Source: path.go in package path

Package path implements utility routines for manipulating slash-separated paths. The path package should only be used for paths separated by forward slashes, such as the paths in URLs. This package does not deal with Windows paths with drive letters or backslashes; to manipulate operating system paths, use the path/filepath package.

package path

A lazybuf is a lazily constructed path buffer. It supports append, reading previously appended bytes, and retrieving the final string. It does not allocate a buffer to hold the output until that output diverges from s.

type lazybuf struct {
	s   string
	buf []byte
	w   int
}

func (b *lazybuf) index(i int) byte {
	if b.buf != nil {
		return b.buf[i]
	}
	return b.s[i]
}

func (b *lazybuf) append(c byte) {
	if b.buf == nil {
		if b.w < len(b.s) && b.s[b.w] == c {
			b.w++
			return
		}
		b.buf = make([]byte, len(b.s))
		copy(b.buf, b.s[:b.w])
	}
	b.buf[b.w] = c
	b.w++
}

func (b *lazybuf) string() string {
	if b.buf == nil {
		return b.s[:b.w]
	}
	return string(b.buf[:b.w])
}

Clean returns the shortest path name equivalent to path by purely lexical processing. It applies the following rules iteratively until no further processing can be done: 1. Replace multiple slashes with a single slash. 2. Eliminate each . path name element (the current directory). 3. Eliminate each inner .. path name element (the parent directory) along with the non-.. element that precedes it. 4. Eliminate .. elements that begin a rooted path: that is, replace "/.." by "/" at the beginning of a path. The returned path ends in a slash only if it is the root "/". If the result of this process is an empty string, Clean returns the string ".". See also Rob Pike, ``Lexical File Names in Plan 9 or Getting Dot-Dot Right,'' https://9p.io/sys/doc/lexnames.html

func Clean(path string) string {
	if path == "" {
		return "."
	}

	rooted := path[0] == '/'
	n := len(path)

Invariants: reading from path; r is index of next byte to process. writing to buf; w is index of next byte to write. dotdot is index in buf where .. must stop, either because it is the leading slash or it is a leading ../../.. prefix.

	out := lazybuf{s: path}
	r, dotdot := 0, 0
	if rooted {
		out.append('/')
		r, dotdot = 1, 1
	}

	for r < n {
		switch {

empty path element

r++

. element

r++

.. element: remove to last /

			r += 2
			switch {

can backtrack

				out.w--
				for out.w > dotdot && out.index(out.w) != '/' {
					out.w--
				}

cannot backtrack, but not rooted, so append .. element.

				if out.w > 0 {
					out.append('/')
				}
				out.append('.')
				out.append('.')
				dotdot = out.w
			}

real path element. add slash if needed

			if rooted && out.w != 1 || !rooted && out.w != 0 {
				out.append('/')

copy element

			for ; r < n && path[r] != '/'; r++ {
				out.append(path[r])
			}
		}
	}

Turn empty string into "."

	if out.w == 0 {
		return "."
	}

	return out.string()
}

lastSlash(s) is strings.LastIndex(s, "/") but we can't import strings.

func lastSlash(s string) int {
	i := len(s) - 1
	for i >= 0 && s[i] != '/' {
		i--
	}
	return i
}

Split splits path immediately following the final slash, separating it into a directory and file name component. If there is no slash in path, Split returns an empty dir and file set to path. The returned values have the property that path = dir+file.

func Split(path string) (dir, file string) {
	i := lastSlash(path)
	return path[:i+1], path[i+1:]
}

Join joins any number of path elements into a single path, separating them with slashes. Empty elements are ignored. The result is Cleaned. However, if the argument list is empty or all its elements are empty, Join returns an empty string.

func Join(elem ...string) string {
	size := 0
	for _, e := range elem {
		size += len(e)
	}
	if size == 0 {
		return ""
	}
	buf := make([]byte, 0, size+len(elem)-1)
	for _, e := range elem {
		if len(buf) > 0 || e != "" {
			if len(buf) > 0 {
				buf = append(buf, '/')
			}
			buf = append(buf, e...)
		}
	}
	return Clean(string(buf))
}

Ext returns the file name extension used by path. The extension is the suffix beginning at the final dot in the final slash-separated element of path; it is empty if there is no dot.

func Ext(path string) string {
	for i := len(path) - 1; i >= 0 && path[i] != '/'; i-- {
		if path[i] == '.' {
			return path[i:]
		}
	}
	return ""
}

Base returns the last element of path. Trailing slashes are removed before extracting the last element. If the path is empty, Base returns ".". If the path consists entirely of slashes, Base returns "/".

func Base(path string) string {
	if path == "" {
		return "."

Strip trailing slashes.

	for len(path) > 0 && path[len(path)-1] == '/' {
		path = path[0 : len(path)-1]

Find the last element

	if i := lastSlash(path); i >= 0 {
		path = path[i+1:]

If empty now, it had only slashes.

	if path == "" {
		return "/"
	}
	return path
}

IsAbs reports whether the path is absolute.

func IsAbs(path string) bool {
	return len(path) > 0 && path[0] == '/'
}

Dir returns all but the last element of path, typically the path's directory. After dropping the final element using Split, the path is Cleaned and trailing slashes are removed. If the path is empty, Dir returns ".". If the path consists entirely of slashes followed by non-slash bytes, Dir returns a single slash. In any other case, the returned path does not end in a slash.

func Dir(path string) string {
	dir, _ := Split(path)
	return Clean(dir)