Go-library for reading and processing data from a io.Reader line by line. Now you can add UNIX text processing principles to its Reader (like with awk, grep, sed ...).

go get -u github.com/msoap/byline

import "github.com/msoap/byline"

// Create new line-by-line Reader from io.Reader:
lr := byline.NewReader(reader)

// Add to the Reader stack of a filter functions:
lr.MapString(func(line string) string {return "prefix_" + line}).GrepByRegexp(regexp.MustCompile("only this"))

// Read all content
result, err := lr.ReadAll()

// Use everywhere instead of io.Reader
_, err := io.Copy(os.Stdout, lr)

// Or in one place
result, err := byline.NewReader(reader).MapString(func(line string) string {return "prefix_" + line}).ReadAll()

• Map(func([]byte) []byte) - processing of each line as []byte.
• MapErr(func([]byte) ([]byte, error)) - processing of each line as []byte, and you can return error, io.EOF or custom error.
• MapString(func(string) string) - processing of each line as string.
• MapStringErr(func(string) (string, error)) - processing of each line as string, and you can return error.
• Each(func([]byte)) - processing each line without changing the line
• EachString(func(string)) - processing each line as string without changing the line
• Grep(func([]byte) bool) - filtering lines by function.
• GrepString(func(string) bool) - filtering lines as string by function.
• GrepByRegexp(re *regexp.Regexp) - filtering lines by regexp.
• AWKMode(func(line string, fields []string, vars AWKVars) (string, error)) - processing of each line in AWK mode. In addition to current line, filterFn gets slice with fields splitted by separator (default is /\s+/) and vars releated to awk (NR, NF, RS, FS). Attention! Use AWKMode() with caution on large data sets, see Overheads below.

Map*Err, AWKMode methods can return byline.ErrOmitLine - error for discard processing of current line.

• SetRS(rs byte) - set line (record) separator, default is newline - \n.
• SetFS(fs *regexp.Regexp) - set field separator for AWK mode, default is \s+.
• Discard() - discard all content from Reader only for side effect of filter functions.
• ReadAll() ([]byte, error) - return all content as slice of bytes.
• ReadAllSlice() ([][]byte, error) - return all content by lines as [][]byte.
• ReadAllString() (string, error) - return all content as string.
• ReadAllSliceString() ([]string, error) - return all content by lines as slice of strings.

Add line number to each line and add suffix at the end of line:

reader := strings.NewReader("111\n222\n333")
// or read file
reader, err := os.Open("file.txt")
// or process response from HTTP client
reader := httpResponse.Body

i := 0
blr := byline.NewReader(reader).MapString(func(line string) string {
	i++
	return fmt.Sprintf("(%d) %s", i, string(line))
}).Map(func(line []byte) []byte {
	return regexp.MustCompile(`\n?$`).ReplaceAll(line, []byte(" suf\n"))
})

result, err := blr.ReadAll()

type StateMachine struct {
	beginRe *regexp.Regexp
	endRe   *regexp.Regexp
	inBlock bool
}

func (sm *StateMachine) SMFilter(line []byte) bool {
	switch {
	case sm.beginRe.Match(line):
		sm.inBlock = true
		return true
	case sm.inBlock && sm.endRe.Match(line):
		sm.inBlock = false
		return true
	default:
		return sm.inBlock
	}
}

func ExampleReader_Grep() {
	file, err := os.Open("byline.go")
	if err != nil {
		fmt.Println(err)
		return
	}

	// get all lines between "^type..." and "^}"
	sm := StateMachine{
		beginRe: regexp.MustCompile(`^type `),
		endRe:   regexp.MustCompile(`^}\s+$`),
	}

	blr := byline.NewReader(file).Grep(sm.SMFilter).Map(func(line []byte) []byte {
		// and remove comments
		return regexp.MustCompile(`\s+//.+`).ReplaceAll(line, []byte{})
	})

	result, err := blr.ReadAllString()
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Print(result)
}

type Reader struct {
	scanner     *bufio.Scanner
	buffer      bytes.Buffer
	existsData  bool
	filterFuncs []func(line []byte) ([]byte, error)
	awkVars     AWKVars
}
type AWKVars struct {
	NR int
	NF int
	RS byte
	FS *regexp.Regexp
}

// CSV with "#" instead of "\n"
reader := strings.NewReader(`1,name one,12.3#2,second row;7.1#3,three row;15.51`)

sum := 0.0
err := byline.NewReader(reader).
	SetRS('#').
	SetFS(regexp.MustCompile(`[,;]`)).
	AWKMode(func(line string, fields []string, vars byline.AWKVars) (string, error) {
		if vars.NF < 3 {
			return "", fmt.Errorf("csv parse failed for %q", line)
		}

		if price, err := strconv.ParseFloat(fields[2], 10); err != nil {
			return "", err
		} else if price < 10 {
			return "", byline.ErrOmitLine
		} else {
			sum += price
			return "", nil
		}
	}).Discard()

if err != nil {
	fmt.Println("Price sum:", sum)
}

Price sum: 27.81

An example in which we get odd lines (for io.Reader with 10000 lines):

❯ make benchmark
go test -benchtime 5s -benchmem -bench .
Benchmark_NativeScannerBytes-4       	   20000	    312502 ns/op	  215080 B/op	      24 allocs/op
Benchmark_NativeScannerOnlyCount-4   	   30000	    217491 ns/op	    4160 B/op	       4 allocs/op
Benchmark_MapBytes-4                 	   10000	    567421 ns/op	  135184 B/op	      17 allocs/op
Benchmark_MapString-4                	    5000	   1408956 ns/op	  374000 B/op	   15018 allocs/op
Benchmark_Grep-4                     	   10000	    592100 ns/op	  135200 B/op	      18 allocs/op
Benchmark_GrepString-4               	    5000	   1151309 ns/op	  294416 B/op	   10019 allocs/op
Benchmark_Each-4                     	   10000	    562337 ns/op	    6201 B/op	      13 allocs/op
Benchmark_EachString-4               	   10000	    991528 ns/op	  165427 B/op	   10013 allocs/op
Benchmark_AWKMode-4                  	     500	  11865482 ns/op	 3410392 B/op	   55466 allocs/op
PASS

See benchmark_test.go for benchmark code

• io, bufio - Go packages for work with Readers.
• go-linereader - package that reads lines from an io.Reader and puts them onto a channel.
• AWK - programming language and great UNIX tool.