/*
Copyright 2014 The Perkeep 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 readerutil

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
}