Round up bits_precision when creating BoxedUint (#365)

fjarri · web-flow · commit 7d174f02b80e · 2023-12-02T12:03:12.000-07:00
diff --git a/src/limb/encoding.rs b/src/limb/encoding.rs
@@ -34,16 +34,30 @@ impl Encoding for Limb {
 impl Limb {
     /// Decode limb from a big endian byte slice.
     ///
-    /// Panics if the slice is not the same size as [`Limb::Repr`].
+    /// Panics if the slice is larger than [`Limb::Repr`].
     pub(crate) fn from_be_slice(bytes: &[u8]) -> Self {
-        Self::from_be_bytes(bytes.try_into().expect("slice not limb-sized"))
+        let mut repr = Self::ZERO.to_be_bytes();
+        let repr_len = repr.len();
+        assert!(
+            bytes.len() <= repr_len,
+            "The given slice is larger than the limb size"
+        );
+        repr[(repr_len - bytes.len())..].copy_from_slice(bytes);
+        Self::from_be_bytes(repr)
     }
 
     /// Decode limb from a little endian byte slice.
     ///
     /// Panics if the slice is not the same size as [`Limb::Repr`].
     pub(crate) fn from_le_slice(bytes: &[u8]) -> Self {
-        Self::from_le_bytes(bytes.try_into().expect("slice not limb-sized"))
+        let mut repr = Self::ZERO.to_le_bytes();
+        let repr_len = repr.len();
+        assert!(
+            bytes.len() <= repr_len,
+            "The given slice is larger than the limb size"
+        );
+        repr[..bytes.len()].copy_from_slice(bytes);
+        Self::from_le_bytes(repr)
     }
 }
 
diff --git a/src/macros.rs b/src/macros.rs
@@ -59,7 +59,7 @@ macro_rules! const_assert_ne {
 #[macro_export]
 macro_rules! nlimbs {
     ($bits:expr) => {
-        $bits / $crate::Limb::BITS as usize
+        (($bits + $crate::Limb::BITS - 1) / $crate::Limb::BITS) as usize
     };
 }
 
diff --git a/src/uint/boxed.rs b/src/uint/boxed.rs
@@ -50,6 +50,10 @@ pub struct BoxedUint {
 }
 
 impl BoxedUint {
+    fn limbs_for_precision(at_least_bits_precision: u32) -> usize {
+        ((at_least_bits_precision + Limb::BITS - 1) / Limb::BITS) as usize
+    }
+
     /// Get the value `0` represented as succinctly as possible.
     pub fn zero() -> Self {
         Self {
@@ -59,15 +63,9 @@ impl BoxedUint {
 
     /// Get the value `0` with the given number of bits of precision.
     ///
-    /// Panics if the precision is not a multiple of [`Limb::BITS`].
-    pub fn zero_with_precision(bits_precision: u32) -> Self {
-        assert_eq!(
-            bits_precision % Limb::BITS,
-            0,
-            "precision is not a multiple of limb size"
-        );
-
-        vec![Limb::ZERO; (bits_precision / Limb::BITS) as usize].into()
+    /// `at_least_bits_precision` is rounded up to a multiple of [`Limb::BITS`].
+    pub fn zero_with_precision(at_least_bits_precision: u32) -> Self {
+        vec![Limb::ZERO; Self::limbs_for_precision(at_least_bits_precision)].into()
     }
 
     /// Get the value `1`, represented as succinctly as possible.
@@ -79,11 +77,9 @@ impl BoxedUint {
 
     /// Get the value `1` with the given number of bits of precision.
     ///
-    /// Panics if the precision is not at least [`Limb::BITS`] or if it is not
-    /// a multiple thereof.
-    pub fn one_with_precision(bits_precision: u32) -> Self {
-        assert!(bits_precision >= Limb::BITS, "precision too small");
-        let mut ret = Self::zero_with_precision(bits_precision);
+    /// `at_least_bits_precision` is rounded up to a multiple of [`Limb::BITS`].
+    pub fn one_with_precision(at_least_bits_precision: u32) -> Self {
+        let mut ret = Self::zero_with_precision(at_least_bits_precision);
         ret.limbs[0] = Limb::ONE;
         ret
     }
@@ -125,17 +121,12 @@ impl BoxedUint {
         !self.is_odd()
     }
 
