use std::io;
use super::BlobReader;
/// This implements [io::Seek] for and [io::Read] by simply skipping over some
/// bytes, keeping track of the position.
/// It fails whenever you try to seek backwards.
pub struct DumbSeeker<R: io::Read> {
r: R,
pos: u64,
}
impl<R: io::Read> DumbSeeker<R> {
pub fn new(r: R) -> Self {
DumbSeeker { r, pos: 0 }
}
}
impl<R: io::Read> io::Read for DumbSeeker<R> {
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
let bytes_read = self.r.read(buf)?;
self.pos += bytes_read as u64;
Ok(bytes_read)
}
}
impl<R: io::Read> io::Seek for DumbSeeker<R> {
fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
let absolute_offset: u64 = match pos {
io::SeekFrom::Start(start_offset) => {
if start_offset < self.pos {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
format!("can't seek backwards ({} -> {})", self.pos, start_offset),
));
} else {
start_offset
}
}
// we don't know the total size, can't support this.
io::SeekFrom::End(_end_offset) => {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
"can't seek from end",
));
}
io::SeekFrom::Current(relative_offset) => {
if relative_offset < 0 {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
"can't seek backwards relative to current position",
));
} else {
self.pos + relative_offset as u64
}
}
};
// we already know absolute_offset is larger than self.pos
debug_assert!(
absolute_offset > self.pos,
"absolute_offset is larger than self.pos"
);
// calculate bytes to skip
let bytes_to_skip: u64 = absolute_offset - self.pos;
// discard these bytes. We can't use take() as it requires ownership of
// self.r, but we only have &mut self.
let mut buf = [0; 1024];
let mut bytes_skipped: u64 = 0;
while bytes_skipped < bytes_to_skip {
let len = std::cmp::min(bytes_to_skip - bytes_skipped, buf.len() as u64);
match self.r.read(&mut buf[..len as usize]) {
Ok(0) => break,
Ok(n) => bytes_skipped += n as u64,
Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => {}
Err(e) => return Err(e),
}
}
debug_assert_eq!(bytes_to_skip, bytes_skipped);
self.pos = absolute_offset;
// return the new position from the start of the stream
Ok(absolute_offset)
}
}
/// A Cursor<Vec<u8>> can be used as a BlobReader.
impl<R: io::Read + Send + 'static> BlobReader for DumbSeeker<R> {}
