Add CanonPath::makeRelative()

src/libutil/canon-path.cc

@@ -100,4 +100,30 @@ std::ostream & operator << (std::ostream & stream, const CanonPath & path)
     return stream;
 }
 
+std::string CanonPath::makeRelative(const CanonPath & path) const
+{
+    auto p1 = begin();
+    auto p2 = path.begin();
+
+    for (; p1 != end() && p2 != path.end() && *p1 == *p2; ++p1, ++p2) ;
+
+    if (p1 == end() && p2 == path.end())
+        return ".";
+    else if (p1 == end())
+        return std::string(p2.remaining);
+    else {
+        std::string res;
+        while (p1 != end()) {
+            ++p1;
+            if (!res.empty()) res += '/';
+            res += "..";
+        }
+        if (p2 != path.end()) {
+            if (!res.empty()) res += '/';
+            res += p2.remaining;
+        }
+        return res;
+    }
+}
+
 }