Improve sourcemap output for prepended texts

Fixes #879
Should work for all prepended texts (like top-comments)

emitter.cpp

@@ -88,7 +88,7 @@ namespace Sass {
   // prepend some text or token to the buffer
   void Emitter::prepend_string(const string& text)
   {
-    // update source-map for new text
+    wbuf.smap.prepend(Offset(text));
     wbuf.buffer = text + wbuf.buffer;
   }
 

position.cpp

@@ -4,6 +4,18 @@ namespace Sass {
 
   using namespace std;
 
+  Offset::Offset(const char* string)
+  : line(0), column(0)
+  {
+    *this = inc(string, string + strlen(string));
+  }
+
+  Offset::Offset(const string& text)
+  : line(0), column(0)
+  {
+    *this = inc(text.c_str(), text.c_str() + text.size());
+  }
+
   Offset::Offset(const size_t line, const size_t column)
   : line(line), column(column) { }
 
@@ -34,7 +46,12 @@ namespace Sass {
     return line != pos.line || column != pos.column;
   }
 
-  const Offset Offset::operator+ (const Offset &off) const
+  void Offset::operator+= (const Offset &off)
+  {
+    *this = Offset(line + off.line, off.line > 0 ? off.column : off.column + column);
+  }
+
+  Offset Offset::operator+ (const Offset &off) const
   {
     return Offset(line + off.line, off.line > 0 ? off.column : off.column + column);
   }
@@ -67,7 +84,7 @@ namespace Sass {
   Position Position::inc(const char* begin, const char* end) const
   {
     Offset offset(line, column);
-    offset.inc(begin, end);
+    offset = offset.inc(begin, end);
     return Position(file, offset);
   }
 
@@ -81,6 +98,11 @@ namespace Sass {
     return file == pos.file || line != pos.line || column != pos.column;
   }
 
+  void Position::operator+= (const Offset &off)
+  {
+    *this = Position(file, line + off.line, off.line > 0 ? off.column : off.column + column);
+  }
+
   const Position Position::operator+ (const Offset &off) const
   {
     return Position(file, line + off.line, off.line > 0 ? off.column : off.column + column);

position.hpp

@@ -14,15 +14,18 @@ namespace Sass {
   class Offset {
 
     public: // c-tor
+      Offset(const char* string);
+      Offset(const string& text);
       Offset(const size_t line, const size_t column);
 
       // return new position, incremented by the given string
       Offset inc(const char* begin, const char* end) const;
 
     public: // overload operators for position
+      void operator+= (const Offset &pos);
       bool operator== (const Offset &pos) const;
       bool operator!= (const Offset &pos) const;
-      const Offset operator+ (const Offset &off) const;
+      Offset operator+ (const Offset &off) const;
 
     public: // overload output stream operator
       // friend ostream& operator<<(ostream& strm, const Offset& off);
@@ -45,6 +48,7 @@ namespace Sass {
       Position(const size_t file, const size_t line, const size_t column);
 
     public: // overload operators for position
+      void operator+= (const Offset &off);
       bool operator== (const Position &pos) const;
       bool operator!= (const Position &pos) const;
       const Position operator+ (const Offset &off) const;

source_map.cpp

@@ -112,6 +112,24 @@ namespace Sass {
     return result;
   }
 
+  void SourceMap::prepend(const Offset& offset)
+  {
+    if (offset.line != 0 || offset.column != 0) {
+      for (Mapping& mapping : mappings) {
+        // move stuff on the first old line
+        if (mapping.generated_position.line == 0) {
+          mapping.generated_position.column += offset.column;
+        }
+        // make place for the new lines
+        mapping.generated_position.line += offset.line;
+      }
+    }
+    if (current_position.line == 0) {
+      current_position.column += offset.column;
+    }
+    current_position.line += offset.line;
+  }
+
   void SourceMap::update_column(const string& str)
   {
     const ptrdiff_t new_line_count = std::count(str.begin(), str.end(), '\n');

source_map.hpp

@@ -5,6 +5,7 @@
 
 #include "ast_fwd_decl.hpp"
 #include "base64vlq.hpp"
+#include "position.hpp"
 #include "mapping.hpp"
 
 namespace Sass {
@@ -22,6 +23,8 @@ namespace Sass {
     void setFile(const string& str) {
       file = str;
     }
+    // void append(const Offset& offset);
+    void prepend(const Offset& offset);
     void update_column(const string& str);
     void add_open_mapping(AST_Node* node);
     void add_close_mapping(AST_Node* node);