use "sourceRoot" in map instead to concatenate it to every source file name

src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java

@@ -63,8 +63,6 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
 
     private final boolean               createSourceMap;
 
-    private final String                javaSourceMapBase;
-
     private WasmOutputStream            codeStream          = new WasmOutputStream();
 
     private List<TypeEntry>             functionTypes       = new ArrayList<>();
@@ -104,7 +102,6 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
         this.target = target;
         // for now we build the source map together with debug names
         createSourceMap = options.debugNames();
-        javaSourceMapBase = options.getSourceMapBase();
     }
 
     /**
@@ -294,7 +291,7 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
         }
         wasm.writeSection( SectionType.Code, stream );
 
-        SourceMapWriter sourceMap = createSourceMap ? new SourceMapWriter() : null;
+        SourceMapWriter sourceMap = createSourceMap ? new SourceMapWriter( options.getSourceMapBase() ) : null;
         if( sourceMap != null ) {
             int offset = wasm.size() - start - stream.size();
             for( Function func : functions.values() ) {
@@ -563,7 +560,7 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
     protected void writeMethodStart( FunctionName name, String sourceFile ) throws IOException {
         if( createSourceMap ) {
             int idx = name.className.lastIndexOf( '/' );
-            this.javaSourceFile = javaSourceMapBase + name.className.substring( 0, idx + 1 ) + sourceFile;
+            this.javaSourceFile = name.className.substring( 0, idx + 1 ) + sourceFile;
         }
         codeStream.reset();
     }

src/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriter.java

@@ -21,6 +21,8 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map.Entry;
 
+import javax.annotation.Nullable;
+
 /**
  * Generates Source Map version 3.
  *
@@ -28,12 +30,24 @@ import java.util.Map.Entry;
  */
 public class SourceMapWriter {
 
-    private List<SourceMapping>                  mappings        = new ArrayList<>();
+    private final String                         sourceRoot;
 
-    private LinkedHashMap<String, Integer> sourceFileNames = new LinkedHashMap<String, Integer>();
+    private final List<SourceMapping>            mappings        = new ArrayList<>();
+
+    private final LinkedHashMap<String, Integer> sourceFileNames = new LinkedHashMap<String, Integer>();
 
     private int                                  nextSourceFileNameIndex;
 
+    /**
+     * Create a new instance of the writer for a single map file.
+     * 
+     * @param sourceRoot
+     *            optional absolute or relative path to the sources
+     */
+    public SourceMapWriter( @Nullable String sourceRoot ) {
+        this.sourceRoot = sourceRoot;
+    }
+
     /**
      * Adds a mapping for the given node. Mappings must be added in order.
      * 
@@ -61,6 +75,11 @@ public class SourceMapWriter {
         out.append( "{\n" );
         appendJsonField( out, "version", "3" );
 
+        // sourceRoot
+        if( sourceRoot != null && !sourceRoot.isEmpty() ) {
+            appendJsonField( out, "sourceRoot", sourceRoot );
+        }
+
         // the source file names
         out.append( ",\n" );
         appendJsonField( out, "sources", "[" );

test/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriterTest.java

@@ -10,7 +10,7 @@ public class SourceMapWriterTest {
 
     @Test
     public void simple() throws IOException {
-        SourceMapWriter map = new SourceMapWriter();
+        SourceMapWriter map = new SourceMapWriter( null );
 
         map.addMapping( new SourceMapping( 0, 1, "Test1.java" ) );
         map.addMapping( new SourceMapping( 5, 2, "Test1.java" ) );