perkeep/pkg/types/fakeseeker.go

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/*
Copyright 2014 The Camlistore Authors
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
package types
import (
"errors"
"fmt"
"io"
"os"
)
// fakeSeeker can seek to the ends but any read not at the current
// position will fail.
type fakeSeeker struct {
r io.Reader
size int64
fakePos int64
realPos int64
}
// NewFakeSeeker returns a ReadSeeker that can pretend to Seek (based
// on the provided total size of the reader's content), but any reads
// will fail if the fake seek position doesn't match reality.
func NewFakeSeeker(r io.Reader, size int64) io.ReadSeeker {
return &fakeSeeker{r: r, size: size}
}
func (fs *fakeSeeker) Seek(offset int64, whence int) (int64, error) {
var newo int64
switch whence {
default:
return 0, errors.New("invalid whence")
case os.SEEK_SET:
newo = offset
case os.SEEK_CUR:
newo = fs.fakePos + offset
case os.SEEK_END:
newo = fs.size + offset
}
if newo < 0 {
return 0, errors.New("negative seek")
}
fs.fakePos = newo
return newo, nil
}
func (fs *fakeSeeker) Read(p []byte) (n int, err error) {
if fs.fakePos != fs.realPos {
return 0, fmt.Errorf("attempt to read from fake seek offset %d; real offset is %d", fs.fakePos, fs.realPos)
}
n, err = fs.r.Read(p)
fs.fakePos += int64(n)
fs.realPos += int64(n)
return
}