Source: archive.go in package golang.org/x/tools/txtar

Package txtar implements a trivial text-based file archive format. The goals for the format are: - be trivial enough to create and edit by hand. - be able to store trees of text files describing go command test cases. - diff nicely in git history and code reviews. Non-goals include being a completely general archive format, storing binary data, storing file modes, storing special files like symbolic links, and so on. Txtar format A txtar archive is zero or more comment lines and then a sequence of file entries. Each file entry begins with a file marker line of the form "-- FILENAME --" and is followed by zero or more file content lines making up the file data. The comment or file content ends at the next file marker line. The file marker line must begin with the three-byte sequence "-- " and end with the three-byte sequence " --", but the enclosed file name can be surrounding by additional white space, all of which is stripped. If the txtar file is missing a trailing newline on the final line, parsers should consider a final newline to be present anyway. There are no possible syntax errors in a txtar archive.

package txtar

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"strings"
)

An Archive is a collection of files.

type Archive struct {
	Comment []byte
	Files   []File
}

A File is a single file in an archive.

type File struct {
	Name string // name of file ("foo/bar.txt")
	Data []byte // text content of file
}

Format returns the serialized form of an Archive. It is assumed that the Archive data structure is well-formed: a.Comment and all a.File[i].Data contain no file marker lines, and all a.File[i].Name is non-empty.

func Format(a *Archive) []byte {
	var buf bytes.Buffer
	buf.Write(fixNL(a.Comment))
	for _, f := range a.Files {
		fmt.Fprintf(&buf, "-- %s --\n", f.Name)
		buf.Write(fixNL(f.Data))
	}
	return buf.Bytes()
}

ParseFile parses the named file as an archive.

func ParseFile(file string) (*Archive, error) {
	data, err := ioutil.ReadFile(file)
	if err != nil {
		return nil, err
	}
	return Parse(data), nil
}

Parse parses the serialized form of an Archive. The returned Archive holds slices of data.

func Parse(data []byte) *Archive {
	a := new(Archive)
	var name string
	a.Comment, name, data = findFileMarker(data)
	for name != "" {
		f := File{name, nil}
		f.Data, name, data = findFileMarker(data)
		a.Files = append(a.Files, f)
	}
	return a
}

var (
	newlineMarker = []byte("\n-- ")
	marker        = []byte("-- ")
	markerEnd     = []byte(" --")
)

findFileMarker finds the next file marker in data, extracts the file name, and returns the data before the marker, the file name, and the data after the marker. If there is no next marker, findFileMarker returns before = fixNL(data), name = "", after = nil.

func findFileMarker(data []byte) (before []byte, name string, after []byte) {
	var i int
	for {
		if name, after = isMarker(data[i:]); name != "" {
			return data[:i], name, after
		}
		j := bytes.Index(data[i:], newlineMarker)
		if j < 0 {
			return fixNL(data), "", nil
		}
		i += j + 1 // positioned at start of new possible marker
	}
}

isMarker checks whether data begins with a file marker line. If so, it returns the name from the line and the data after the line. Otherwise it returns name == "" with an unspecified after.

func isMarker(data []byte) (name string, after []byte) {
	if !bytes.HasPrefix(data, marker) {
		return "", nil
	}
	if i := bytes.IndexByte(data, '\n'); i >= 0 {
		data, after = data[:i], data[i+1:]
	}
	if !bytes.HasSuffix(data, markerEnd) {
		return "", nil
	}
	return strings.TrimSpace(string(data[len(marker) : len(data)-len(markerEnd)])), after
}

If data is empty or ends in \n, fixNL returns data. Otherwise fixNL returns a new slice consisting of data with a final \n added.

func fixNL(data []byte) []byte {
	if len(data) == 0 || data[len(data)-1] == '\n' {
		return data
	}
	d := make([]byte, len(data)+1)
	copy(d, data)
	d[len(data)] = '\n'
	return d