improve sourceFileName gennerate logic with new relative method

Author: creeperyang

index.js

@@ -8,7 +8,6 @@ var exists = require('./lib/helpers/exists')();
 var read = require('./lib/helpers/read')();
 var resolveRc = require('./lib/resolve-rc.js');
 var pkg = require('./package.json');
-var path = require('path');
 
 var transpile = function(source, options) {
   var result = babel.transform(source, options);
@@ -25,6 +24,46 @@ var transpile = function(source, options) {
   };
 };
 
+/**
+* Make a path relative to a URL or another path.
+* Borrowed from https://github.com/mozilla/source-map/blob/master/lib/util.js
+*
+* @param aRoot The root path or URL.
+* @param aPath The path or URL to be made relative to aRoot.
+*/
+function relative(aRoot, aPath) {
+  if (aRoot === '') {
+    aRoot = '.';
+  }
+
+  aRoot = aRoot.replace(/\/$/, '');
+
+  // It is possible for the path to be above the root. In this case, simply
+  // checking whether the root is a prefix of the path won't work. Instead, we
+  // need to remove components from the root one by one, until either we find
+  // a prefix that fits, or we run out of components to remove.
+  var level = 0;
+  while (aPath.indexOf(aRoot + '/') !== 0) {
+    var index = aRoot.lastIndexOf('/');
+    if (index < 0) {
+      return aPath;
+    }
+
+    // If the only part of the root that is left is the scheme (i.e. http://,
+    // file:///, etc.), one or more slashes (/), or simply nothing at all, we
+    // have exhausted all components, so the path is not relative to the root.
+    aRoot = aRoot.slice(0, index);
+    if (aRoot.match(/^([^\/]+:\/)?\/*$/)) {
+      return aPath;
+    }
+
+    ++level;
+  }
+
+  // Make sure we add a '../' for each component we removed from the root.
+  return Array(level + 1).join('../') + aPath.substr(aRoot.length + 1);
+}
+
 module.exports = function(source, inputSourceMap) {
   var result = {};
 
@@ -57,7 +96,7 @@ module.exports = function(source, inputSourceMap) {
   }
 
   if (options.sourceFileName === undefined) {
-    options.sourceFileName = path.relative(
+    options.sourceFileName = relative(
         options.sourceRoot,
         options.filename
     );