halo2/src/plonk/permutation/keygen.rs

use ff::Field;

use super::{Argument, ProvingKey, VerifyingKey};
use crate::{
    arithmetic::{Curve, CurveAffine, FieldExt},
    plonk::{circuit::ConstraintSystem, Error},
    poly::{
        commitment::{Blind, Params},
        EvaluationDomain, Rotation,
    },
};

pub(crate) struct AssemblyHelper<C: CurveAffine> {
    deltaomega: Vec<Vec<C::Scalar>>,
}

pub(crate) struct Assembly {
    mapping: Vec<Vec<(usize, usize)>>,
    aux: Vec<Vec<(usize, usize)>>,
    sizes: Vec<Vec<usize>>,
}

impl Assembly {
    pub(crate) fn new<C: CurveAffine>(params: &Params<C>, p: &Argument) -> Self {
        // Initialize the copy vector to keep track of copy constraints in all
        // the permutation arguments.
        let mut columns = vec![];
        for i in 0..p.columns.len() {
            // Computes [(i, 0), (i, 1), ..., (i, n - 1)]
            columns.push((0..params.n).map(|j| (i, j as usize)).collect());
        }

        // Before any equality constraints are applied, every cell in the permutation is
        // in a 1-cycle; therefore mapping and aux are identical, because every cell is
        // its own distinguished element.
        Assembly {
            mapping: columns.clone(),
            aux: columns,
            sizes: vec![vec![1usize; params.n as usize]; p.columns.len()],
        }
    }

    pub(crate) fn copy(
        &mut self,
        left_column: usize,
        left_row: usize,
        right_column: usize,
        right_row: usize,
    ) -> Result<(), Error> {
        // Check bounds first
        if left_column >= self.mapping.len()
            || left_row >= self.mapping[left_column].len()
            || right_column >= self.mapping.len()
            || right_row >= self.mapping[right_column].len()
        {
            return Err(Error::BoundsFailure);
        }

        // See book/src/design/permutation.md for a description of this algorithm.

        let mut left_cycle = self.aux[left_column][left_row];
        let mut right_cycle = self.aux[right_column][right_row];

        // If left and right are in the same cycle, do nothing.
        if left_cycle == right_cycle {
            return Ok(());
        }

        if self.sizes[left_cycle.0][left_cycle.1] < self.sizes[right_cycle.0][right_cycle.1] {
            std::mem::swap(&mut left_cycle, &mut right_cycle);
        }

        // Merge the right cycle into the left one.
        self.sizes[left_cycle.0][left_cycle.1] += self.sizes[right_cycle.0][right_cycle.1];
        let mut i = right_cycle;
        loop {
            self.aux[i.0][i.1] = left_cycle;
            i = self.mapping[i.0][i.1];
            if i == right_cycle {
                break;
            }
        }

        let tmp = self.mapping[left_column][left_row];
        self.mapping[left_column][left_row] = self.mapping[right_column][right_row];
        self.mapping[right_column][right_row] = tmp;

        Ok(())
    }

    pub(crate) fn build_helper<C: CurveAffine>(
        params: &Params<C>,
        cs: &ConstraintSystem<C::Scalar>,
        domain: &EvaluationDomain<C::Scalar>,
    ) -> AssemblyHelper<C> {
        // Get the largest permutation argument length in terms of the number of
        // advice columns involved.
        let largest_permutation_length = cs
            .permutations
            .iter()
            .map(|p| p.columns.len())
            .max()
            .unwrap_or_default();

        // Compute [omega^0, omega^1, ..., omega^{params.n - 1}]
        let mut omega_powers = Vec::with_capacity(params.n as usize);
        {
            let mut cur = C::Scalar::one();
            for _ in 0..params.n {
                omega_powers.push(cur);
                cur *= &domain.get_omega();
            }
        }

        // Compute [omega_powers * \delta^0, omega_powers * \delta^1, ..., omega_powers * \delta^m]
        let mut deltaomega = Vec::with_capacity(largest_permutation_length);
        {
            let mut cur = C::Scalar::one();
            for _ in 0..largest_permutation_length {
                let mut omega_powers = omega_powers.clone();
                for o in &mut omega_powers {
                    *o *= &cur;
                }

                deltaomega.push(omega_powers);

                cur *= &C::Scalar::DELTA;
            }
        }

        AssemblyHelper { deltaomega }
    }

    pub(crate) fn build_keys<C: CurveAffine>(
        self,
        params: &Params<C>,
        domain: &EvaluationDomain<C::Scalar>,
        helper: &AssemblyHelper<C>,
        p: &Argument,
    ) -> (ProvingKey<C>, VerifyingKey<C>) {
        // Compute permutation polynomials, convert to coset form and
        // pre-compute commitments for the SRS.
        let mut commitments = vec![];
        let mut permutations = vec![];
        let mut polys = vec![];
        let mut cosets = vec![];
        for i in 0..p.columns.len() {
            // Computes the permutation polynomial based on the permutation
            // description in the assembly.
            let mut permutation_poly = domain.empty_lagrange();
            for (j, p) in permutation_poly.iter_mut().enumerate() {
                let (permuted_i, permuted_j) = self.mapping[i][j];
                *p = helper.deltaomega[permuted_i][permuted_j];
            }

            // Compute commitment to permutation polynomial
            commitments.push(
                params
                    .commit_lagrange(&permutation_poly, Blind::default())
                    .to_affine(),
            );
            // Store permutation polynomial and precompute its coset evaluation
            permutations.push(permutation_poly.clone());
            let poly = domain.lagrange_to_coeff(permutation_poly);
            polys.push(poly.clone());
            cosets.push(domain.coeff_to_extended(poly, Rotation::default()));
        }
        (
            ProvingKey {
                permutations,
                polys,
                cosets,
            },
            VerifyingKey { commitments },
        )
    }
}