cmDependsC: Collapse relative include paths

While calculating dependencies collapse sequences such as

 ../../../a/b/c/../../d/e/../../e/f

to avoid total path lengths over the Windows path length limit as much
as possible.

Source/cmDependsC.cxx

@@ -193,17 +193,8 @@ bool cmDependsC::WriteDependencies(const std::set<std::string>& sources,
           // Construct the name of the file as if it were in the current
           // include directory.  Avoid using a leading "./".
 
-          tempPathStr = "";
-          if((*i) == ".")
-            {
-            tempPathStr += current.FileName;
-            }
-          else
-            {
-            tempPathStr += *i;
-            tempPathStr+="/";
-            tempPathStr+=current.FileName;
-            }
+          tempPathStr =
+            cmSystemTools::CollapseCombinedPath(*i, current.FileName);
 
           // Look for the file in this location.
           if(cmSystemTools::FileExists(tempPathStr.c_str(), true))
@@ -458,9 +449,8 @@ void cmDependsC::Scan(std::istream& is, const char* directory,
         // This was a double-quoted include with a relative path.  We
         // must check for the file in the directory containing the
         // file we are scanning.
-        entry.QuotedLocation = directory;
-        entry.QuotedLocation += "/";
-        entry.QuotedLocation += entry.FileName;
+        entry.QuotedLocation =
+          cmSystemTools::CollapseCombinedPath(directory, entry.FileName);
         }
 
       // Queue the file if it has not yet been encountered and it

Source/cmSystemTools.cxx

@@ -1594,6 +1594,40 @@ std::string cmSystemTools::RelativePath(const char* local, const char* remote)
   return cmsys::SystemTools::RelativePath(local, remote);
 }
 
+std::string cmSystemTools::CollapseCombinedPath(std::string const& dir,
+                                                std::string const& file)
+{
+  if(dir.empty() || dir == ".")
+    {
+    return file;
+    }
+
+  std::vector<std::string> dirComponents;
+  std::vector<std::string> fileComponents;
+  cmSystemTools::SplitPath(dir.c_str(), dirComponents);
+  cmSystemTools::SplitPath(file.c_str(), fileComponents);
+
+  if(fileComponents.empty())
+    {
+    return dir;
+    }
+  if(fileComponents[0] != "")
+    {
+    // File is not a relative path.
+    return file;
+    }
+
+  std::vector<std::string>::iterator i = fileComponents.begin()+1;
+  while(i != fileComponents.end() && *i == ".." && dirComponents.size() > 1)
+    {
+    ++i; // Remove ".." file component.
+    dirComponents.pop_back(); // Remove last dir component.
+    }
+
+  dirComponents.insert(dirComponents.end(), i, fileComponents.end());
+  return cmSystemTools::JoinPath(dirComponents);
+}
+
 #ifdef CMAKE_BUILD_WITH_CMAKE
 //----------------------------------------------------------------------
 bool cmSystemTools::UnsetEnv(const char* value)

Source/cmSystemTools.h

@@ -365,6 +365,12 @@ public:
   */
   static std::string RelativePath(const char* local, const char* remote);
 
+  /** Joins two paths while collapsing x/../ parts
+   * For example CollapseCombinedPath("a/b/c", "../../d") results in "a/d"
+   */
+  static std::string CollapseCombinedPath(std::string const& dir,
+                                          std::string const& file);
+
 #ifdef CMAKE_BUILD_WITH_CMAKE
   /** Remove an environment variable */
   static bool UnsetEnv(const char* value);