// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package filepath implements utility routines for manipulating filename paths
// in a way compatible with the target operating system-defined file paths.
package filepath

import (
	"bytes"
	"os"
	"runtime"
	"sort"
	"strings"
)

const (
	Separator     = os.PathSeparator
	ListSeparator = os.PathListSeparator
)

// 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 Separator elements with a single one.
//	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,
//         assuming Separator is '/'.
//
// 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,''
// http://plan9.bell-labs.com/sys/doc/lexnames.html
func Clean(path string) string {
	vol := VolumeName(path)
	path = path[len(vol):]
	if path == "" {
		if len(vol) > 1 && vol[1] != ':' {
			// should be UNC
			return FromSlash(vol)
		}
		return vol + "."
	}
	rooted := os.IsPathSeparator(path[0])

	// 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.
	n := len(path)
	buf := []byte(path)
	r, w, dotdot := 0, 0, 0
	if rooted {
		buf[0] = Separator
		r, w, dotdot = 1, 1, 1
	}

	for r < n {
		switch {
		case os.IsPathSeparator(path[r]):
			// empty path element
			r++
		case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
			// . element
			r++
		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
			// .. element: remove to last separator
			r += 2
			switch {
			case w > dotdot:
				// can backtrack
				w--
				for w > dotdot && !os.IsPathSeparator(buf[w]) {
					w--
				}
			case !rooted:
				// cannot backtrack, but not rooted, so append .. element.
				if w > 0 {
					buf[w] = Separator
					w++
				}
				buf[w] = '.'
				w++
				buf[w] = '.'
				w++
				dotdot = w
			}
		default:
			// real path element.
			// add slash if needed
			if rooted && w != 1 || !rooted && w != 0 {
				buf[w] = Separator
				w++
			}
			// copy element
			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
				buf[w] = path[r]
				w++
			}
		}
	}

	// Turn empty string into "."
	if w == 0 {
		buf[w] = '.'
		w++
	}

	return FromSlash(vol + string(buf[0:w]))
}

// ToSlash returns the result of replacing each separator character
// in path with a slash ('/') character.
func ToSlash(path string) string {
	if Separator == '/' {
		return path
	}
	return strings.Replace(path, string(Separator), "/", -1)
}

// FromSlash returns the result of replacing each slash ('/') character
// in path with a separator character.
func FromSlash(path string) string {
	if Separator == '/' {
		return path
	}
	return strings.Replace(path, "/", string(Separator), -1)
}

// SplitList splits a list of paths joined by the OS-specific ListSeparator.
func SplitList(path string) []string {
	if path == "" {
		return []string{}
	}
	return strings.Split(path, string(ListSeparator))
}

// Split splits path immediately following the final Separator,
// separating it into a directory and file name component.
// If there is no Separator in path, Split returns an empty dir
// and file set to path.
func Split(path string) (dir, file string) {
	vol := VolumeName(path)
	i := len(path) - 1
	for i >= len(vol) && !os.IsPathSeparator(path[i]) {
		i--
	}
	return path[:i+1], path[i+1:]
}

// Join joins any number of path elements into a single path, adding
// a Separator if necessary.  All empty strings are ignored.
func Join(elem ...string) string {
	for i, e := range elem {
		if e != "" {
			return Clean(strings.Join(elem[i:], string(Separator)))
		}
	}
	return ""
}

// Ext returns the file name extension used by path.
// The extension is the suffix beginning at the final dot
// in the final element of path; it is empty if there is
// no dot.
func Ext(path string) string {
	for i := len(path) - 1; i >= 0 && !os.IsPathSeparator(path[i]); i-- {
		if path[i] == '.' {
			return path[i:]
		}
	}
	return ""
}

// EvalSymlinks returns the path name after the evaluation of any symbolic
// links.
// If path is relative it will be evaluated relative to the current directory.
func EvalSymlinks(path string) (string, os.Error) {
	if runtime.GOOS == "windows" {
		// Symlinks are not supported under windows.
		_, err := os.Lstat(path)
		if err != nil {
			return "", err
		}
		return Clean(path), nil
	}
	const maxIter = 255
	originalPath := path
	// consume path by taking each frontmost path element,
	// expanding it if it's a symlink, and appending it to b
	var b bytes.Buffer
	for n := 0; path != ""; n++ {
		if n > maxIter {
			return "", os.NewError("EvalSymlinks: too many links in " + originalPath)
		}

		// find next path component, p
		i := strings.IndexRune(path, Separator)
		var p string
		if i == -1 {
			p, path = path, ""
		} else {
			p, path = path[:i], path[i+1:]
		}

		if p == "" {
			if b.Len() == 0 {
				// must be absolute path
				b.WriteRune(Separator)
			}
			continue
		}

		fi, err := os.Lstat(b.String() + p)
		if err != nil {
			return "", err
		}
		if !fi.IsSymlink() {
			b.WriteString(p)
			if path != "" {
				b.WriteRune(Separator)
			}
			continue
		}

		// it's a symlink, put it at the front of path
		dest, err := os.Readlink(b.String() + p)
		if err != nil {
			return "", err
		}
		if IsAbs(dest) {
			b.Reset()
		}
		path = dest + string(Separator) + path
	}
	return Clean(b.String()), nil
}

// Abs returns an absolute representation of path.
// If the path is not absolute it will be joined with the current
// working directory to turn it into an absolute path.  The absolute
// path name for a given file is not guaranteed to be unique.
func Abs(path string) (string, os.Error) {
	if IsAbs(path) {
		return Clean(path), nil
	}
	wd, err := os.Getwd()
	if err != nil {
		return "", err
	}
	return Join(wd, path), nil
}

