Replace BoxedUint::new with ::zero_with_precision (#327)

The two APIs are nearly identical. The only difference is the latter is
infallible and panics if an invalid precision is given.

This also changes `BoxedUint::max` to have the same behavior.

Whether they should panic is a debatable point. Indeed having both
options would be good, but it should probably return `CtOption` where
`BoxedUint::max` was returning `Option`.

Regardless, this removes duplication and makes the API at least more
consistent with itself.

Author: tarcieri

src/uint/boxed.rs

@@ -78,34 +78,17 @@ impl BoxedUint {
             .fold(Choice::from(1), |acc, limb| acc & limb.is_zero())
     }
 
-    /// Create a new [`BoxedUint`] with the given number of bits of precision.
-    ///
-    /// Returns `None` if the number of bits is not a multiple of the
-    /// [`Limb`] size.
-    pub fn new(bits_precision: usize) -> Option<Self> {
-        if bits_precision == 0 || bits_precision % Limb::BITS != 0 {
-            return None;
-        }
-
-        let nlimbs = bits_precision / Limb::BITS;
-
-        Some(Self {
-            limbs: vec![Limb::ZERO; nlimbs].into(),
-        })
-    }
-
     /// Get the maximum value for a given number of bits of precision.
     ///
-    /// Returns `None` if the number of bits is not a multiple of the
-    /// [`Limb`] size.
-    pub fn max(bits_precision: usize) -> Option<Self> {
-        let mut ret = Self::new(bits_precision)?;
-
-        for limb in &mut *ret.limbs {
-            *limb = Limb::MAX;
-        }
+    /// Panics if the precision is not a multiple of [`Limb::BITS`].
+    pub fn max(bits_precision: usize) -> Self {
+        assert_eq!(
+            bits_precision % Limb::BITS,
+            0,
+            "precision is not a multiple of limb size"
+        );
 
-        Some(ret)
+        vec![Limb::MAX; bits_precision / Limb::BITS].into()
     }
 
     /// Create a [`BoxedUint`] from an array of [`Word`]s (i.e. word-sized unsigned

src/uint/boxed/add.rs

@@ -39,9 +39,7 @@ mod tests {
 
     #[test]
     fn adc_with_carry() {
-        let (res, carry) = BoxedUint::max(Limb::BITS)
-            .unwrap()
-            .adc(&BoxedUint::one(), Limb::ZERO);
+        let (res, carry) = BoxedUint::max(Limb::BITS).adc(&BoxedUint::one(), Limb::ZERO);
         assert_eq!(res, BoxedUint::zero());
         assert_eq!(carry, Limb::ONE);
     }
@@ -54,9 +52,7 @@ mod tests {
 
     #[test]
     fn checked_add_overflow() {
-        let result = BoxedUint::max(Limb::BITS)
-            .unwrap()
-            .checked_add(&BoxedUint::one());
+        let result = BoxedUint::max(Limb::BITS).checked_add(&BoxedUint::one());
         assert!(!bool::from(result.is_some()));
     }
 }

src/uint/boxed/bit_and.rs

@@ -140,7 +140,7 @@ mod tests {
 
     #[test]
     fn overlapping_and_ok() {
-        let result = BoxedUint::max(128).unwrap().wrapping_and(&BoxedUint::one());
+        let result = BoxedUint::max(128).wrapping_and(&BoxedUint::one());
         assert_eq!(result, BoxedUint::one());
     }
 }

src/uint/boxed/cmp.rs

@@ -91,7 +91,7 @@ mod tests {
     fn ct_gt() {
         let a = BoxedUint::zero();
         let b = BoxedUint::one();
-        let c = BoxedUint::max(64).unwrap();
+        let c = BoxedUint::max(64);
 
         assert!(bool::from(b.ct_gt(&a)));
         assert!(bool::from(c.ct_gt(&a)));
@@ -110,7 +110,7 @@ mod tests {
     fn ct_lt() {
         let a = BoxedUint::zero();
         let b = BoxedUint::one();
-        let c = BoxedUint::max(64).unwrap();
+        let c = BoxedUint::max(64);
 
         assert!(bool::from(a.ct_lt(&b)));
         assert!(bool::from(a.ct_lt(&c)));
@@ -129,7 +129,7 @@ mod tests {
     fn cmp() {
         let a = BoxedUint::zero();
         let b = BoxedUint::one();
-        let c = BoxedUint::max(64).unwrap();
+        let c = BoxedUint::max(64);
 
         assert_eq!(a.cmp(&b), Ordering::Less);
         assert_eq!(a.cmp(&c), Ordering::Less);

src/uint/boxed/sub.rs

@@ -44,7 +44,7 @@ mod tests {
     #[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!(res, BoxedUint::max(Limb::BITS));
         assert_eq!(borrow, Limb::MAX);
     }
 

tests/boxed_uint_proptests.rs

@@ -42,7 +42,7 @@ proptest! {
         match Option::<BoxedUint>::from(a.checked_add(&b)) {
             Some(actual) => prop_assert_eq!(expected, to_biguint(&actual)),
             None => {
-                let max = BoxedUint::max(a.bits()).unwrap();
+                let max = BoxedUint::max(a.bits());
                 prop_assert!(expected > to_biguint(&max));
             }
         }