-    /// Get the maximum value for a given number of bits of precision.
+    /// Get the maximum value for a `BoxedUint` created with `at_least_bits_precision`
+    /// precision bits requested.
     ///
-    /// Panics if the precision is not a multiple of [`Limb::BITS`].
-    pub fn max(bits_precision: u32) -> Self {
-        assert_eq!(
-            bits_precision % Limb::BITS,
-            0,
-            "precision is not a multiple of limb size"
-        );
-
-        vec![Limb::MAX; (bits_precision / Limb::BITS) as usize].into()
+    /// That is, returns the value `2^self.bits_precision() - 1`.
+    pub fn max(at_least_bits_precision: u32) -> Self {
+        vec![Limb::MAX; Self::limbs_for_precision(at_least_bits_precision)].into()
     }
 
     /// Create a [`BoxedUint`] from an array of [`Word`]s (i.e. word-sized unsigned
@@ -235,25 +226,21 @@ impl BoxedUint {
 
     /// Widen this type's precision to the given number of bits.
     ///
-    /// Panics if `bits_precision` is not a multiple of `Limb::BITS` or smaller than the current
-    /// precision.
-    pub fn widen(&self, bits_precision: u32) -> BoxedUint {
-        assert!(bits_precision % Limb::BITS == 0);
-        assert!(bits_precision >= self.bits_precision());
+    /// Panics if `at_least_bits_precision` is smaller than the current precision.
+    pub fn widen(&self, at_least_bits_precision: u32) -> BoxedUint {
+        assert!(at_least_bits_precision >= self.bits_precision());
 
-        let mut ret = BoxedUint::zero_with_precision(bits_precision);
+        let mut ret = BoxedUint::zero_with_precision(at_least_bits_precision);
         ret.limbs[..self.nlimbs()].copy_from_slice(&self.limbs);
         ret
     }
 
     /// Shortens this type's precision to the given number of bits.
     ///
-    /// Panics if `bits_precision` is not a multiple of `Limb::BITS` or smaller than the current
-    /// precision.
-    pub fn shorten(&self, bits_precision: u32) -> BoxedUint {
-        assert!(bits_precision % Limb::BITS == 0);
-        assert!(bits_precision <= self.bits_precision());
-        let mut ret = BoxedUint::zero_with_precision(bits_precision);
+    /// Panics if `at_least_bits_precision` is larger than the current precision.
+    pub fn shorten(&self, at_least_bits_precision: u32) -> BoxedUint {
+        assert!(at_least_bits_precision <= self.bits_precision());
+        let mut ret = BoxedUint::zero_with_precision(at_least_bits_precision);
         let nlimbs = ret.nlimbs();
         ret.limbs.copy_from_slice(&self.limbs[..nlimbs]);
         ret
diff --git a/src/uint/boxed/encoding.rs b/src/uint/boxed/encoding.rs
@@ -8,18 +8,21 @@ use core::fmt;
 /// Decoding errors for [`BoxedUint`].
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum DecodeError {
-    /// Input is not a valid size.
+    /// Input size is too small to fit in the given precision.
     InputSize,
 
-    /// Precision is not a multiple of [`Limb::BYTES`].
+    /// The deserialized number is larger than the given precision.
     Precision,
 }
 
