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package reversereader
import (
"io"
"os"
"github.com/pkg/errors"
)
// ReverseReader structure for reading a file backwards
type ReverseReader struct {
reader *os.File
offset int64
readSize int64
}
// NewReverseReader returns a reader that reads from the end of a file
// rather than the beginning. It sets the readsize to pagesize and determines
// the first offset using using modulus.
func NewReverseReader(reader *os.File) (*ReverseReader, error) {
// pagesize should be safe for memory use and file reads should be on page
// boundaries as well
pageSize := int64(os.Getpagesize())
stat, err := reader.Stat()
if err != nil {
return nil, err
}
// figure out the last page boundary
remainder := stat.Size() % pageSize
end, err := reader.Seek(0, 2)
if err != nil {
return nil, err
}
// set offset (starting position) to the last page boundary or
// zero if fits in one page
startOffset := end - remainder
if startOffset < 0 {
startOffset = 0
}
rr := ReverseReader{
reader: reader,
offset: startOffset,
readSize: pageSize,
}
return &rr, nil
}
// ReverseReader reads from a given offset to the previous offset and
// then sets the newoff set one pagesize less than the previous read.
func (r *ReverseReader) Read() (string, error) {
if r.offset < 0 {
return "", errors.Wrap(io.EOF, "at beginning of file")
}
// Read from given offset
b := make([]byte, r.readSize)
n, err := r.reader.ReadAt(b, r.offset)
if err != nil && errors.Cause(err) != io.EOF {
return "", err
}
if int64(n) < r.readSize {
b = b[0:n]
}
// Set to the next page boundary
r.offset = -r.readSize
return string(b), nil
}
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