/
multireaderat.go
113 lines (100 loc) · 3.19 KB
/
multireaderat.go
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// Copyright (c) 2012 The Go Authors. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
package store
import (
"io"
"sort"
)
// This code is taken from https://talks.golang.org/2013/oscon-dl/sizereaderat.go
// A SizeReaderAt is a ReaderAt with a Size method.
//
// An io.SectionReader implements SizeReaderAt.
type SizeReaderAt interface {
Size() int64
io.ReaderAt
}
// NewMultiReaderAt is like io.MultiReader but produces a ReaderAt
// (and Size), instead of just a reader.
func NewMultiReaderAt(parts ...SizeReaderAt) SizeReaderAt {
m := &multi{
parts: make([]offsetAndSource, 0, len(parts)),
}
var off int64
for _, p := range parts {
m.parts = append(m.parts, offsetAndSource{off, p})
off += p.Size()
}
m.size = off
return m
}
type offsetAndSource struct {
off int64
SizeReaderAt
}
type multi struct {
parts []offsetAndSource
size int64
}
func (m *multi) Size() int64 { return m.size }
func (m *multi) ReadAt(p []byte, off int64) (n int, err error) {
wantN := len(p)
// Skip past the requested offset.
skipParts := sort.Search(len(m.parts), func(i int) bool {
// This function returns whether parts[i] will
// contribute any bytes to our output.
part := m.parts[i]
return part.off+part.Size() > off
})
parts := m.parts[skipParts:]
// How far to skip in the first part.
needSkip := off
if len(parts) > 0 {
needSkip -= parts[0].off
}
for len(parts) > 0 && len(p) > 0 {
readP := p
partSize := parts[0].Size()
if int64(len(readP)) > partSize-needSkip {
readP = readP[:partSize-needSkip]
}
pn, err0 := parts[0].ReadAt(readP, needSkip)
if err0 != nil {
return n, err0
}
n += pn
p = p[pn:]
if int64(pn)+needSkip == partSize {
parts = parts[1:]
}
needSkip = 0
}
if n != wantN {
err = io.ErrUnexpectedEOF
}
return
}