sub.rs

use crate::bigint::BigIntImpl;
use crate::treepp::*;

impl<const N_BITS: u32, const LIMB_SIZE: u32> BigIntImpl<N_BITS, LIMB_SIZE> {
    /// Compute the difference of two BigInts
    pub fn sub(a: u32, b: u32) -> Script {
        script! {
            {Self::zip(a, b)}

            { 1 << LIMB_SIZE }

            // A0 - B0
            limb_sub_borrow OP_TOALTSTACK

            // from     A1      - (B1        + borrow_0)
            //   to     A{N-2}  - (B{N-2}    + borrow_{N-3})
            for _ in 0..Self::N_LIMBS - 2 {
                OP_ROT
                OP_ADD
                OP_SWAP
                limb_sub_borrow OP_TOALTSTACK
            }

            // A{N-1} - (B{N-1} + borrow_{N-2})
            OP_NIP
            OP_ADD
            { limb_sub_noborrow(Self::HEAD_OFFSET) }

            for _ in 0..Self::N_LIMBS - 1 {
                OP_FROMALTSTACK
            }
        }
    }
}

/// Compute the difference of two limbs, including the carry bit
///
/// Author: @stillsaiko
pub fn limb_sub_borrow() -> Script {
    script! {
        OP_ROT OP_ROT
        OP_SUB
        OP_DUP
        0
        OP_LESSTHAN
        OP_TUCK
        OP_IF
            2 OP_PICK OP_ADD
        OP_ENDIF
    }
}

/// Compute the sum of two limbs, dropping the carry bit
///
/// Author: @weikengchen
pub fn limb_sub_noborrow(head_offset: u32) -> Script {
    script! {
        OP_SUB
        OP_DUP
        0
        OP_LESSTHAN
        OP_IF
            { head_offset }
            OP_ADD
        OP_ENDIF
    }
}

#[cfg(test)]
mod test {
    use crate::bigint::{U254, U64};
    use crate::treepp::*;
    use core::ops::{Rem, Shl};
    use num_bigint::{BigUint, RandomBits};
    use num_traits::One;
    use rand::{Rng, SeedableRng};
    use rand_chacha::ChaCha20Rng;

    #[test]
    fn test_sub() {
        let mut prng = ChaCha20Rng::seed_from_u64(0);

        for _ in 0..100 {
            let a: BigUint = prng.sample(RandomBits::new(254));
            let b: BigUint = prng.sample(RandomBits::new(254));
            let mut c: BigUint = BigUint::one().shl(254) + &a - &b;
            c = c.rem(BigUint::one().shl(254));

            let script = script! {
                { U254::push_u32_le(&a.to_u32_digits()) }
                { U254::push_u32_le(&b.to_u32_digits()) }
                { U254::sub(1, 0) }
                { U254::push_u32_le(&c.to_u32_digits()) }
                { U254::equalverify(1, 0) }
                OP_TRUE
            };
            run(script);

            let script = script! {
                { U254::push_u32_le(&b.to_u32_digits()) }
                { U254::push_u32_le(&a.to_u32_digits()) }
                { U254::sub(0, 1) }
                { U254::push_u32_le(&c.to_u32_digits()) }
                { U254::equalverify(1, 0) }
                OP_TRUE
            };
            run(script);
        }

        for _ in 0..100 {
            let a: BigUint = prng.sample(RandomBits::new(64));
            let b: BigUint = prng.sample(RandomBits::new(64));
            let mut c: BigUint = BigUint::one().shl(64) + &a - &b;
            c = c.rem(BigUint::one().shl(64));

            let script = script! {
                { U64::push_u32_le(&a.to_u32_digits()) }
                { U64::push_u32_le(&b.to_u32_digits()) }
                { U64::sub(1, 0) }
                { U64::push_u32_le(&c.to_u32_digits()) }
                { U64::equalverify(1, 0) }
                OP_TRUE
            };
            run(script);

            let script = script! {
                { U64::push_u32_le(&b.to_u32_digits()) }
                { U64::push_u32_le(&a.to_u32_digits()) }
                { U64::sub(0, 1) }
                { U64::push_u32_le(&c.to_u32_digits()) }
                { U64::equalverify(1, 0) }
                OP_TRUE
            };
            run(script);
        }
    }
}