test.file

Copyright (c) 2014 Charles Banning <clbanning@gmail.com>.  All rights reserved.

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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

===============================================================================

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rfile
=====

Read a file in reverse line-by-line.

Inspired by: https://groups.google.com/forum/?fromgroups#!topic/golang-nuts/a0NRB-spRhc.

<pre>
...
  f, err := Open(file)
  if err != nil {
    // handle err
  }
  for {
    line, err := f.ReadLine()
    if err != nil {
      if err != io.EOF {
        // handle error
      }
      break // may be io.EOF
    }
    // do something with "line"
  }
  f.Close()
  ...
</pre>
// Copyright (c) 2014 Charles Banning <clbanning@gmail.com>.  All rights reserved.
// See LICENSE file for terms of use.

// Package rfile reads a file line-by-line backwards.
package rfile

import (
	"bytes"
	"io"
	"os"
)

type Rfile struct {
	fh            *os.File
	offset        int64
	bufsize       int
	lines         [][]byte
	i             int
}

// Open a file to be read in reverse line-by-line.
// (Use Open(). Kept for backwards compatibility.)
func NewReverseFile(file string) (*Rfile, error) {
	var err error
	rf := new(Rfile)
	if rf.fh, err = os.Open(file); err != nil {
		return nil, err
	}
	fi, _ := rf.fh.Stat()
	rf.offset = fi.Size()
	rf.bufsize = 4096
	return rf, nil
}

// Open a file to be read in reverse line-by-line.
func Open(file string) (*Rfile, error) {
	var err error
	rf := new(Rfile)
	if rf.fh, err = os.Open(file); err != nil {
		return nil, err
	}
	fi, _ := rf.fh.Stat()
	rf.offset = fi.Size()
	rf.bufsize = 4096
	return rf, nil
}

// Close file that was opened.
func (rf *Rfile) Close() {
	rf.fh.Close()
}

// Read next previous line, beginning with the last line in the file.
// When the beginning of the file is reached: "", io.EOF is returned.
func (rf *Rfile) ReadLine() (string, error) {
	if rf.i > 0 {
		rf.i--
		return string(rf.lines[rf.i+1]), nil
	}
	if rf.i < 0 {
		return "", io.EOF
	}
	if rf.offset == 0 {
		rf.i-- // use as flag to send EOF on next call
		return string(rf.lines[0]), nil
	}

	// get more from file - back up from end-of-file
	rf.offset -= int64(rf.bufsize)
	if rf.offset < 0 {
		rf.bufsize += int(rf.offset) // rf.offset is negative
		rf.offset = 0
	}
	_, err := rf.fh.Seek(rf.offset, 0)
	if err != nil {
		return "", err
	}

	// compute buffer size
	buf := make([]byte, rf.bufsize)
	if n, err := rf.fh.Read(buf); err != nil && err != io.EOF {
		return "", err
	} else if n != rf.bufsize { // shouldn't happen
		buf = buf[:n]
	}

	// get the lines in the buffer, append what was carried over
	if len(rf.lines) > 0 {
		buf = append(buf, rf.lines[0]...)
	}
	rf.lines = bytes.Split(buf, []byte("\n"))
	rf.i = len(rf.lines) - 1

	return rf.ReadLine() // now read the next line back
}
// reverse_test.go - read a file line-by-line backwards

package rfile

import (
	"fmt"
	"io"
	"testing"
)

func TestRfile(t *testing.T) {
		rf, err := Open("reverse.go")
		if err != nil {
			t.Fatal(err)
		}
		for {
			line, err := rf.ReadLine()
			if err != nil {
				if err != io.EOF {
					t.Fatal(err)
				}
				break
			}
			fmt.Println(line)
		}
		rf.Close()
}