Add a fixed signed writer to endianness trait

This eliminates another runtime check in favor of something that can be evaluated at compile-time.
tuffy · Jun 14, 2024 · bdeedfd · bdeedfd
1 parent be2db77
commit bdeedfd
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Showing 2 changed files with 38 additions and 2 deletions.
diff --git a/src/lib.rs b/src/lib.rs
@@ -359,6 +359,12 @@ pub trait Endianness: Sized {
         W: BitWrite,
         S: SignedNumeric;
 
+    /// Writes signed value to writer in this endianness
+    fn write_signed_fixed<W, const B: u32, S>(w: &mut W, value: S) -> io::Result<()>
+    where
+        W: BitWrite,
+        S: SignedNumeric;
+
     /// Reads convertable numeric value from reader in this endianness
     fn read_primitive<R, V>(r: &mut R) -> io::Result<V>
     where
@@ -518,6 +524,21 @@ impl Endianness for BigEndian {
         }
     }
 
+    fn write_signed_fixed<W, const B: u32, S>(w: &mut W, value: S) -> io::Result<()>
+    where
+        W: BitWrite,
+        S: SignedNumeric,
+    {
+        if B == S::BITS_SIZE {
+            w.write_bytes(value.to_be_bytes().as_ref())
+        } else if value.is_negative() {
+            w.write_bit(true)
+                .and_then(|()| w.write(B - 1, value.as_unsigned(B)))
+        } else {
+            w.write_bit(false).and_then(|()| w.write(B - 1, value))
+        }
+    }
+
     #[inline]
     fn read_primitive<R, V>(r: &mut R) -> io::Result<V>
     where
@@ -685,6 +706,21 @@ impl Endianness for LittleEndian {
         }
     }
 
+    fn write_signed_fixed<W, const B: u32, S>(w: &mut W, value: S) -> io::Result<()>
+    where
+        W: BitWrite,
+        S: SignedNumeric,
+    {
+        if B == S::BITS_SIZE {
+            w.write_bytes(value.to_le_bytes().as_ref())
+        } else if value.is_negative() {
+            w.write(B - 1, value.as_unsigned(B))
+                .and_then(|()| w.write_bit(true))
+        } else {
+            w.write(B - 1, value).and_then(|()| w.write_bit(false))
+        }
+    }
+
     #[inline]
     fn read_primitive<R, V>(r: &mut R) -> io::Result<V>
     where

diff --git a/src/write.rs b/src/write.rs
@@ -775,7 +775,7 @@ impl<W: io::Write, E: Endianness> BitWrite for BitWriter<W, E> {
         const {
             assert!(BITS <= S::BITS_SIZE, "excessive bits for type written");
         }
-        E::write_signed(self, BITS, value)
+        E::write_signed_fixed::<_, BITS, S>(self, value)
     }
 
     #[inline]
@@ -956,7 +956,7 @@ where
         const {
             assert!(BITS <= S::BITS_SIZE, "excessive bits for type written");
         }
-        E::write_signed(self, BITS, value)
+        E::write_signed_fixed::<_, BITS, S>(self, value)
     }
 
     #[inline]