1 files changed, 69 insertions, 0 deletions
diff --git a/src/pkg/path/filepath/path.go b/src/pkg/path/filepath/path.go
index c40d9ff564..afb8f102d5 100644
--- a/src/pkg/path/filepath/path.go
+++ b/src/pkg/path/filepath/path.go
@@ -258,6 +258,75 @@ func Abs(path string) (string, os.Error) {
 	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.