// Rel returns a relative path that is lexically equivalent to targpath when
// joined to basepath with an intervening separator. That is,
// Join(basepath, Rel(basepath, targpath)) is equivalent to targpath itself.
// An error is returned if targpath can't be made relative to basepath or if
// knowing the current working directory would be necessary to compute it.
func Rel(basepath, targpath string) (string, os.Error) {
	baseVol := VolumeName(basepath)
	targVol := VolumeName(targpath)
	base := Clean(basepath)
	targ := Clean(targpath)
	if targ == base {
		return ".", nil
	}
	base = base[len(baseVol):]
	targ = targ[len(targVol):]
	if base == "." {
		base = ""
	}
	// Can't use IsAbs - `\a` and `a` are both relative in Windows.
	baseSlashed := len(base) > 0 && base[0] == Separator
	targSlashed := len(targ) > 0 && targ[0] == Separator
	if baseSlashed != targSlashed || baseVol != targVol {
		return "", os.NewError("Rel: can't make " + targ + " relative to " + base)
	}
	// Position base[b0:bi] and targ[t0:ti] at the first differing elements.
	bl := len(base)
	tl := len(targ)
	var b0, bi, t0, ti int
	for {
		for bi < bl && base[bi] != Separator {
			bi++
		}
		for ti < tl && targ[ti] != Separator {
			ti++
		}
		if targ[t0:ti] != base[b0:bi] {
			break
		}
		if bi < bl {
			bi++
		}
		if ti < tl {
			ti++
		}
		b0 = bi
		t0 = ti
	}
	if base[b0:bi] == ".." {
		return "", os.NewError("Rel: can't make " + targ + " relative to " + base)
	}
	if b0 != bl {
		// Base elements left. Must go up before going down.
		seps := strings.Count(base[b0:bl], string(Separator))
		buf := make([]byte, 3+seps*3+tl-t0)
		n := copy(buf, "..")
		for i := 0; i < seps; i++ {
			buf[n] = Separator
			copy(buf[n+1:], "..")
			n += 3
		}
		if t0 != tl {
			buf[n] = Separator
			copy(buf[n+1:], targ[t0:])
		}
		return string(buf), nil
	}
	return targ[t0:], nil
}

// SkipDir is used as a return value from WalkFuncs to indicate that
// the directory named in the call is to be skipped. It is not returned
// as an error by any function.
var SkipDir = os.NewError("skip this directory")

// WalkFunc is the type of the function called for each file or directory
// visited by Walk.  If there was a problem walking to the file or directory
// named by path, the incoming error will describe the problem and the
// function can decide how to handle that error (and Walk will not descend
// into that directory).  If an error is returned, processing stops.  The
// sole exception is that if path is a directory and the function returns the
// special value SkipDir, the contents of the directory are skipped
// and processing continues as usual on the next file.
type WalkFunc func(path string, info *os.FileInfo, err os.Error) os.Error

// walk recursively descends path, calling w.
func walk(path string, info *os.FileInfo, walkFn WalkFunc) os.Error {
	err := walkFn(path, info, nil)
	if err != nil {
		if info.IsDirectory() && err == SkipDir {
			return nil
		}
		return err
	}

	if !info.IsDirectory() {
		return nil
	}

	list, err := readDir(path)
	if err != nil {
		return walkFn(path, info, err)
	}

	for _, fileInfo := range list {
		if err = walk(Join(path, fileInfo.Name), fileInfo, walkFn); err != nil {
			return err
		}
	}
	return nil
}

// Walk walks the file tree rooted at root, calling walkFn for each file or
// directory in the tree, including root. All errors that arise visiting files
// and directories are filtered by walkFn. The files are walked in lexical
// order, which makes the output deterministic but means that for very
// large directories Walk can be inefficient.
func Walk(root string, walkFn WalkFunc) os.Error {
	info, err := os.Lstat(root)
	if err != nil {
		return walkFn(root, nil, err)
	}
	return walk(root, info, walkFn)
}

// readDir reads the directory named by dirname and returns
// a sorted list of directory entries.
// Copied from io/ioutil to avoid the circular import.
func readDir(dirname string) ([]*os.FileInfo, os.Error) {
	f, err := os.Open(dirname)
	if err != nil {
		return nil, err
	}
	list, err := f.Readdir(-1)
	f.Close()
	if err != nil {
		return nil, err
	}
	fi := make(fileInfoList, len(list))
	for i := range list {
		fi[i] = &list[i]
	}
	sort.Sort(fi)
	return fi, nil
}

// A dirList implements sort.Interface.
type fileInfoList []*os.FileInfo

func (f fileInfoList) Len() int           { return len(f) }
func (f fileInfoList) Less(i, j int) bool { return f[i].Name < f[j].Name }
func (f fileInfoList) Swap(i, j int)      { f[i], f[j] = f[j], f[i] }

// Base returns the last element of path.
// Trailing path separators are removed before extracting the last element.
// If the path is empty, Base returns ".".
// If the path consists entirely of separators, Base returns a single separator.
func Base(path string) string {
	if path == "" {
		return "."
	}
	// Strip trailing slashes.
	for len(path) > 0 && os.IsPathSeparator(path[len(path)-1]) {
		path = path[0 : len(path)-1]
	}
	// Find the last element
	i := len(path) - 1
	for i >= 0 && !os.IsPathSeparator(path[i]) {
		i--
	}
	if i >= 0 {
		path = path[i+1:]
	}
	// If empty now, it had only slashes.
	if path == "" {
		return string(Separator)
	}
	return path
}