Source file src/path/path.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package path implements utility routines for manipulating slash-separated
     6  // paths.
     7  //
     8  // The path package should only be used for paths separated by forward
     9  // slashes, such as the paths in URLs. This package does not deal with
    10  // Windows paths with drive letters or backslashes; to manipulate
    11  // operating system paths, use the path/filepath package.
    12  package path
    13  
    14  // A lazybuf is a lazily constructed path buffer.
    15  // It supports append, reading previously appended bytes,
    16  // and retrieving the final string. It does not allocate a buffer
    17  // to hold the output until that output diverges from s.
    18  type lazybuf struct {
    19  	s   string
    20  	buf []byte
    21  	w   int
    22  }
    23  
    24  func (b *lazybuf) index(i int) byte {
    25  	if b.buf != nil {
    26  		return b.buf[i]
    27  	}
    28  	return b.s[i]
    29  }
    30  
    31  func (b *lazybuf) append(c byte) {
    32  	if b.buf == nil {
    33  		if b.w < len(b.s) && b.s[b.w] == c {
    34  			b.w++
    35  			return
    36  		}
    37  		b.buf = make([]byte, len(b.s))
    38  		copy(b.buf, b.s[:b.w])
    39  	}
    40  	b.buf[b.w] = c
    41  	b.w++
    42  }
    43  
    44  func (b *lazybuf) string() string {
    45  	if b.buf == nil {
    46  		return b.s[:b.w]
    47  	}
    48  	return string(b.buf[:b.w])
    49  }
    50  
    51  // Clean returns the shortest path name equivalent to path
    52  // by purely lexical processing. It applies the following rules
    53  // iteratively until no further processing can be done:
    54  //
    55  //  1. Replace multiple slashes with a single slash.
    56  //  2. Eliminate each . path name element (the current directory).
    57  //  3. Eliminate each inner .. path name element (the parent directory)
    58  //     along with the non-.. element that precedes it.
    59  //  4. Eliminate .. elements that begin a rooted path:
    60  //     that is, replace "/.." by "/" at the beginning of a path.
    61  //
    62  // The returned path ends in a slash only if it is the root "/".
    63  //
    64  // If the result of this process is an empty string, Clean
    65  // returns the string ".".
    66  //
    67  // See also Rob Pike, ā€œLexical File Names in Plan 9 or
    68  // Getting Dot-Dot Right,ā€
    69  // https://9p.io/sys/doc/lexnames.html
    70  func Clean(path string) string {
    71  	if path == "" {
    72  		return "."
    73  	}
    74  
    75  	rooted := path[0] == '/'
    76  	n := len(path)
    77  
    78  	// Invariants:
    79  	//	reading from path; r is index of next byte to process.
    80  	//	writing to buf; w is index of next byte to write.
    81  	//	dotdot is index in buf where .. must stop, either because
    82  	//		it is the leading slash or it is a leading ../../.. prefix.
    83  	out := lazybuf{s: path}
    84  	r, dotdot := 0, 0
    85  	if rooted {
    86  		out.append('/')
    87  		r, dotdot = 1, 1
    88  	}
    89  
    90  	for r < n {
    91  		switch {
    92  		case path[r] == '/':
    93  			// empty path element
    94  			r++
    95  		case path[r] == '.' && (r+1 == n || path[r+1] == '/'):
    96  			// . element
    97  			r++
    98  		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || path[r+2] == '/'):
    99  			// .. element: remove to last /
   100  			r += 2
   101  			switch {
   102  			case out.w > dotdot:
   103  				// can backtrack
   104  				out.w--
   105  				for out.w > dotdot && out.index(out.w) != '/' {
   106  					out.w--
   107  				}
   108  			case !rooted:
   109  				// cannot backtrack, but not rooted, so append .. element.
   110  				if out.w > 0 {
   111  					out.append('/')
   112  				}
   113  				out.append('.')
   114  				out.append('.')
