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/// Calculate a - b - borrow, returning the result and modifying
/// the borrow value.
#[inline(always)]
pub fn sbb(a: u64, b: u64, borrow: &mut u64) -> u64 {
    let tmp = (1u128 << 64) + u128::from(a) - u128::from(b) - u128::from(*borrow);

    *borrow = if tmp >> 64 == 0 { 1 } else { 0 };

    tmp as u64
}

/// Calculate a + b + carry, returning the sum and modifying the
/// carry value.
#[inline(always)]
pub fn adc(a: u64, b: u64, carry: &mut u64) -> u64 {
    let tmp = u128::from(a) + u128::from(b) + u128::from(*carry);

    *carry = (tmp >> 64) as u64;

    tmp as u64
}

/// Calculate a + (b * c) + carry, returning the least significant digit
/// and setting carry to the most significant digit.
#[inline(always)]
pub fn mac_with_carry(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 {
    let tmp = (u128::from(a)) + u128::from(b) * u128::from(c) + u128::from(*carry);

    *carry = (tmp >> 64) as u64;

    tmp as u64
}