Function poly_commitment::commitment::shift_scalar

pub fn shift_scalar<G: AffineRepr>(x: G::ScalarField) -> G::ScalarFieldwhere
    G::BaseField: PrimeField,

Inside the circuit, we have a specialized scalar multiplication which computes either

|g: G, x: G::ScalarField| g.scale(x + 2^n)

if the scalar field of G is greater than the size of the base field and

|g: G, x: G::ScalarField| g.scale(2*x + 2^n)

otherwise. So, if we want to actually scale by x, we need to apply the inverse function of |x| x + 2^n (or of |x| 2*x + 2^n in the other case), before supplying the scalar to our in-circuit scalar-multiplication function. This computes that inverse function. Namely,

|x: G::ScalarField| x - 2^n

when the scalar field is larger than the base field and

|x: G::ScalarField| (x - 2^n) / 2

in the other case.