Uint: const fn encoders (#453)

Adds `const fn` encoders which can serialize to big or little endian
byte arrays.

Author: tarcieri

src/uint/encoding.rs

@@ -7,7 +7,10 @@ mod der;
 mod rlp;
 
 use super::Uint;
-use crate::{Encoding, Limb, Word};
+use crate::{Limb, Word};
+
+#[cfg(feature = "generic-array")]
+use crate::Encoding;
 
 impl<const LIMBS: usize> Uint<LIMBS> {
     /// Create a new [`Uint`] from the provided big endian bytes.
@@ -124,6 +127,7 @@ impl<const LIMBS: usize> Uint<LIMBS> {
 
     /// Serialize this [`Uint`] as big-endian, writing it into the provided
     /// byte slice.
+    #[cfg(feature = "generic-array")]
     #[inline]
     pub(crate) fn write_be_bytes(&self, out: &mut [u8]) {
         debug_assert_eq!(out.len(), Limb::BYTES * LIMBS);
@@ -141,6 +145,7 @@ impl<const LIMBS: usize> Uint<LIMBS> {
 
     /// Serialize this [`Uint`] as little-endian, writing it into the provided
     /// byte slice.
+    #[cfg(feature = "generic-array")]
     #[inline]
     pub(crate) fn write_le_bytes(&self, out: &mut [u8]) {
         debug_assert_eq!(out.len(), Limb::BYTES * LIMBS);
@@ -156,6 +161,58 @@ impl<const LIMBS: usize> Uint<LIMBS> {
     }
 }
 
+/// Encode a [`Uint`] to a big endian byte array of the given size.
+pub(crate) const fn uint_to_be_bytes<const LIMBS: usize, const BYTES: usize>(
+    uint: &Uint<LIMBS>,
+) -> [u8; BYTES] {
+    if BYTES != LIMBS * Limb::BYTES {
+        panic!("BYTES != LIMBS * Limb::BYTES");
+    }
+
+    let mut ret = [0u8; BYTES];
+    let mut i = 0;
+
+    while i < LIMBS {
+        let limb_bytes = uint.limbs[LIMBS - i - 1].0.to_be_bytes();
+        let mut j = 0;
+
+        while j < Limb::BYTES {
+            ret[i * Limb::BYTES + j] = limb_bytes[j];
+            j += 1;
+        }
+
+        i += 1;
+    }
+
+    ret
+}
+
+/// Encode a [`Uint`] to a little endian byte array of the given size.
+pub(crate) const fn uint_to_le_bytes<const LIMBS: usize, const BYTES: usize>(
+    uint: &Uint<LIMBS>,
+) -> [u8; BYTES] {
+    if BYTES != LIMBS * Limb::BYTES {
+        panic!("BYTES != LIMBS * Limb::BYTES");
+    }
+
+    let mut ret = [0u8; BYTES];
+    let mut i = 0;
+
+    while i < LIMBS {
+        let limb_bytes = uint.limbs[i].0.to_le_bytes();
+        let mut j = 0;
+
+        while j < Limb::BYTES {
+            ret[i * Limb::BYTES + j] = limb_bytes[j];
+            j += 1;
+        }
+
+        i += 1;
+    }
+
+    ret
+}
+
 /// Decode a single nibble of upper or lower hex
 #[inline(always)]
 const fn decode_nibble(src: u8) -> u16 {

src/uint/macros.rs

@@ -34,7 +34,14 @@ macro_rules! impl_uint_aliases {
         $(
             #[doc = $doc]
             #[doc="unsigned big integer."]
-            pub type $name = Uint<{nlimbs!($bits)}>;
+            pub type $name = Uint<{ nlimbs!($bits) }>;
+
+            impl $name {
+                /// Serialize as big endian bytes.
+                pub const fn to_be_bytes(&self) -> [u8; $bits / 8] {
+                    encoding::uint_to_be_bytes::<{ nlimbs!($bits) }, { $bits / 8 }>(self)
+                }
+            }
 
             impl Encoding for $name {
                 type Repr = [u8; $bits / 8];
@@ -51,16 +58,12 @@ macro_rules! impl_uint_aliases {
 
                 #[inline]
                 fn to_be_bytes(&self) -> Self::Repr {
-                    let mut result = [0u8; $bits / 8];
-                    self.write_be_bytes(&mut result);
-                    result
+                    encoding::uint_to_be_bytes(self)
                 }
 
                 #[inline]
                 fn to_le_bytes(&self) -> Self::Repr {
-                    let mut result = [0u8; $bits / 8];
-                    self.write_le_bytes(&mut result);
-                    result
+                    encoding::uint_to_le_bytes(self)
                 }
             }