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use crate::{BigInt, UBigInt};

impl BigInt {

    #[must_use = "method returns a new number and does not mutate the original value"]
    pub fn sub_bi(&self, other: &BigInt) -> Self {
        let result = if self.is_neg() != other.is_neg() {
            BigInt::from_ubi(self.val.add_ubi(&other.val), self.is_neg())
        }

        else {
            let self_less = self.val.lt_ubi(&other.val);
            let abs_diff = if self_less {
                other.val.sub_ubi(&self.val)
            } else {
                self.val.sub_ubi(&other.val)
            };
            let is_diff_zero = abs_diff.is_zero();

            // self - other      self_less       self.is_neg       diff.val == 0     result.is_neg()
            //    3 - 4            true            false               false             true
            //    4 - 3            false           false               false             false
            // (-3) - (-4)         true            true                false             false
            // (-4) - (-3)         false           true                false             true
            //    3 - 3            false           false               true              false
            // (-3) - (-3)         false           true                true              false

            BigInt::from_ubi(abs_diff, (self_less ^ self.is_neg()) & !is_diff_zero)
        };

        #[cfg(test)] assert!(result.is_valid());

        result
    }

    pub fn sub_bi_mut(&mut self, other: &BigInt) {

        if self.is_neg() != other.is_neg() {
            self.val.add_ubi_mut(&other.val);
        }

        else {
            let self_less = self.val.lt_ubi(&other.val);

            if self_less {
                self.val = other.val.sub_ubi(&self.val);
            }

            else {
                self.val.sub_ubi_mut(&other.val);
            }

            let is_diff_zero = self.val.is_zero();

            self._is_neg = (self_less ^ self.is_neg()) & !is_diff_zero;
        }

        #[cfg(test)] assert!(self.is_valid());
    }

    #[must_use = "method returns a new number and does not mutate the original value"]
    pub fn sub_i32(&self, other: i32) -> Self {
        let result = if self.is_neg() != (other < 0) {
            BigInt::from_ubi(self.val.add_u32(other.abs() as u32), self.is_neg())
        }

        else {
            let other_abs = other.abs() as u32;
            let self_less = self.val.lt_u32(other_abs);
            let abs_diff = if self_less {
                UBigInt::from_u32(other_abs - self.val.to_u32().unwrap())
            } else {
                self.val.sub_u32(other_abs)
            };
            let is_diff_zero = abs_diff.is_zero();

            BigInt::from_ubi(abs_diff, (self_less ^ self.is_neg()) & !is_diff_zero)
        };

        #[cfg(test)] assert!(result.is_valid());

        result
    }

    pub fn sub_i32_mut(&mut self, other: i32) {

        if self.is_neg() != (other < 0) {
            self.val.add_u32_mut(other.abs() as u32);
        }

        else {
            let other_abs = other.abs() as u32;
            let self_less = self.val.lt_u32(other_abs);

            if self_less {
                self.val = UBigInt::from_u32(other_abs - self.val.to_u32().unwrap());
            }

            else {
                self.val.sub_u32_mut(other_abs);
            }

            let is_diff_zero = self.val.is_zero();
            self._is_neg = (self_less ^ self.is_neg()) & !is_diff_zero;
        }

        #[cfg(test)] assert!(self.is_valid());
    }

}

#[cfg(test)]
mod tests {
    use crate::BigInt;

    #[test]
    fn sign_test() {

        for x in -7..8 {

            for y in -7..8 {
                let mut x1 = BigInt::from_i32(x);
                let y1 = BigInt::from_i32(y);
                let mut x2 = BigInt::from_i32(x);
                let res1 = x1.sub_bi(&y1);
                let res2 = x1.sub_i32(y);
                let res3 = BigInt::from_i32(x - y);
                x1.sub_bi_mut(&y1);
                x2.sub_i32_mut(y);

                assert_eq!(x1, x2);
                assert_eq!(res1, res2);
                assert_eq!(res2, res3);
                assert_eq!(res1, x1);
                assert_eq!(res2, x2);
            }

        }

    }

}