,
/// Indicate to the `ConstraintSystem` that the high-level variable should
/// be allocated as a public input to the `ConstraintSystem`.
Input = 1,
/// Indicate to the `ConstraintSystem` that the high-level variable should
/// be allocated as a private witness to the `ConstraintSystem`.
Witness = 2,
}
impl AllocationMode {
/// Outputs the maximum according to the relation `Constant < Input <
/// Witness`.
pub fn max(&self, other: Self) -> Self {
use AllocationMode::*;
match (self, other) {
(Constant, _) => other,
(Input, Constant) => *self,
(Input, _) => other,
(Witness, _) => *self,
}
}
}
/// Specifies how variables of type `Self` should be allocated in a
/// `ConstraintSystem`.
pub trait AllocVar<V: ?Sized, F: Field>: Sized {
/// Allocates a new variable of type `Self` in the `ConstraintSystem` `cs`.
/// The mode of allocation is decided by `mode`.
fn new_variable<T: Borrow<V>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
mode: AllocationMode,
) -> Result<Self, SynthesisError>;
/// Allocates a new constant of type `Self` in the `ConstraintSystem` `cs`.
///
/// This should *not* allocate any new variables or constraints in `cs`.
#[tracing::instrument(target = "gr1cs", skip(cs, t))]
fn new_constant(
cs: impl Into<Namespace<F>>,
t: impl Borrow<V>,
) -> Result<Self, SynthesisError> {
Self::new_variable(cs, || Ok(t), AllocationMode::Constant)
}
/// Allocates a new public input of type `Self` in the `ConstraintSystem`
/// `cs`.
#[tracing::instrument(target = "gr1cs", skip(cs, f))]
fn new_input<T: Borrow<V>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
) -> Result<Self, SynthesisError> {
Self::new_variable(cs, f, AllocationMode::Input)
}
/// Allocates a new private witness of type `Self` in the `ConstraintSystem`
/// `cs`.
#[tracing::instrument(target = "gr1cs", skip(cs, f))]
fn new_witness<T: Borrow<V>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
) -> Result<Self, SynthesisError> {
Self::new_variable(cs, f, AllocationMode::Witness)
}
/// Allocates a new constant or private witness of type `Self` in the
/// `ConstraintSystem` `cs` with the allocation mode inferred from `cs`.
/// A constant is allocated if `cs` is `None`, and a private witness is
/// allocated otherwise.
///
/// A common use case is the creation of non-deterministic advice (a.k.a.
/// hints) in the circuit, where this method can avoid boilerplate code
/// while allowing optimization on circuit size.
///
/// For example, to compute `x_var / y_var` where `y_var` is a non-zero
/// variable, one can write:
/// ```
/// use ark_ff::PrimeField;
/// use ark_r1cs_std::{alloc::AllocVar, fields::{fp::FpVar, FieldVar}, GR1CSVar};
/// use ark_relations::gr1cs::SynthesisError;
///
/// fn div<F: PrimeField>(x_var: &FpVar<F>, y_var: &FpVar<F>) -> Result<FpVar<F>, SynthesisError> {
/// let cs = x_var.cs().or(y_var.cs());
/// let z_var = FpVar::new_variable_with_inferred_mode(cs, || Ok(x_var.value()? / y_var.value()?))?;
/// z_var.mul_equals(y_var, x_var)?;
/// Ok(z_var)
/// }
/// ```
/// In this example, if either `x_var` or `y_var` is a witness variable,
/// then `z_var` is also a witness variable. On the other hand, `z_var`
/// is a constant if both `x_var` and `y_var` are constants (i.e., `cs`
/// is `None`), and future operations on `z_var` do not generate any
/// constraints.
///
/// (Note that we use division as an example for simplicity. You may
/// call `x_var.mul_by_inverse(y_var)?` directly, which internally works
/// similarly to the above code.)
#[tracing::instrument(target = "r1cs", skip(cs, f))]
fn new_variable_with_inferred_mode<T: Borrow<V>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
) -> Result<Self, SynthesisError> {
let ns: Namespace<F> = cs.into();
let cs = ns.cs();
let mode = if cs.is_none() {
AllocationMode::Constant
} else {
AllocationMode::Witness
};
Self::new_variable(cs, f, mode)
}
}
/// This blanket implementation just allocates variables in `Self`
/// element by element.
impl<I, F: Field, A: AllocVar<I, F>> AllocVar<[I], F> for Vec<A> {
fn new_variable<T: Borrow<[I]>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
mode: AllocationMode,
) -> Result<Self, SynthesisError> {
let ns = cs.into();
let cs = ns.cs();
f().and_then(|v| {
v.borrow()
.iter()
.map(|e| A::new_variable(cs.clone(), || Ok(e), mode))
.collect()
})
}
}
/// Dummy impl for `()`.
impl<F: Field> AllocVar<(), F> for () {
fn new_variable<T: Borrow<()>>(
_cs: impl Into<Namespace<F>>,
_f: impl FnOnce() -> Result<T, SynthesisError>,
_mode: AllocationMode,
) -> Result<Self, SynthesisError> {
Ok(())
}
}
/// This blanket implementation just allocates variables in `Self`
/// element by element.
impl<I, F: Field, A: AllocVar<I, F>, const N: usize> AllocVar<[I; N], F> for [A; N] {
fn new_variable<T: Borrow<[I; N]>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
mode: AllocationMode,
) -> Result<Self, SynthesisError> {
let ns = cs.into();
let cs = ns.cs();
f().map(|v| {
let v = v.borrow();
core::array::from_fn(|i| A::new_variable(cs.clone(), || Ok(&v[i]), mode).unwrap())
})
}
}
/// This blanket implementation just allocates variables in `Self`
/// element by element.
impl<I, F: Field, A: AllocVar<I, F>, const N: usize> AllocVar<[I], F> for [A; N] {
fn new_variable<T: Borrow<[I]>>(
cs: impl Into<Namespace<F>>,
f: impl FnOnce() -> Result<T, SynthesisError>,
mode: AllocationMode,
) -> Result<Self, SynthesisError> {
let ns = cs.into();
let cs = ns.cs();
f().map(|v| {
let v = v.borrow();
core::array::from_fn(|i| A::new_variable(cs.clone(), || Ok(&v[i]), mode).unwrap())
})
}
}