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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: 3 days ago
JSON representation
A scanner similar to bufio.Scanner, but it reads and returns lines in reverse order, starting at a given position and going backward.
- Host: GitHub
- URL: https://github.com/icza/backscanner
- Owner: icza
- License: apache-2.0
- Created: 2017-10-18T07:59:07.000Z (about 7 years ago)
- Default Branch: master
- Last Pushed: 2024-03-28T21:58:59.000Z (9 months ago)
- Last Synced: 2024-07-31T20:53:21.695Z (5 months ago)
- Topics: log, reverse, scanner
- Language: Go
- Homepage:
- Size: 31.3 KB
- Stars: 64
- Watchers: 6
- Forks: 10
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Funding: .github/FUNDING.yml
- License: LICENSE
Awesome Lists containing this project
- awesome-go - backscanner - A scanner similar to bufio.Scanner, but it reads and returns lines in reverse order, starting at a given position and going backward. (Utilities / Utility/Miscellaneous)
- awesome-go - backscanner - A scanner similar to bufio.Scanner, but it reads and returns lines in reverse order, starting at a given position and going backward. - ★ 6 (Utilities)
- awesome-go-extra - backscanner - 10-18T07:59:07Z|2022-08-12T13:37:52Z| (Utilities / Fail injection)
README
# backscanner
[](https://pkg.go.dev/github.com/icza/backscanner)
[](https://goreportcard.com/report/github.com/icza/backscanner)
[](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:
```shell
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:
```go
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"`):
```go
file, err := os.Open("mylog.txt")
if err != nil {
panic(err)
}
fileStatus, err := file.Stat()
if err != nil {
panic(err)
}
defer file.Close()
scanner := backscanner.New(file, int(fileStatus.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
}
}
```