go4.org@v0.0.0-20230225012048-214862532bf5/readerutil/fakeseeker.go (about)

     1  /*
     2  Copyright 2014 The Perkeep Authors
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8       http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package readerutil
    18  
    19  import (
    20  	"errors"
    21  	"fmt"
    22  	"io"
    23  	"os"
    24  )
    25  
    26  // fakeSeeker can seek to the ends but any read not at the current
    27  // position will fail.
    28  type fakeSeeker struct {
    29  	r    io.Reader
    30  	size int64
    31  
    32  	fakePos int64
    33  	realPos int64
    34  }
    35  
    36  // NewFakeSeeker returns a ReadSeeker that can pretend to Seek (based
    37  // on the provided total size of the reader's content), but any reads
    38  // will fail if the fake seek position doesn't match reality.
    39  func NewFakeSeeker(r io.Reader, size int64) io.ReadSeeker {
    40  	return &fakeSeeker{r: r, size: size}
    41  }
    42  
    43  func (fs *fakeSeeker) Seek(offset int64, whence int) (int64, error) {
    44  	var newo int64
    45  	switch whence {
    46  	default:
    47  		return 0, errors.New("invalid whence")
    48  	case os.SEEK_SET:
    49  		newo = offset
    50  	case os.SEEK_CUR:
    51  		newo = fs.fakePos + offset
    52  	case os.SEEK_END:
    53  		newo = fs.size + offset
    54  	}
    55  	if newo < 0 {
    56  		return 0, errors.New("negative seek")
    57  	}
    58  	fs.fakePos = newo
    59  	return newo, nil
    60  }
    61  
    62  func (fs *fakeSeeker) Read(p []byte) (n int, err error) {
    63  	if fs.fakePos != fs.realPos {
    64  		return 0, fmt.Errorf("attempt to read from fake seek offset %d; real offset is %d", fs.fakePos, fs.realPos)
    65  	}
    66  	n, err = fs.r.Read(p)
    67  	fs.fakePos += int64(n)
    68  	fs.realPos += int64(n)
    69  	return
    70  }