 impl fmt::Display for DecodeError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::InputSize => write!(f, "input is not a valid size"),
-            Self::Precision => write!(f, "precision is not a multiple of the word size"),
+            Self::InputSize => write!(f, "input size is too small to fit in the given precision"),
+            Self::Precision => write!(
+                f,
+                "the deserialized number is larger than the given precision"
+            ),
         }
     }
 }
@@ -31,51 +34,53 @@ impl BoxedUint {
     /// Create a new [`BoxedUint`] from the provided big endian bytes.
     ///
     /// The `bits_precision` argument represents the precision of the resulting integer, which is
-    /// fixed as this type is not arbitrary-precision. It MUST be a multiple of the limb size, i.e.
-    /// [`Limb::BITS`], or otherwise this function will return [`DecodeError::Precision`].
+    /// fixed as this type is not arbitrary-precision.
+    /// The new [`BoxedUint`] will be created with `bits_precision`
+    /// rounded up to a multiple of [`Limb::BITS`].
     ///
-    /// If the length of `bytes` (when interpreted as bits) is larger than `bits_precision`, this
-    /// function will return [`DecodeError::InputSize`].
+    /// If the length of `bytes` is larger than `bits_precision` (rounded up to a multiple of 8)
+    /// this function will return [`DecodeError::InputSize`].
+    /// If the size of the decoded integer is larger than `bits_precision`,
+    /// this function will return [`DecodeError::Precision`].
     pub fn from_be_slice(bytes: &[u8], bits_precision: u32) -> Result<Self, DecodeError> {
         if bytes.is_empty() && bits_precision == 0 {
             return Ok(Self::zero());
         }
 
-        if bits_precision % Limb::BITS != 0 {
-            return Err(DecodeError::Precision);
-        }
-
-        if bytes.len() % Limb::BYTES != 0 || bytes.len() * 8 > bits_precision as usize {
+        if bytes.len() > (bits_precision as usize + 7) / 8 {
             return Err(DecodeError::InputSize);
         }
 
         let mut ret = Self::zero_with_precision(bits_precision);
 
-        for (chunk, limb) in bytes.chunks(Limb::BYTES).rev().zip(ret.limbs.iter_mut()) {
+        for (chunk, limb) in bytes.rchunks(Limb::BYTES).zip(ret.limbs.iter_mut()) {
             *limb = Limb::from_be_slice(chunk);
         }
 
+        if bits_precision < ret.bits() {
+            return Err(DecodeError::Precision);
+        }
+
         Ok(ret)
     }
 
     /// Create a new [`BoxedUint`] from the provided little endian bytes.
     ///
     /// The `bits_precision` argument represents the precision of the resulting integer, which is
-    /// fixed as this type is not arbitrary-precision. It MUST be a multiple of the limb size, i.e.
-    /// [`Limb::BITS`], or otherwise this function will return [`DecodeError::Precision`].
+    /// fixed as this type is not arbitrary-precision.
+    /// The new [`BoxedUint`] will be created with `bits_precision`
+    /// rounded up to a multiple of [`Limb::BITS`].
     ///
-    /// If the length of `bytes` (when interpreted as bits) is larger than `bits_precision`, this
-    /// function will return [`DecodeError::InputSize`].
+    /// If the length of `bytes` is larger than `bits_precision` (rounded up to a multiple of 8)
+    /// this function will return [`DecodeError::InputSize`].
+    /// If the size of the decoded integer is larger than `bits_precision`,
+    /// this function will return [`DecodeError::Precision`].
     pub fn from_le_slice(bytes: &[u8], bits_precision: u32) -> Result<Self, DecodeError> {
         if bytes.is_empty() && bits_precision == 0 {
             return Ok(Self::zero());
         }
 
-        if bits_precision % Limb::BITS != 0 {
-            return Err(DecodeError::Precision);
-        }
-
-        if bytes.len() % Limb::BYTES != 0 || bytes.len() * 8 > bits_precision as usize {
+        if bytes.len() > (bits_precision as usize + 7) / 8 {
             return Err(DecodeError::InputSize);
         }
 
@@ -85,6 +90,10 @@ impl BoxedUint {
             *limb = Limb::from_le_slice(chunk);
         }
 
+        if bits_precision < ret.bits() {
+            return Err(DecodeError::Precision);
+        }
+
         Ok(ret)
     }
 
