plonky2/src/fri/prover.rs

use rayon::prelude::*;

use crate::field::extension_field::{flatten, unflatten, Extendable};
use crate::field::field_types::RichField;
use crate::fri::proof::{FriInitialTreeProof, FriProof, FriQueryRound, FriQueryStep};
use crate::fri::FriConfig;
use crate::hash::hash_types::HashOut;
use crate::hash::hashing::hash_n_to_1;
use crate::hash::merkle_tree::MerkleTree;
use crate::iop::challenger::Challenger;
use crate::plonk::circuit_data::CircuitConfig;
use crate::plonk::plonk_common::reduce_with_powers;
use crate::polynomial::polynomial::{PolynomialCoeffs, PolynomialValues};
use crate::timed;
use crate::util::reverse_index_bits_in_place;
use crate::util::timing::TimingTree;

/// Builds a FRI proof.
pub fn fri_proof<F: RichField + Extendable<D>, const D: usize>(
    initial_merkle_trees: &[&MerkleTree<F>],
    // Coefficients of the polynomial on which the LDT is performed. Only the first `1/rate` coefficients are non-zero.
    lde_polynomial_coeffs: PolynomialCoeffs<F::Extension>,
    // Evaluation of the polynomial on the large domain.
    lde_polynomial_values: PolynomialValues<F::Extension>,
    challenger: &mut Challenger<F>,
    config: &CircuitConfig,
    timing: &mut TimingTree,
) -> FriProof<F, D> {
    let n = lde_polynomial_values.values.len();
    assert_eq!(lde_polynomial_coeffs.coeffs.len(), n);

    // Commit phase
    let (trees, final_coeffs) = timed!(
        timing,
        "fold codewords in the commitment phase",
        fri_committed_trees(
            lde_polynomial_coeffs,
            lde_polynomial_values,
            challenger,
            config,
        )
    );

    // PoW phase
    let current_hash = challenger.get_hash();
    let pow_witness = timed!(
        timing,
        "find for proof-of-work witness",
        fri_proof_of_work(current_hash, &config.fri_config)
    );

    // Query phase
    let query_round_proofs = fri_prover_query_rounds(
        initial_merkle_trees,
        &trees,
        challenger,
        n,
        &config.fri_config,
    );

    FriProof {
        commit_phase_merkle_caps: trees.iter().map(|t| t.cap.clone()).collect(),
        query_round_proofs,
        final_poly: final_coeffs,
        pow_witness,
        is_compressed: false,
    }
}

fn fri_committed_trees<F: RichField + Extendable<D>, const D: usize>(
    mut coeffs: PolynomialCoeffs<F::Extension>,
    mut values: PolynomialValues<F::Extension>,
    challenger: &mut Challenger<F>,
    config: &CircuitConfig,
) -> (Vec<MerkleTree<F>>, PolynomialCoeffs<F::Extension>) {
    let mut trees = Vec::new();

    let mut shift = F::MULTIPLICATIVE_GROUP_GENERATOR;
    let num_reductions = config.fri_config.reduction_arity_bits.len();
    for i in 0..num_reductions {
        let arity = 1 << config.fri_config.reduction_arity_bits[i];

        reverse_index_bits_in_place(&mut values.values);
        let chunked_values = values
            .values
            .par_chunks(arity)
            .map(|chunk: &[F::Extension]| flatten(chunk))
            .collect();
        let tree = MerkleTree::new(chunked_values, config.cap_height);

        challenger.observe_cap(&tree.cap);
        trees.push(tree);

        let beta = challenger.get_extension_challenge();
        // P(x) = sum_{i<r} x^i * P_i(x^r) becomes sum_{i<r} beta^i * P_i(x).
        coeffs = PolynomialCoeffs::new(
            coeffs
                .coeffs
                .par_chunks_exact(arity)
                .map(|chunk| reduce_with_powers(chunk, beta))
                .collect::<Vec<_>>(),
        );
        shift = shift.exp_u64(arity as u64);
        values = coeffs.coset_fft(shift.into())
    }

    // The coefficients being removed here should always be zero.
    coeffs.coeffs.truncate(coeffs.len() >> config.rate_bits);

    challenger.observe_extension_elements(&coeffs.coeffs);
    (trees, coeffs)
}

fn fri_proof_of_work<F: RichField>(current_hash: HashOut<F>, config: &FriConfig) -> F {
    (0..=F::NEG_ONE.to_canonical_u64())
        .into_par_iter()
        .find_any(|&i| {
            hash_n_to_1(
                current_hash
                    .elements
                    .iter()
                    .copied()
                    .chain(Some(F::from_canonical_u64(i)))
                    .collect(),
                false,
            )
            .to_canonical_u64()
            .leading_zeros()
                >= config.proof_of_work_bits + (64 - F::order().bits()) as u32
        })
        .map(F::from_canonical_u64)
        .expect("Proof of work failed. This is highly unlikely!")
}

fn fri_prover_query_rounds<F: RichField + Extendable<D>, const D: usize>(
    initial_merkle_trees: &[&MerkleTree<F>],
    trees: &[MerkleTree<F>],
    challenger: &mut Challenger<F>,
    n: usize,
    config: &FriConfig,
) -> Vec<FriQueryRound<F, D>> {
    (0..config.num_query_rounds)
        .map(|_| fri_prover_query_round(initial_merkle_trees, trees, challenger, n, config))
        .collect()
}

fn fri_prover_query_round<F: RichField + Extendable<D>, const D: usize>(
    initial_merkle_trees: &[&MerkleTree<F>],
    trees: &[MerkleTree<F>],
    challenger: &mut Challenger<F>,
    n: usize,
    config: &FriConfig,
) -> FriQueryRound<F, D> {
    let mut query_steps = Vec::new();
    let x = challenger.get_challenge();
    let mut x_index = x.to_canonical_u64() as usize % n;
    let initial_proof = initial_merkle_trees
        .iter()
        .map(|t| (t.get(x_index).to_vec(), t.prove(x_index)))
        .collect::<Vec<_>>();
    for (i, tree) in trees.iter().enumerate() {
        let arity_bits = config.reduction_arity_bits[i];
        let evals = unflatten(tree.get(x_index >> arity_bits));
        let merkle_proof = tree.prove(x_index >> arity_bits);

        query_steps.push(FriQueryStep {
            evals,
            merkle_proof,
        });

        x_index >>= arity_bits;
    }
    FriQueryRound {
        initial_trees_proof: FriInitialTreeProof {
            evals_proofs: initial_proof,
        },
        steps: query_steps,
    }
}