fst 0.1.9

Use finite state transducers to compactly represents sets or maps of many strings (> 1 billion is possible).
Documentation 
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use std::io;

/// Wraps any writer and counts bytes written.
pub struct CountingWriter<W> {
    wtr: W,
    cnt: u64,
}

impl<W: io::Write> CountingWriter<W> {
    /// Wrap the given writer with a counter.
    pub fn new(wtr: W) -> CountingWriter<W> {
        CountingWriter {
            wtr: wtr,
            cnt: 0,
        }
    }

    /// Return the total number of bytes written to the underlying writer.
    ///
    /// The count returned is the sum of all counts resulting from a call
    /// to `write`.
    pub fn count(&self) -> u64 {
        self.cnt
    }

    /// Unwrap the counting writer and return the inner writer.
    pub fn into_inner(self) -> W {
        self.wtr
    }
}

impl<W: io::Write> io::Write for CountingWriter<W> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let n = try!(self.wtr.write(buf));
        self.cnt += n as u64;
        Ok(n)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.wtr.flush()
    }
}

#[cfg(test)]
mod tests {
    use std::io::Write;
    use super::CountingWriter;

    #[test]
    fn counts_bytes() {
        let mut wtr = CountingWriter::new(vec![]);
        wtr.write_all(b"foobar").unwrap();
        assert_eq!(wtr.count(), 6);
    }
}