@@ -186,19 +195,39 @@ mod tests {
     }
 
     #[test]
+    #[cfg(target_pointer_width = "32")]
     fn from_be_slice_not_word_sized() {
-        let bytes = hex!("00112233445566778899aabbccddee");
+        let bytes = hex!("112233445566778899aabbccddeeff");
+        let n = BoxedUint::from_be_slice(&bytes, 127).unwrap();
         assert_eq!(
-            BoxedUint::from_be_slice(&bytes, 128),
-            Err(DecodeError::InputSize)
+            n.as_limbs(),
+            &[
+                Limb(0xccddeeff),
+                Limb(0x8899aabb),
+                Limb(0x44556677),
+                Limb(0x00112233)
+            ]
         );
+        assert_eq!(n.bits_precision(), 128);
     }
 
     #[test]
-    fn from_be_slice_bad_precision() {
-        let bytes = hex!("00112233445566778899aabbccddeeff");
+    #[cfg(target_pointer_width = "64")]
+    fn from_be_slice_not_word_sized() {
+        let bytes = hex!("112233445566778899aabbccddeeff");
+        let n = BoxedUint::from_be_slice(&bytes, 127).unwrap();
         assert_eq!(
-            BoxedUint::from_be_slice(&bytes, 127),
+            n.as_limbs(),
+            &[Limb(0x8899aabbccddeeff), Limb(0x0011223344556677)]
+        );
+        assert_eq!(n.bits_precision(), 128);
+    }
+
+    #[test]
+    fn from_be_slice_non_multiple_precision() {
+        let bytes = hex!("0f112233445566778899aabbccddeeff");
+        assert_eq!(
+            BoxedUint::from_be_slice(&bytes, 121),
             Err(DecodeError::Precision)
         );
     }
@@ -259,19 +288,39 @@ mod tests {
     }
 
     #[test]
+    #[cfg(target_pointer_width = "32")]
     fn from_le_slice_not_word_sized() {
         let bytes = hex!("ffeeddccbbaa998877665544332211");
+        let n = BoxedUint::from_le_slice(&bytes, 127).unwrap();
         assert_eq!(
-            BoxedUint::from_be_slice(&bytes, 128),
-            Err(DecodeError::InputSize)
+            n.as_limbs(),
+            &[
+                Limb(0xccddeeff),
+                Limb(0x8899aabb),
+                Limb(0x44556677),
+                Limb(0x00112233)
+            ]
         );
+        assert_eq!(n.bits_precision(), 128);
     }
 
     #[test]
-    fn from_le_slice_bad_precision() {
-        let bytes = hex!("ffeeddccbbaa99887766554433221100");
+    #[cfg(target_pointer_width = "64")]
+    fn from_le_slice_not_word_sized() {
+        let bytes = hex!("ffeeddccbbaa998877665544332211");
+        let n = BoxedUint::from_le_slice(&bytes, 127).unwrap();
+        assert_eq!(
+            n.as_limbs(),
+            &[Limb(0x8899aabbccddeeff), Limb(0x0011223344556677)]
+        );
+        assert_eq!(n.bits_precision(), 128);
+    }
+
+    #[test]
+    fn from_le_slice_non_multiple_precision() {
+        let bytes = hex!("ffeeddccbbaa998877665544332211f0");
         assert_eq!(
-            BoxedUint::from_le_slice(&bytes, 127),
+            BoxedUint::from_le_slice(&bytes, 121),
             Err(DecodeError::Precision)
         );
     }
diff --git a/src/uint/boxed/rand.rs b/src/uint/boxed/rand.rs
diff --git a/src/uint/rand.rs b/src/uint/rand.rs

Original file line number	Diff line number	Diff line change
`@@ -59,7 +59,7 @@ macro_rules! const_assert_ne {`
`59`	`59`	`#[macro_export]`
`60`	`60`	`macro_rules! nlimbs {`
`61`	`61`	`($bits:expr) => {`
`62`		`- $bits / $crate::Limb::BITS as usize`
	`62`	`+ (($bits + $crate::Limb::BITS - 1) / $crate::Limb::BITS) as usize`
`63`	`63`	`};`
`64`	`64`	`}`
`65`	`65`