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/* This file is part of DarkFi (https://dark.fi)
 *
 * Copyright (C) 2020-2024 Dyne.org foundation
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use halo2_proofs::{
    circuit::{Region, Value},
    pasta::group::ff::WithSmallOrderMulGroup,
    plonk::{Advice, Column, ConstraintSystem, Error, Expression, VirtualCells},
    poly::Rotation,
};

#[derive(Clone, Debug)]
pub struct IsZeroConfig<F> {
    pub value_inv: Column<Advice>,
    pub is_zero_expr: Expression<F>,
}

impl<F: WithSmallOrderMulGroup<3> + Ord> IsZeroConfig<F> {
    pub fn expr(&self) -> Expression<F> {
        self.is_zero_expr.clone()
    }
}

pub struct IsZeroChip<F: WithSmallOrderMulGroup<3> + Ord> {
    config: IsZeroConfig<F>,
}

impl<F: WithSmallOrderMulGroup<3> + Ord> IsZeroChip<F> {
    pub fn construct(config: IsZeroConfig<F>) -> Self {
        Self { config }
    }

    pub fn configure(
        meta: &mut ConstraintSystem<F>,
        q_enable: impl FnOnce(&mut VirtualCells<'_, F>) -> Expression<F>,
        value: impl FnOnce(&mut VirtualCells<'_, F>) -> Expression<F>,
        value_inv: Column<Advice>,
    ) -> IsZeroConfig<F> {
        let mut is_zero_expr = Expression::Constant(F::ZERO);

        meta.create_gate("is_zero", |meta| {
            //
            // valid | value |  value_inv |  1 - value * value_inv | value * (1 - value* value_inv)
            // ------+-------+------------+------------------------+-------------------------------
            //  yes  |   x   |    1/x     |         0              |  0
            //  no   |   x   |    0       |         1              |  x
            //  yes  |   0   |    0       |         1              |  0
            //  yes  |   0   |    y       |         1              |  0
            //
            let value = value(meta);
            let q_enable = q_enable(meta);
            let value_inv = meta.query_advice(value_inv, Rotation::cur());

            is_zero_expr = Expression::Constant(F::ONE) - value.clone() * value_inv;
            vec![q_enable * value * is_zero_expr.clone()]
        });

        IsZeroConfig { value_inv, is_zero_expr }
    }

    pub fn assign(
        &self,
        region: &mut Region<'_, F>,
        offset: usize,
        value: Value<F>,
    ) -> Result<(), Error> {
        let value_inv = value.map(|value| value.invert().unwrap_or(F::ZERO));
        region.assign_advice(|| "value inv", self.config.value_inv, offset, || value_inv)?;
        Ok(())
    }
}