combine signed reading

src/is_signed.rs

@@ -0,0 +1,25 @@
+pub trait IsSigned {
+    fn is_signed() -> bool;
+}
+
+macro_rules! impl_is_signed {
+    ($type:ty, $signed:expr) => {
+        impl IsSigned for $type {
+            #[inline]
+            fn is_signed() -> bool {
+                $signed
+            }
+        }
+    }
+}
+
+impl_is_signed!(u8, false);
+impl_is_signed!(u16, false);
+impl_is_signed!(u32, false);
+impl_is_signed!(u64, false);
+impl_is_signed!(usize, false);
+impl_is_signed!(i8, true);
+impl_is_signed!(i16, true);
+impl_is_signed!(i32, true);
+impl_is_signed!(i64, true);
+impl_is_signed!(isize, true);

src/lib.rs

@@ -2,13 +2,15 @@
 
 extern crate test;
 
-use num_traits::{PrimInt, Signed};
+use is_signed::IsSigned;
+use num_traits::{PrimInt};
 use std::cmp::min;
 use std::mem::size_of;
 use std::ops::BitOrAssign;
 
 #[cfg(test)]
 mod tests;
+mod is_signed;
 
 #[derive(Debug, PartialEq, Copy, Clone)]
 pub enum ReadError {
@@ -66,7 +68,7 @@ impl<'a> BitBuffer<'a> {
         self.byte_len
     }
 
-    pub fn read_usize(&self, position: usize, count: usize) -> Result<usize> {
+    fn read_usize(&self, position: usize, count: usize) -> Result<usize> {
         if position + count > self.bit_len {
             return Err(ReadError::NotEnoughData {
                 requested: count,
@@ -103,16 +105,26 @@ impl<'a> BitBuffer<'a> {
     }
 
     pub fn read<T>(&self, position: usize, count: usize) -> Result<T>
-        where T: PrimInt + BitOrAssign
+        where T: PrimInt + BitOrAssign + IsSigned
     {
-        if size_of::<T>() * 8 < count {
+        let value = {
+            let type_bit_size = size_of::<T>() * 8;
+
+            if type_bit_size < count {
                 return Err(ReadError::TooManyBits {
                     requested: count,
                     max: size_of::<T>() * 8,
                 });
             }
+
             if size_of::<usize>() > size_of::<T>() || (count / 8) < size_of::<usize>() {
-            Ok(T::from(self.read_usize(position, count)?).unwrap())
+                let raw = self.read_usize(position, count)?;
+                let max_signed_value = (1 << (type_bit_size - 1)) - 1;
+                if T::is_signed() && raw > max_signed_value {
+                    return Ok(T::zero() - T::from(raw & max_signed_value).unwrap())
+                } else {
+                    T::from(raw).unwrap()
+                }
             } else {
                 let mut bits_left = count;
                 let mut partial = T::zero();
@@ -127,17 +139,18 @@ impl<'a> BitBuffer<'a> {
                     bits_left -= read;
                 }
 
-            Ok(partial)
+                partial
-        }
             }
+        };
 
-    pub fn read_signed<T>(&self, position: usize, count: usize) -> Result<T>
+        if T::is_signed() {
-        where T: PrimInt + BitOrAssign + Signed
-    {
-        let value = self.read::<T>(position, count)?;
-
             let sign_bit = value >> (count - 1) & T::one();
-        Ok(value | (T::zero() - sign_bit) ^ ((T::one() << count) - T::one()))
+            let absolute_value = value & !(T::max_value() << (count - 1));
+            let sign = T::one() - sign_bit - sign_bit;
+            Ok(absolute_value * sign)
+        } else {
+            Ok(value)
+        }
     }
 
     pub fn read_bytes(&self, position: usize, byte_count: usize) -> Result<Vec<u8>> {

src/tests.rs

@@ -2,7 +2,6 @@ use std::fs;
 use super::*;
 use test::Bencher;
 
-
 const BYTES: &'static[u8] = &[
     0b1011_0101, 0b0110_1010, 0b1010_1100, 0b1001_1001,
     0b1001_1001, 0b1001_1001, 0b1001_1001, 0b1110_0111,
@@ -17,6 +16,7 @@ fn read_u8() {
     assert_eq!(buffer.read::<u8>(0, 1).unwrap(), 0b1);
     assert_eq!(buffer.read::<u8>(1, 1).unwrap(), 0b0);
     assert_eq!(buffer.read::<u8>(2, 2).unwrap(), 0b01);
+    assert_eq!(buffer.read::<u8>(0, 3).unwrap(), 0b101);
     assert_eq!(buffer.read::<u8>(7, 5).unwrap(), 0b1010_1);
     assert_eq!(buffer.read::<u8>(6, 5).unwrap(), 0b010_10);
 }
@@ -43,6 +43,14 @@ fn read_u64() {
     assert_eq!(buffer.read::<u64>(6, 60).unwrap(), 0b00_1110_01111001_1001_1001_1001_1001_1001_1001_1001_1010_1100_0110_1010_10);
 }
 
+#[test]
+fn read_i8() {
+    let buffer = BitBuffer::from_padded_slice(BYTES, 12);
+
+    assert_eq!(buffer.read::<i8>(0, 3).unwrap(), -0b1);
+    assert_eq!(buffer.read::<i8>(0, 8).unwrap(), -0b011_0101);
+}
+
 fn read_perf(buffer: BitBuffer) -> u16 {
     let size = 5;
     let mut pos = 0;