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package gitio
import (
"fmt"
"io"
)
// TrailerReader models the behavior of Git hashfiles where the last N
// bytes of the underlying reader are not part of the content.
// TrailerReader acts like an io.Reader but will always hold back the
// last N bytes. Once the underlying reader has reached EOF, the trailer
// (the last N bytes) can be retrieved with the Trailer() method.
type TrailerReader struct {
r io.Reader
start, end int
trailerSize int
buf []byte
atEOF bool
}
// NewTrailerReader returns a new TrailerReader. The returned
// TrailerReader will never return the last trailerSize bytes of r; to
// get to those bytes, first read the TrailerReader to EOF and then call
// Trailer().
func NewTrailerReader(r io.Reader, trailerSize int) *TrailerReader {
const bufSize = 8192
if trailerSize >= bufSize {
panic("trailerSize too large for TrailerReader")
}
return &TrailerReader{
r: r,
trailerSize: trailerSize,
buf: make([]byte, bufSize),
}
}
// Trailer yields the last trailerSize bytes of the underlying reader of
// tr. If the underlying reader has not reached EOF yet Trailer will
// return an error.
func (tr *TrailerReader) Trailer() ([]byte, error) {
bufLen := tr.end - tr.start
if !tr.atEOF || bufLen > tr.trailerSize {
return nil, fmt.Errorf("cannot get trailer before reader has reached EOF")
}
if bufLen < tr.trailerSize {
return nil, fmt.Errorf("not enough bytes to yield trailer")
}
return tr.buf[tr.end-tr.trailerSize : tr.end], nil
}
func (tr *TrailerReader) Read(p []byte) (int, error) {
if bufLen := tr.end - tr.start; !tr.atEOF && bufLen <= tr.trailerSize {
copy(tr.buf, tr.buf[tr.start:tr.end])
tr.start = 0
tr.end = bufLen
n, err := tr.r.Read(tr.buf[tr.end:])
if err != nil {
if err != io.EOF {
return 0, err
}
tr.atEOF = true
}
tr.end += n
}
if tr.end-tr.start <= tr.trailerSize {
if tr.atEOF {
return 0, io.EOF
}
return 0, nil
}
n := copy(p, tr.buf[tr.start:tr.end-tr.trailerSize])
tr.start += n
return n, nil
}
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