Function chunk_precompute_p_from_hash

bitvm/src/chunk/taps_ext_miller.rs:117–199

precompute P

(
    hint_in_p: ark_bn254::G1Affine,
)

// precompute P
pub(crate) fn chunk_precompute_p_from_hash(
    hint_in_p: ark_bn254::G1Affine,
) -> (ark_bn254::G1Affine, bool, Script, Vec<Hint>) {
    let mut hints = vec![];

    let mut px: ark_bn254::Fq = ark_bn254::Fq::ONE;
    let mut py: ark_bn254::Fq = ark_bn254::Fq::ONE;

    let py_is_not_zero = hint_in_p.y != ark_bn254::Fq::ZERO;
    if py_is_not_zero {
        px = hint_in_p.x;
        py = hint_in_p.y;
    }

    let (on_curve_scr, on_curve_hint) = G1Affine::hinted_is_on_curve(px, py);
    if py_is_not_zero {
        hints.extend_from_slice(&on_curve_hint);
    }

    let p = ark_bn254::G1Affine::new_unchecked(px, py);
    let (eval_xy, eval_hints) = hinted_from_eval_points(p);

    let valid_point = py_is_not_zero && p.is_on_curve();
    let mock_pd = ark_bn254::G1Affine::new_unchecked(ark_bn254::Fq::ONE, ark_bn254::Fq::ONE);

    let pd = if valid_point {
        hints.extend_from_slice(&eval_hints);

        let pdy = p.y.inverse().unwrap();
        let pdx = -p.x * pdy;

        ark_bn254::G1Affine::new_unchecked(pdx, pdy)
    } else {
        mock_pd
    };

    let drop_and_return_scr = script! {
        // [px, py] [pdhash, phash]
        {G1Affine::push(mock_pd)} // mock values for pd,these values won't be useful as we add {0} <- skip output hash check for invalid input
        // [p, pd] [pdhash, phash]
        {0} // skip output hash check because input was invalid
    };
    let scr = script! {
        // [hints, px, py] [pdhash, phash]

        // Validity checks
        { Fq::check_validity() }
        { Fq::check_validity() }
        { Fq::fromaltstack() } { Fq::fromaltstack() }

        // [hints, px, py] [pdhash, phash]

        {Fq::is_zero_keep_element(0)}
        OP_IF // PY = 0
            {drop_and_return_scr.clone()}
        OP_ELSE // PY != 0
            // [hints, px, py]
            {Fq2::copy(0)}