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reader_support.go
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/
reader_support.go
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// MIT license · Gregor Noczinski · gregor [at] noczinski [dot] eu · 12/2021
package jsonlex
import (
"fmt"
"io"
)
// EnsureAtLeastSingleByteUnreadableReader will provide an instance of the
// provided io.Reader which can at least unread a single byte.
func EnsureAtLeastSingleByteUnreadableReader(r io.Reader) UnreadableReader {
if ur, ok := r.(UnreadableReader); ok {
return ur
}
return &singleByteUnreadableReader{
delegate: r,
}
}
type singleByteUnreadableReader struct {
delegate io.Reader
lastByte byte
state singleByteUnreadableReaderState
}
func (r *singleByteUnreadableReader) Read(p []byte) (int, error) {
if len(p) == 0 {
return 0, nil
}
switch r.state {
case singleByteUnreadableReaderStateBuffered, singleByteUnreadableReaderStateEmpty:
n, err := r.delegate.Read(p)
if n > 0 {
r.lastByte = p[n-1]
r.state = singleByteUnreadableReaderStateBuffered
}
return n, err
case singleByteUnreadableReaderStateRewind:
p[0] = r.lastByte
r.state = singleByteUnreadableReaderStateEmpty
if len(p) == 1 {
return 1, nil
}
n, err := r.delegate.Read(p[1:])
return n + 1, err
default:
panic(fmt.Sprintf("unknown state: %d", r.state))
}
}
func (r *singleByteUnreadableReader) UnreadByte() error {
switch r.state {
case singleByteUnreadableReaderStateBuffered:
r.state = singleByteUnreadableReaderStateRewind
return nil
case singleByteUnreadableReaderStateEmpty, singleByteUnreadableReaderStateRewind:
return io.ErrShortBuffer
default:
panic(fmt.Sprintf("unknown state: %d", r.state))
}
}
type singleByteUnreadableReaderState uint8
const (
singleByteUnreadableReaderStateEmpty singleByteUnreadableReaderState = iota
singleByteUnreadableReaderStateBuffered
singleByteUnreadableReaderStateRewind
)