lib: add String::relpath for advanced path manipulation

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/lib/standard/file.nit b/lib/standard/file.nit
index 3499c89..02edcaf 100644 (file)
--- a/lib/standard/file.nit
+++ b/lib/standard/file.nit
@@ -364,6 +364,70 @@ redef class String
        #     assert a/b/c == "/bar/baz/foobar"
        fun /(path: String): String do return join_path(path)
 
+       # Returns the relative path needed to go from `self` to `dest`.
+       #
+       #     assert "/foo/bar".relpath("/foo/baz") == "../baz"
+       #     assert "/foo/bar".relpath("/baz/bar") == "../../baz/bar"
+       #
+       # If `self` or `dest` is relative, they are considered relatively to `getcwd`.
+       #
+       # In some cases, the result is still independent of the current directory:
+       #
+       #     assert "foo/bar".relpath("..") == "../../.."
+       #
+       # In other cases, parts of the current directory may be exhibited:
+       #
+       #     var p = "../foo/bar".relpath("baz")
+       #     var c = getcwd.basename("")
+       #     assert p == "../../{c}/baz"
+       #
+       # For path resolution independent of the current directory (eg. for paths in URL),
+       # or to use an other starting directory than the current directory,
+       # just force absolute paths:
+       #
+       #     var start = "/a/b/c/d"
+       #     var p2 = (start/"../foo/bar").relpath(start/"baz")
+       #     assert p2 == "../../d/baz"
+       #
+       #
+       # Neither `self` or `dest` has to be real paths or to exist in directories since
+       # the resolution is only done with string manipulations and without any access to
+       # the underlying file system.
+       #
+       # If `self` and `dest` are the same directory, the empty string is returned:
+       #
+       #     assert "foo".relpath("foo") == ""
+       #     assert "foo/../bar".relpath("bar") == ""
+       #
+       # The empty string and "." designate both the current directory:
+       #
+       #     assert "".relpath("foo/bar")  == "foo/bar"
+       #     assert ".".relpath("foo/bar") == "foo/bar"
+       #     assert "foo/bar".relpath("")  == "../.."
+       #     assert "/" + "/".relpath(".") == getcwd
+       fun relpath(dest: String): String
+       do
+               var cwd = getcwd
+               var from = (cwd/self).simplify_path.split("/")
+               if from.last.is_empty then from.pop # case for the root directory
+               var to = (cwd/dest).simplify_path.split("/")
+               if to.last.is_empty then to.pop # case for the root directory
+
+               # Remove common prefixes
+               while not from.is_empty and not to.is_empty and from.first == to.first do
+                       from.shift
+                       to.shift
+               end
+
+               # Result is going up in `from` with ".." then going down following `to`
+               var from_len = from.length
+               if from_len == 0 then return to.join("/")
+               var up = "../"*(from_len-1) + ".."
+               if to.is_empty then return up
+               var res = up + "/" + to.join("/")
+               return res
+       end
+
        # Create a directory (and all intermediate directories if needed)
        fun mkdir
        do