iphone - Objective-C code to generate a relative path given a file and a directory -

given file path , directory path nsstrings, have objective-c code generate path file, relative directory?

for example, given directory /tmp/foo , file /tmp/bar/test.txt, code should produce ../bar/test.txt.

i know python, @ least, has method this: os.path.relpath.

rather continue defend why need this, decided write , share. based off of implementation of python's os.path.relpath @ http://mail.python.org/pipermail/python-list/2009-august/1215220.html

@implementation nsstring (paths)  - (nsstring*)stringwithpathrelativeto:(nsstring*)anchorpath {     nsarray *pathcomponents = [self pathcomponents];     nsarray *anchorcomponents = [anchorpath pathcomponents];      nsinteger componentsincommon = min([pathcomponents count], [anchorcomponents count]);     (nsinteger = 0, n = componentsincommon; < n; i++) {         if (![[pathcomponents objectatindex:i] isequaltostring:[anchorcomponents objectatindex:i]]) {             componentsincommon = i;             break;         }     }      nsuinteger numberofparentcomponents = [anchorcomponents count] - componentsincommon;     nsuinteger numberofpathcomponents = [pathcomponents count] - componentsincommon;      nsmutablearray *relativecomponents = [nsmutablearray arraywithcapacity:                                           numberofparentcomponents + numberofpathcomponents];     (nsinteger = 0; < numberofparentcomponents; i++) {         [relativecomponents addobject:@".."];     }     [relativecomponents addobjectsfromarray:      [pathcomponents subarraywithrange:nsmakerange(componentsincommon, numberofpathcomponents)]];     return [nsstring pathwithcomponents:relativecomponents]; }  @end 

note there cases won't correctly handle. happens handle cases need. here skimpy unit test used verify correctness:

@implementation nsstringpathstests  - (void)testrelativepaths {     stassertequalobjects([@"/a" stringwithpathrelativeto:@"/"], @"a", @"");     stassertequalobjects([@"a/b" stringwithpathrelativeto:@"a"], @"b", @"");     stassertequalobjects([@"a/b/c" stringwithpathrelativeto:@"a"], @"b/c", @"");     stassertequalobjects([@"a/b/c" stringwithpathrelativeto:@"a/b"], @"c", @"");     stassertequalobjects([@"a/b/c" stringwithpathrelativeto:@"a/d"], @"../b/c", @"");     stassertequalobjects([@"a/b/c" stringwithpathrelativeto:@"a/d/e"], @"../../b/c", @"");     stassertequalobjects([@"/a/b/c" stringwithpathrelativeto:@"/d/e/f"], @"../../../a/b/c", @""); }  @end