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multireader.go
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/
multireader.go
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package ioutils
import (
"bytes"
"fmt"
"io"
"os"
)
type pos struct {
idx int
offset int64
}
type multiReadSeeker struct {
readers []io.ReadSeeker
pos *pos
posIdx map[io.ReadSeeker]int
}
func (r *multiReadSeeker) Seek(offset int64, whence int) (int64, error) {
var tmpOffset int64
switch whence {
case os.SEEK_SET:
for i, rdr := range r.readers {
// get size of the current reader
s, err := rdr.Seek(0, os.SEEK_END)
if err != nil {
return -1, err
}
if offset > tmpOffset+s {
if i == len(r.readers)-1 {
rdrOffset := s + (offset - tmpOffset)
if _, err := rdr.Seek(rdrOffset, os.SEEK_SET); err != nil {
return -1, err
}
r.pos = &pos{i, rdrOffset}
return offset, nil
}
tmpOffset += s
continue
}
rdrOffset := offset - tmpOffset
idx := i
rdr.Seek(rdrOffset, os.SEEK_SET)
// make sure all following readers are at 0
for _, rdr := range r.readers[i+1:] {
rdr.Seek(0, os.SEEK_SET)
}
if rdrOffset == s && i != len(r.readers)-1 {
idx += 1
rdrOffset = 0
}
r.pos = &pos{idx, rdrOffset}
return offset, nil
}
case os.SEEK_END:
for _, rdr := range r.readers {
s, err := rdr.Seek(0, os.SEEK_END)
if err != nil {
return -1, err
}
tmpOffset += s
}
r.Seek(tmpOffset+offset, os.SEEK_SET)
return tmpOffset + offset, nil
case os.SEEK_CUR:
if r.pos == nil {
return r.Seek(offset, os.SEEK_SET)
}
// Just return the current offset
if offset == 0 {
return r.getCurOffset()
}
curOffset, err := r.getCurOffset()
if err != nil {
return -1, err
}
rdr, rdrOffset, err := r.getReaderForOffset(curOffset + offset)
if err != nil {
return -1, err
}
r.pos = &pos{r.posIdx[rdr], rdrOffset}
return curOffset + offset, nil
default:
return -1, fmt.Errorf("Invalid whence: %d", whence)
}
return -1, fmt.Errorf("Error seeking for whence: %d, offset: %d", whence, offset)
}
func (r *multiReadSeeker) getReaderForOffset(offset int64) (io.ReadSeeker, int64, error) {
var rdr io.ReadSeeker
var rdrOffset int64
for i, rdr := range r.readers {
offsetTo, err := r.getOffsetToReader(rdr)
if err != nil {
return nil, -1, err
}
if offsetTo > offset {
rdr = r.readers[i-1]
rdrOffset = offsetTo - offset
break
}
if rdr == r.readers[len(r.readers)-1] {
rdrOffset = offsetTo + offset
break
}
}
return rdr, rdrOffset, nil
}
func (r *multiReadSeeker) getCurOffset() (int64, error) {
var totalSize int64
for _, rdr := range r.readers[:r.pos.idx+1] {
if r.posIdx[rdr] == r.pos.idx {
totalSize += r.pos.offset
break
}
size, err := getReadSeekerSize(rdr)
if err != nil {
return -1, fmt.Errorf("error getting seeker size: %v", err)
}
totalSize += size
}
return totalSize, nil
}
func (r *multiReadSeeker) getOffsetToReader(rdr io.ReadSeeker) (int64, error) {
var offset int64
for _, r := range r.readers {
if r == rdr {
break
}
size, err := getReadSeekerSize(rdr)
if err != nil {
return -1, err
}
offset += size
}
return offset, nil
}
func (r *multiReadSeeker) Read(b []byte) (int, error) {
if r.pos == nil {
r.pos = &pos{0, 0}
}
bCap := int64(cap(b))
buf := bytes.NewBuffer(nil)
var rdr io.ReadSeeker
for _, rdr = range r.readers[r.pos.idx:] {
readBytes, err := io.CopyN(buf, rdr, bCap)
if err != nil && err != io.EOF {
return -1, err
}
bCap -= readBytes
if bCap == 0 {
break
}
}
rdrPos, err := rdr.Seek(0, os.SEEK_CUR)
if err != nil {
return -1, err
}
r.pos = &pos{r.posIdx[rdr], rdrPos}
return buf.Read(b)
}
func getReadSeekerSize(rdr io.ReadSeeker) (int64, error) {
// save the current position
pos, err := rdr.Seek(0, os.SEEK_CUR)
if err != nil {
return -1, err
}
// get the size
size, err := rdr.Seek(0, os.SEEK_END)
if err != nil {
return -1, err
}
// reset the position
if _, err := rdr.Seek(pos, os.SEEK_SET); err != nil {
return -1, err
}
return size, nil
}
// MultiReadSeeker returns a ReadSeeker that's the logical concatenation of the provided
// input readseekers. After calling this method the initial position is set to the
// beginning of the first ReadSeeker. At the end of a ReadSeeker, Read always advances
// to the beginning of the next ReadSeeker and returns EOF at the end of the last ReadSeeker.
// Seek can be used over the sum of lengths of all readseekers.
//
// When a MultiReadSeeker is used, no Read and Seek operations should be made on
// its ReadSeeker components. Also, users should make no assumption on the state
// of individual readseekers while the MultiReadSeeker is used.
func MultiReadSeeker(readers ...io.ReadSeeker) io.ReadSeeker {
if len(readers) == 1 {
return readers[0]
}
idx := make(map[io.ReadSeeker]int)
for i, rdr := range readers {
idx[rdr] = i
}
return &multiReadSeeker{
readers: readers,
posIdx: idx,
}
}