kraken

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// Copyright (c) 2016-2019 Uber Technologies, Inc.
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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 piecereader
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import (
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	"fmt"
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	"io"
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	"os"
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	"github.com/uber/kraken/lib/store"
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)
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// Opener opens files.
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type Opener interface {
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	Open() (store.FileReader, error)
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}
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// FileReader is a storage.PieceReader which reads a piece from a file.
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type FileReader struct {
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	offset int64
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	length int64
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	opener Opener
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	closer io.Closer
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	reader io.Reader
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}
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// NewFileReader creates a FileReader which reads a piece from f. f should not
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// be used once it is given to a FileReader.
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func NewFileReader(offset, length int64, opener Opener) *FileReader {
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	return &FileReader{
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		offset: offset,
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		length: length,
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		opener: opener,
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	}
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}
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// Read reads a piece in p.
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func (r *FileReader) Read(p []byte) (int, error) {
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	if r.reader == nil {
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		f, err := r.opener.Open()
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		if err != nil {
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			return 0, fmt.Errorf("open: %s", err)
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		}
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		if _, err := f.Seek(r.offset, os.SEEK_SET); err != nil {
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			return 0, fmt.Errorf("seek: %s", err)
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		}
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		r.reader = io.LimitReader(f, r.length)
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		r.closer = f
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	}
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	return r.reader.Read(p)
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}
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// Close closes the underlying file.
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func (r *FileReader) Close() error {
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	if r.closer == nil {
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		return nil
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	}
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	return r.closer.Close()
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}
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// Length returns the length of the piece.
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func (r *FileReader) Length() int {
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	return int(r.length)
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}
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