fix: apply sign extension when decoding int (#732)

Closes #730

src/buf/buf_impl.rs

@@ -66,6 +66,12 @@ macro_rules! buf_get_impl {
     }};
 }
 
+// https://en.wikipedia.org/wiki/Sign_extension
+fn sign_extend(val: u64, nbytes: usize) -> i64 {
+    let shift = (8 - nbytes) * 8;
+    (val << shift) as i64 >> shift
+}
+
 /// Read bytes from a buffer.
 ///
 /// A buffer stores bytes in memory such that read operations are infallible.
@@ -923,7 +929,7 @@ pub trait Buf {
     /// This function panics if there is not enough remaining data in `self`, or
     /// if `nbytes` is greater than 8.
     fn get_int(&mut self, nbytes: usize) -> i64 {
-        buf_get_impl!(be => self, i64, nbytes);
+        sign_extend(self.get_uint(nbytes), nbytes)
     }
 
     /// Gets a signed n-byte integer from `self` in little-endian byte order.
@@ -944,7 +950,7 @@ pub trait Buf {
     /// This function panics if there is not enough remaining data in `self`, or
     /// if `nbytes` is greater than 8.
     fn get_int_le(&mut self, nbytes: usize) -> i64 {
-        buf_get_impl!(le => self, i64, nbytes);
+        sign_extend(self.get_uint_le(nbytes), nbytes)
     }
 
     /// Gets a signed n-byte integer from `self` in native-endian byte order.

tests/test_buf.rs

@@ -36,6 +36,19 @@ fn test_get_u16() {
     assert_eq!(0x5421, buf.get_u16_le());
 }
 
+#[test]
+fn test_get_int() {
+    let mut buf = &b"\xd6zomg"[..];
+    assert_eq!(-42, buf.get_int(1));
+    let mut buf = &b"\xd6zomg"[..];
+    assert_eq!(-42, buf.get_int_le(1));
+
+    let mut buf = &b"\xfe\x1d\xc0zomg"[..];
+    assert_eq!(0xffffffffffc01dfeu64 as i64, buf.get_int_le(3));
+    let mut buf = &b"\xfe\x1d\xc0zomg"[..];
+    assert_eq!(0xfffffffffffe1dc0u64 as i64, buf.get_int(3));
+}
+
 #[test]
 #[should_panic]
 fn test_get_u16_buffer_underflow() {