106 lines
3.1 KiB
Go
106 lines
3.1 KiB
Go
// Copyright 2018 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package secio provides support for sectioned I/O.
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package secio
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import (
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"errors"
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"io"
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)
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// ErrReachedLimit is returned when SectionReader.Read or SectionWriter.Write
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// reaches its limit.
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var ErrReachedLimit = errors.New("reached limit")
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// SectionReader implements io.Reader on a section of an underlying io.ReaderAt.
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// It is similar to io.SectionReader, but:
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//
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// - Reading beyond the limit returns ErrReachedLimit, not io.EOF.
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//
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// - Limit overflow is handled correctly.
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type SectionReader struct {
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r io.ReaderAt
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off int64
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limit int64
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}
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// Read implements io.Reader.Read.
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func (r *SectionReader) Read(dst []byte) (int, error) {
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if r.limit >= 0 {
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if max := r.limit - r.off; max < int64(len(dst)) {
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dst = dst[:max]
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}
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}
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n, err := r.r.ReadAt(dst, r.off)
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r.off += int64(n)
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if err == nil && r.off == r.limit {
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err = ErrReachedLimit
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}
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return n, err
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}
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// NewOffsetReader returns an io.Reader that reads from r starting at offset
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// off.
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func NewOffsetReader(r io.ReaderAt, off int64) *SectionReader {
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return &SectionReader{r, off, -1}
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}
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// NewSectionReader returns an io.Reader that reads from r starting at offset
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// off and stops with ErrReachedLimit after n bytes.
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func NewSectionReader(r io.ReaderAt, off int64, n int64) *SectionReader {
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// If off + n overflows, it will be < 0 such that no limit applies, but
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// this is the correct behavior as long as r prohibits reading at offsets
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// beyond MaxInt64.
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return &SectionReader{r, off, off + n}
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}
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// SectionWriter implements io.Writer on a section of an underlying
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// io.WriterAt. Writing beyond the limit returns ErrReachedLimit.
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type SectionWriter struct {
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w io.WriterAt
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off int64
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limit int64
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}
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// Write implements io.Writer.Write.
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func (w *SectionWriter) Write(src []byte) (int, error) {
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if w.limit >= 0 {
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if max := w.limit - w.off; max < int64(len(src)) {
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src = src[:max]
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}
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}
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n, err := w.w.WriteAt(src, w.off)
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w.off += int64(n)
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if err == nil && w.off == w.limit {
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err = ErrReachedLimit
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}
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return n, err
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}
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// NewOffsetWriter returns an io.Writer that writes to w starting at offset
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// off.
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func NewOffsetWriter(w io.WriterAt, off int64) *SectionWriter {
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return &SectionWriter{w, off, -1}
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}
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// NewSectionWriter returns an io.Writer that writes to w starting at offset
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// off and stops with ErrReachedLimit after n bytes.
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func NewSectionWriter(w io.WriterAt, off int64, n int64) *SectionWriter {
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// If off + n overflows, it will be < 0 such that no limit applies, but
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// this is the correct behavior as long as w prohibits writing at offsets
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// beyond MaxInt64.
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return &SectionWriter{w, off, off + n}
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}
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