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// Package delta implements the Delta filter.
package delta
import (
"errors"
"fmt"
"io"
)
type readCloser struct {
rc io.ReadCloser
state [stateSize]byte
delta int
}
const (
stateSize = 256
)
var (
errAlreadyClosed = errors.New("delta: already closed")
errNeedOneReader = errors.New("delta: need exactly one reader")
errInsufficientProperties = errors.New("delta: not enough properties")
)
func (rc *readCloser) Close() error {
if rc.rc == nil {
return errAlreadyClosed
}
if err := rc.rc.Close(); err != nil {
return fmt.Errorf("delta: error closing: %w", err)
}
rc.rc = nil
return nil
}
func (rc *readCloser) Read(p []byte) (int, error) {
if rc.rc == nil {
return 0, errAlreadyClosed
}
n, err := rc.rc.Read(p)
if err != nil && !errors.Is(err, io.EOF) {
return n, fmt.Errorf("delta: error reading: %w", err)
}
var (
buffer [stateSize]byte
j int
)
copy(buffer[:], rc.state[:rc.delta])
for i := 0; i < n; {
for j = 0; j < rc.delta && i < n; i++ {
p[i] = buffer[j] + p[i]
buffer[j] = p[i]
j++
}
}
if j == rc.delta {
j = 0
}
copy(rc.state[:], buffer[j:rc.delta])
copy(rc.state[rc.delta-j:], buffer[:j])
return n, nil
}
// NewReader returns a new Delta io.ReadCloser.
func NewReader(p []byte, _ uint64, readers []io.ReadCloser) (io.ReadCloser, error) {
if len(readers) != 1 {
return nil, errNeedOneReader
}
if len(p) != 1 {
return nil, errInsufficientProperties
}
return &readCloser{
rc: readers[0],
delta: int(p[0] + 1),
}, nil
}
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