Clean up the API for reading bits

src/lib.rs

@@ -11,6 +11,7 @@ pub enum KError<'a> {
     Encoding { expected: &'static str },
     MissingRoot,
     MissingParent,
+    ReadBitsTooLarge { requested: usize },
     UnexpectedContents { actual: &'a [u8] },
     UnknownVariant(i64),
 }
@@ -76,7 +77,7 @@ pub trait KStream {
     fn read_f8le(&self) -> KResult<f64>;
 
     fn align_to_byte(&self) -> KResult<()>;
-    fn read_bits_int(&self, n: u32) -> KResult<u64>;
+    fn read_bits_int(&self, n: usize) -> KResult<u64>;
 
     fn read_bytes(&self, len: usize) -> KResult<&[u8]>;
     fn read_bytes_full(&self) -> KResult<&[u8]>;
@@ -243,9 +244,13 @@ impl<'a> KStream for BytesReader<'a> {
         Ok(())
     }
 
-    // TODO: Clean up the casting nightmare
-    fn read_bits_int(&self, n: u32) -> KResult<u64> {
-        let bits_needed = n as i64 - self.state.borrow().bits_left;
+    fn read_bits_int(&self, n: usize) -> KResult<u64> {
+        if n > 64 {
+            return Err(KError::ReadBitsTooLarge { requested: n });
+        }
+
+        let n = n as i64;
+        let bits_needed = n - self.state.borrow().bits_left;
         if bits_needed > 0 {
             // 1 bit => 1 byte
             // 8 bits => 1 byte
@@ -253,7 +258,6 @@ impl<'a> KStream for BytesReader<'a> {
             let bytes_needed = ((bits_needed - 1) / 8) + 1;
             // Need to be careful here, because `read_bytes` will borrow our state as mutable,
             // which panics if we're currently holding a borrow
-            // TODO: Return error for bytes_needed > 8
             let buf = self.read_bytes(bytes_needed as usize)?;
             let mut inner = self.state.borrow_mut();
             for b in buf {
@@ -265,12 +269,12 @@ impl<'a> KStream for BytesReader<'a> {
 
         let mut inner = self.state.borrow_mut();
         let mut mask = (1u64 << n) - 1;
-        let shift_bits = inner.bits_left - n as i64;
+        let shift_bits = inner.bits_left - n;
         mask <<= shift_bits;
 
         let result: u64 = (inner.bits & mask) >> shift_bits;
 
-        inner.bits_left -= n as i64;
+        inner.bits_left -= n;
         mask = (1u64 << inner.bits_left) - 1;
         inner.bits &= mask;
 
@@ -364,4 +368,12 @@ mod tests {
 
         assert_eq!(reader.read_bits_int(9).unwrap(), 3);
     }
+
+    #[test]
+    fn read_bits_too_large() {
+        let b: Vec<u8> = vec![1, 2, 3, 4, 5, 6, 7, 8, 9];
+        let mut reader = BytesReader::new(&b[..]);
+
+        assert_eq!(reader.read_bits_int(65).unwrap_err(), KError::ReadBitsTooLarge { requested: 65 })
+    }
 }