add stream_size method

this is just a temporary alternative to the currently unstable
stream_len method, because i don't currently want to depend on unstable
rust versions if i don't need to.

libretrogd/src/utils/io.rs

@@ -0,0 +1,22 @@
+use std::io::{Error, SeekFrom};
+
+/// Provides a convenience method for determining the total size of a stream. This is provided
+/// as a temporary alternative to [std::io::Seek::stream_len] which is currently marked unstable.
+pub trait StreamSize {
+    fn stream_size(&mut self) -> Result<u64, std::io::Error>;
+}
+
+impl<T: std::io::Read + std::io::Seek> StreamSize for T {
+    fn stream_size(&mut self) -> Result<u64, Error> {
+        let old_pos = self.stream_position()?;
+        let len = self.seek(SeekFrom::End(0))?;
+
+        // Avoid seeking a third time when we were already at the end of the
+        // stream. The branch is usually way cheaper than a seek operation.
+        if old_pos != len {
+            self.seek(SeekFrom::Start(old_pos))?;
+        }
+
+        Ok(len)
+    }
+}

libretrogd/src/utils/mod.rs

@@ -5,6 +5,7 @@ use rand::distributions::uniform::SampleUniform;
 use rand::Rng;
 
 pub mod bytes;
+pub mod io;
 pub mod packbits;
 
 pub fn rnd_value<N: SampleUniform + PartialOrd>(low: N, high: N) -> N {