   115  				dotdot = out.w
   116  			}
   117  		default:
   118  			// real path element.
   119  			// add slash if needed
   120  			if rooted && out.w != 1 || !rooted && out.w != 0 {
   121  				out.append('/')
   122  			}
   123  			// copy element
   124  			for ; r < n && path[r] != '/'; r++ {
   125  				out.append(path[r])
   126  			}
   127  		}
   128  	}
   129  
   130  	// Turn empty string into "."
   131  	if out.w == 0 {
   132  		return "."
   133  	}
   134  
   135  	return out.string()
   136  }
   137  
   138  // lastSlash(s) is strings.LastIndex(s, "/") but we can't import strings.
   139  func lastSlash(s string) int {
   140  	i := len(s) - 1
   141  	for i >= 0 && s[i] != '/' {
   142  		i--
   143  	}
   144  	return i
   145  }
   146  
   147  // Split splits path immediately following the final slash,
   148  // separating it into a directory and file name component.
   149  // If there is no slash in path, Split returns an empty dir and
   150  // file set to path.
   151  // The returned values have the property that path = dir+file.
   152  func Split(path string) (dir, file string) {
   153  	i := lastSlash(path)
   154  	return path[:i+1], path[i+1:]
   155  }
   156  
   157  // Join joins any number of path elements into a single path,
   158  // separating them with slashes. Empty elements are ignored.
   159  // The result is Cleaned. However, if the argument list is
   160  // empty or all its elements are empty, Join returns
   161  // an empty string.
   162  func Join(elem ...string) string {
   163  	size := 0
   164  	for _, e := range elem {
   165  		size += len(e)
   166  	}
   167  	if size == 0 {
   168  		return ""
   169  	}
   170  	buf := make([]byte, 0, size+len(elem)-1)
   171  	for _, e := range elem {
   172  		if len(buf) > 0 || e != "" {
   173  			if len(buf) > 0 {
   174  				buf = append(buf, '/')
   175  			}
   176  			buf = append(buf, e...)
   177  		}
   178  	}
   179  	return Clean(string(buf))
   180  }
   181  
   182  // Ext returns the file name extension used by path.
   183  // The extension is the suffix beginning at the final dot
   184  // in the final slash-separated element of path;
   185  // it is empty if there is no dot.
   186  func Ext(path string) string {
   187  	for i := len(path) - 1; i >= 0 && path[i] != '/'; i-- {
   188  		if path[i] == '.' {
   189  			return path[i:]
   190  		}
   191  	}
   192  	return ""
   193  }
   194  
   195  // Base returns the last element of path.
   196  // Trailing slashes are removed before extracting the last element.
   197  // If the path is empty, Base returns ".".
   198  // If the path consists entirely of slashes, Base returns "/".
   199  func Base(path string) string {
   200  	if path == "" {
   201  		return "."
   202  	}
   203  	// Strip trailing slashes.
   204  	for len(path) > 0 && path[len(path)-1] == '/' {
   205  		path = path[0 : len(path)-1]
   206  	}
   207  	// Find the last element
   208  	if i := lastSlash(path); i >= 0 {
   209  		path = path[i+1:]
   210  	}
   211  	// If empty now, it had only slashes.
   212  	if path == "" {
   213  		return "/"
   214  	}
   215  	return path
   216  }
   217  
   218  // IsAbs reports whether the path is absolute.
   219  func IsAbs(path string) bool {
   220  	return len(path) > 0 && path[0] == '/'
   221  }
   222  
   223  // Dir returns all but the last element of path, typically the path's directory.
   224  // After dropping the final element using Split, the path is Cleaned and trailing
   225  // slashes are removed.
   226  // If the path is empty, Dir returns ".".
   227  // If the path consists entirely of slashes followed by non-slash bytes, Dir
   228  // returns a single slash. In any other case, the returned path does not end in a
   229  // slash.
   230  func Dir(path string) string {
   231  	dir, _ := Split(path)
   232  	return Clean(dir)
   233  }
   234  

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