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lazyseek.go
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lazyseek.go
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package gateway
import (
"errors"
"fmt"
"io"
)
// The HTTP server uses seek to determine the file size. Actually _seeking_ can
// be slow so we wrap the seeker in a _lazy_ seeker.
type lazySeeker struct {
reader io.ReadSeeker
size int64
offset int64
realOffset int64
}
func (s *lazySeeker) Seek(offset int64, whence int) (int64, error) {
switch whence {
case io.SeekEnd:
return s.Seek(s.size+offset, io.SeekStart)
case io.SeekCurrent:
return s.Seek(s.offset+offset, io.SeekStart)
case io.SeekStart:
if offset < 0 {
return s.offset, errors.New("invalid seek offset")
}
s.offset = offset
return s.offset, nil
default:
return s.offset, fmt.Errorf("invalid whence: %d", whence)
}
}
func (s *lazySeeker) Read(b []byte) (int, error) {
// If we're past the end, EOF.
if s.offset >= s.size {
return 0, io.EOF
}
// actually seek
for s.offset != s.realOffset {
off, err := s.reader.Seek(s.offset, io.SeekStart)
if err != nil {
return 0, err
}
s.realOffset = off
}
off, err := s.reader.Read(b)
s.realOffset += int64(off)
s.offset += int64(off)
return off, err
}
func (s *lazySeeker) Close() error {
if closer, ok := s.reader.(io.Closer); ok {
return closer.Close()
}
return nil
}