Add BoxedUint::{sbb, wrapping_sub, checked_sub}

src/boxed/uint.rs

@@ -2,6 +2,7 @@
 
 mod add;
 mod cmp;
+mod sub;
 
 use crate::{Limb, Word};
 use alloc::{vec, vec::Vec};

src/boxed/uint/sub.rs

@@ -0,0 +1,58 @@
+//! [`BoxedUint`] subtraction operations.
+
+use crate::{BoxedUint, CheckedSub, Limb, Zero};
+use subtle::CtOption;
+
+impl BoxedUint {
+    /// Computes `a + b + carry`, returning the result along with the new carry.
+    #[inline(always)]
+    pub fn sbb(&self, rhs: &Self, carry: Limb) -> (Self, Limb) {
+        Self::chain(self, rhs, carry, |a, b, c| a.sbb(b, c))
+    }
+
+    /// Perform wrapping subition, discarding overflow.
+    pub fn wrapping_sub(&self, rhs: &Self) -> Self {
+        self.sbb(rhs, Limb::ZERO).0
+    }
+}
+
+impl CheckedSub<&BoxedUint> for BoxedUint {
+    type Output = Self;
+
+    fn checked_sub(&self, rhs: &Self) -> CtOption<Self> {
+        let (result, carry) = self.sbb(rhs, Limb::ZERO);
+        CtOption::new(result, carry.is_zero())
+    }
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+mod tests {
+    use super::{BoxedUint, CheckedSub, Limb};
+
+    #[test]
+    fn sbb_no_carry() {
+        let (res, carry) = BoxedUint::one().sbb(&BoxedUint::one(), Limb::ZERO);
+        assert_eq!(res, BoxedUint::zero());
+        assert_eq!(carry, Limb::ZERO);
+    }
+
+    #[test]
+    fn sbb_with_borrow() {
+        let (res, borrow) = BoxedUint::zero().sbb(&BoxedUint::one(), Limb::ZERO);
+        assert_eq!(res, BoxedUint::max(Limb::BITS).unwrap());
+        assert_eq!(borrow, Limb::MAX);
+    }
+
+    #[test]
+    fn checked_sub_ok() {
+        let result = BoxedUint::one().checked_sub(&BoxedUint::one());
+        assert_eq!(result.unwrap(), BoxedUint::zero());
+    }
+
+    #[test]
+    fn checked_sub_overflow() {
+        let result = BoxedUint::zero().checked_sub(&BoxedUint::one());
+        assert!(!bool::from(result.is_some()));
+    }
+}