ENH: added fileName::relative() method

- this currently just strips off the leading parent directory name

     "/this/path/and/subdirs/name"

     relative("/this/path")  -> "and/subdirs/name"
     relative("/this")       -> "path/and/subdirs/name"

applications/test/fileName/Test-fileName.C

@@ -161,6 +161,29 @@ unsigned testEquals
 }
 
 
+unsigned testRelative(std::initializer_list<Pair<std::string>> tests)
+{
+    Info<< nl << "Checking fileName::relative()" << nl << endl;
+
+    unsigned nFail = 0;
+
+    for (const Pair<std::string>& test : tests)
+    {
+        const std::string& dir    = test.first();
+        const std::string& parent = test.second();
+
+        fileName relative = fileName(dir).relative(parent);
+
+        Info<< "directory: " << dir << nl
+            << "parent   : " << parent << nl
+            << "relative = " << relative << nl
+            << endl;
+    }
+
+    return nFail;
+}
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 // Main program:
 
@@ -170,12 +193,13 @@ int main(int argc, char *argv[])
     argList::addBoolOption("validate", "test fileName::validate");
     argList::addBoolOption("ext", "test handing of file extensions");
     argList::addBoolOption("construct", "test constructors");
+    argList::addBoolOption("relative", "test relative operations");
     argList::addBoolOption("default", "reinstate default tests");
     argList::addNote("runs default tests or specified ones only");
 
     #include "setRootCase.H"
 
-    // Run default tests, unless only specific tests are requested
+    // Run default tests, unless specific tests are requested
     const bool defaultTests =
         args.found("default") || args.options().empty();
 
@@ -418,6 +442,49 @@ int main(int argc, char *argv[])
     }
 
 
+    if (args.found("relative"))
+    {
+        unsigned nFail = 0;
+
+        nFail += testRelative
+        (
+            {
+                { "/some/dir/subdir/name",  "" },
+                { "/some/dir/subdir/name",  "/" },
+                { "/some/dir/subdir/name",  "/some" },
+                { "/some/dir/subdir/name",  "/some/" },
+                { "/some/dir/subdir/name",  "/some/dir" },
+                { "/some/dir/subdir/name",  "/some/dir/" },
+                { "/some/dir/subdir/name",  "/some/dir/subdir/name" },
+                { "/some/dir/subdir/name",  "/some/dir/subdir/name/" },
+                { "/some/dir/subdir/name",  "/some/other" },
+
+                // With single-char for name
+                { "/some/dir/subdir/a",  "" },
+                { "/some/dir/subdir/a",  "/" },
+                { "/some/dir/subdir/a",  "/some" },
+                { "/some/dir/subdir/a",  "/some/" },
+                { "/some/dir/subdir/a",  "/some/dir" },
+                { "/some/dir/subdir/a",  "/some/dir/" },
+                { "/some/dir/subdir/a",  "/some/dir/subdir/a" },
+                { "/some/dir/subdir/a",  "/some/dir/subdir/a/" },
+                { "/some/dir/subdir/a",  "/some/other" },
+
+                // Bad input (trailing slash)
+                { "/some/dir/subdir/a/",  "" },
+                { "/some/dir/subdir/a/",  "/" },
+                { "/some/dir/subdir/a/",  "/some" },
+                { "/some/dir/subdir/a/",  "/some/" },
+                { "/some/dir/subdir/a/",  "/some/dir" },
+                { "/some/dir/subdir/a/",  "/some/dir/" },
+                { "/some/dir/subdir/a/",  "/some/dir/subdir/a" },
+                { "/some/dir/subdir/a/",  "/some/dir/subdir/a/" },
+                { "/some/dir/subdir/a/",  "/some/other" },
+            }
+        );
+    }
+
+
     if (!defaultTests)
     {
         return 0;

applications/utilities/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -320,31 +320,6 @@ vtk::outputOptions getOutputOptions(const argList& args)
 }
 
 
-fileName relativeName(const fileName& parent, const fileName& file)
-{
-    string::size_type next = parent.size();
-    if
-    (
-        file.startsWith(parent)
-     && next < file.size()
-     && file[next] == '/'
-    )
-    {
-        return file.substr(next+1);
-    }
-    else
-    {
-        return file;
-    }
-}
-
-
-fileName relativeName(const Time& runTime, const fileName& file)
-{
-    return relativeName(runTime.path(), file);
-}
-
-
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 int main(int argc, char *argv[])
@@ -672,7 +647,7 @@ int main(int argc, char *argv[])
               + timeDesc
             );
             Info<< "    faceSet   : "
-                << relativeName(runTime, outputName) << nl;
+                << outputName.relative(runTime.path()) << nl;
 
             vtk::writeFaceSet
             (
@@ -700,7 +675,7 @@ int main(int argc, char *argv[])
               + timeDesc
             );
             Info<< "    pointSet  : "
-                << relativeName(runTime, outputName) << nl;
+                << outputName.relative(runTime.path()) << nl;
 
             vtk::writePointSet
             (
@@ -1095,7 +1070,7 @@ int main(int argc, char *argv[])
               + timeDesc
             );
             Info<< "    Internal  : "
-                << relativeName(runTime, outputName) << endl;
+                << outputName.relative(runTime.path()) << nl;
 
             // Write mesh
             vtk::internalWriter writer
@@ -1255,7 +1230,7 @@ int main(int argc, char *argv[])
               + timeDesc
             );
             Info<< "    Combined patches    : "
-                << relativeName(runTime, outputName) << nl;
+                << outputName.relative(runTime.path()) << nl;
 
             vtk::patchWriter writer
             (
@@ -1325,7 +1300,7 @@ int main(int argc, char *argv[])
                   + timeDesc
                 );
                 Info<< "    Patch     : "
-                    << relativeName(runTime, outputName) << nl;
+                    << outputName.relative(runTime.path()) << nl;
 
                 vtk::patchWriter writer
                 (
@@ -1426,7 +1401,7 @@ int main(int argc, char *argv[])
                   + timeDesc
                 );
                 Info<< "    FaceZone  : "
-                    << relativeName(runTime, outputName) << nl;
+                    << outputName.relative(runTime.path()) << nl;
 
                 indirectPrimitivePatch pp
                 (
@@ -1472,7 +1447,7 @@ int main(int argc, char *argv[])
               + "_" + timeDesc
             );
             Info<< "    Lagrangian: "
-                << relativeName(runTime, outputName) << nl;
+                << outputName.relative(runTime.path()) << nl;
 
             IOobjectList cloudObjs
             (

src/OpenFOAM/primitives/strings/fileName/fileName.C

@@ -404,6 +404,25 @@ Foam::fileName Foam::fileName::path() const
 }
 
 
+Foam::fileName Foam::fileName::relative(const fileName& parent) const
+{
+    const auto top = parent.size();
+    const fileName& f = *this;
+
+    // Everything after "parent/xxx" -> "xxx"
+    if
+    (
+        top && (f.size() > (top+1)) && (*this)[top] == '/'
+     && f.startsWith(parent)
+    )
+    {
+        return f.substr(top+1);
+    }
+
+    return f;
+}
+
+
 Foam::fileName Foam::fileName::lessExt() const
 {
     const auto i = find_ext();

src/OpenFOAM/primitives/strings/fileName/fileName.H

@@ -265,6 +265,9 @@ public:
         //- Return directory path name (part before last /)
         fileName path() const;
 
+        //- Return name after stripping off the parent directory
+        fileName relative(const fileName& parent) const;
+
         //- Return file name without extension (part before last .)
         fileName lessExt() const;