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peekreader.go
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/
peekreader.go
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// seehuhn.de/go/postscript - a rudimentary PostScript interpreter
// Copyright (C) 2023 Jochen Voss <voss@seehuhn.de>
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.
package type1
import "io"
func peek(r io.Reader, n int) ([]byte, io.Reader, error) {
buf := make([]byte, n)
if r, ok := r.(io.ReadSeeker); ok {
pos, err := r.Seek(0, io.SeekCurrent)
if err != nil {
return nil, nil, err
}
k, err := io.ReadFull(r, buf)
if err != nil && err != io.ErrUnexpectedEOF && err != io.EOF {
return nil, nil, err
}
_, err = r.Seek(pos, io.SeekStart)
if err != nil {
return nil, nil, err
}
return buf[:k], r, nil
}
k, err := io.ReadFull(r, buf)
if err != nil && err != io.ErrUnexpectedEOF && err != io.EOF {
return nil, nil, err
}
return buf[:k], &peekReader{r: r, buf: buf[:k]}, nil
}
type peekReader struct {
r io.Reader
buf []byte
}
func (r *peekReader) Read(b []byte) (n int, err error) {
if len(r.buf) == 0 {
return r.r.Read(b)
}
k := len(b)
if k > len(r.buf) {
k = len(r.buf)
}
copy(b, r.buf[:k])
r.buf = r.buf[k:]
return k, nil
}