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Avoid third seek operation in Seek::stream_len when possible

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Lukas Kalbertodt 2019-03-14 13:43:17 +01:00
parent e8ee00a649
commit 598a1b4dd1
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GPG key ID: 3CBAF4153F818627
1 changed files with 12 additions and 5 deletions
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17
src/libstd/io/mod.rs
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@ -1332,10 +1332,11 @@ pub trait Seek {
/// Returns the length of this stream (in bytes).
///
/// This method is implemented using three seek operations. If this method
/// returns successfully, the seek position is unchanged (i.e. the position
/// before calling this method is the same as afterwards). However, if this
/// method returns an error, the seek position is undefined.
/// This method is implemented using up to three seek operations. If this
/// method returns successfully, the seek position is unchanged (i.e. the
/// position before calling this method is the same as afterwards).
/// However, if this method returns an error, the seek position is
/// undefined.
///
/// If you need to obtain the length of *many* streams and you don't care
/// about the seek position afterwards, you can reduce the number of seek
@ -1368,7 +1369,13 @@ pub trait Seek {
fn stream_len(&mut self) -> Result<u64> {
let old_pos = self.stream_position()?;
let len = self.seek(SeekFrom::End(0))?;
self.seek(SeekFrom::Start(old_pos))?;
// 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)
}