Refactor buffer for clarity and avoid regex for performance

lib/6to5/generation/buffer.js

@@ -67,32 +67,58 @@ Buffer.prototype.removeLast = function (cha) {
 
 Buffer.prototype.newline = function (i, removeLast) {
   if (this.format.compact) return;
-
-  if (_.isBoolean(i)) {
-    removeLast = i;
-    i = null;
-  }
+  removeLast = removeLast || false;
 
   if (_.isNumber(i)) {
     if (this.endsWith("{\n")) i--;
     if (this.endsWith(util.repeat(i, "\n"))) return;
 
-    for (var j = 0; j < i; j++) {
-      this.newline(null, removeLast);
+    while (i--) {
+      this._newline(removeLast);
     }
     return;
   }
 
+  if (_.isBoolean(i)) {
+    removeLast = i;
+  }
+
+  this._newline(removeLast);
+};
+
+Buffer.prototype._newline = function (removeLast) {
   if (removeLast && this.isLast("\n")) this.removeLast("\n");
 
   this.removeLast(" ");
 
   // remove whitespace if last character was a newline
-  this.buf = this.buf.replace(/\n +$/, "\n");
+  var trimStart = this._getPostNewlineTrailingSpaceStart();
+  if (trimStart) {
+    this.buf = this.buf.substring(0, trimStart);
+  }
 
   this._push("\n");
 };
 
+Buffer.prototype._getPostNewlineTrailingSpaceStart = function () {
+  var lastNewlineIndex = this.buf.lastIndexOf('\n');
+  if (lastNewlineIndex === -1)
+    return;
+
+  var index = this.buf.length - 1;
+  while (index > lastNewlineIndex) {
+    if (this.buf[index] !== ' ') {
+      break;
+    }
+
+    index--;
+  }
+
+  if (index === lastNewlineIndex) {
+    return index + 1;
+  }
+};
+
 Buffer.prototype.push = function (str, noIndent) {
   if (this._indent && !noIndent && str !== "\n") {
     // we have an indent level and we aren't pushing a newline