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https://github.com/icza/backscanner

A scanner similar to bufio.Scanner, but it reads and returns lines in reverse order, starting at a given position and going backward.
https://github.com/icza/backscanner

log reverse scanner

Last synced: 24 days ago
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A scanner similar to bufio.Scanner, but it reads and returns lines in reverse order, starting at a given position and going backward.

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# backscanner



![Build Status](https://github.com/icza/backscanner/actions/workflows/go.yml/badge.svg)

[![Go Reference](https://pkg.go.dev/badge/github.com/icza/backscanner.svg)](https://pkg.go.dev/github.com/icza/backscanner)

[![Go Report Card](https://goreportcard.com/badge/github.com/icza/backscanner)](https://goreportcard.com/report/github.com/icza/backscanner)

[![codecov](https://codecov.io/gh/icza/backscanner/branch/master/graph/badge.svg)](https://codecov.io/gh/icza/backscanner)



Ever needed or wondered how to efficiently search for something in a log file,

but starting at the end and going backward? Here's your solution.



Package `backscanner` provides a scanner similar to `bufio.Scanner`, but it reads

and returns lines in reverse order, starting at a given position (which may be

the end of the input) and going backward.



This library only uses the standard library, but the test uses an external package.

To install this library (along with the test dependency), simply run:



    go get -t github.com/icza/backscanner



Advancing and accessing lines of the input is done by calling `Scanner.Line()`,

which returns the next line (previous in the source) as a `string`.



For maximum efficiency there is `Scanner.LineBytes()`. It returns the next line

as a byte slice, which shares its backing array with the internal buffer of

`Scanner`. This is because no copy is made from the line data; but this also

means you can only inspect or search in the slice before calling `Line()` or

`LineBytes()` again, as the content of the internal buffer–and thus slices

returned by `LineBytes()`–may be overwritten. If you need to retain the line

data, make a copy of it or use `Line()`.



Example using it:



	input := "Line1\nLine2\nLine3"

	scanner := backscanner.New(strings.NewReader(input), len(input))

	for {

		line, pos, err := scanner.Line()

		if err != nil {

			fmt.Println("Error:", err)

			break

		}

		fmt.Printf("Line position: %2d, line: %q\n", pos, line)

	}



Output:



	Line position: 12, line: "Line3"

	Line position:  6, line: "Line2"

	Line position:  0, line: "Line1"

	Error: EOF



Using it to efficiently scan a file, finding last occurrence of a string (`"error"`):



	f, err := os.Open("mylog.txt")

	if err != nil {

		panic(err)

	}

	fi, err := f.Stat()

	if err != nil {

		panic(err)

	}

	defer f.Close()



	scanner := backscanner.New(f, int(fi.Size()))

	what := []byte("error")

	for {

		line, pos, err := scanner.LineBytes()

		if err != nil {

			if err == io.EOF {

				fmt.Printf("%q is not found in file.\n", what)

			} else {

				fmt.Println("Error:", err)

			}

			break

		}

		if bytes.Contains(line, what) {

			fmt.Printf("Found %q at line position: %d, line: %q\n", what, pos, line)

			break

		}

	}