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package tailfile
import (
"bytes"
"errors"
"os"
)
const blockSize = 1024
var eol = []byte("\n")
var ErrNonPositiveLinesNumber = errors.New("Lines number must be positive")
//TailFile returns last n lines of file f
func TailFile(f *os.File, n int) ([][]byte, error) {
if n <= 0 {
return nil, ErrNonPositiveLinesNumber
}
size, err := f.Seek(0, os.SEEK_END)
if err != nil {
return nil, err
}
block := -1
var data []byte
var cnt int
for {
var b []byte
step := int64(block * blockSize)
left := size + step // how many bytes to beginning
if left < 0 {
if _, err := f.Seek(0, os.SEEK_SET); err != nil {
return nil, err
}
b = make([]byte, blockSize+left)
if _, err := f.Read(b); err != nil {
return nil, err
}
data = append(b, data...)
break
} else {
b = make([]byte, blockSize)
if _, err := f.Seek(step, os.SEEK_END); err != nil {
return nil, err
}
if _, err := f.Read(b); err != nil {
return nil, err
}
data = append(b, data...)
}
cnt += bytes.Count(b, eol)
if cnt > n {
break
}
block--
}
lines := bytes.Split(data, eol)
if n < len(lines) {
return lines[len(lines)-n-1 : len(lines)-1], nil
}
return lines[:len(lines)-1], nil
}
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