Introduce method DocumentUtil#getIndent

added test

platform/core-impl/src/com/intellij/util/DocumentUtil.java

@@ -146,9 +146,9 @@ public final class DocumentUtil {
 
   /**
    * Calculates indent of the line containing {@code offset}
-   * @return amount of characters in indent
+   * @return Whitespaces at the beginning of the line
    */
-  public static int getIndent(@NotNull Document document, int offset) {
+  public static CharSequence getIndent(@NotNull Document document, int offset) {
     int lineOffset = getLineStartOffset(offset, document);
     int result = 0;
     while (lineOffset + result < document.getTextLength() &&
@@ -158,6 +158,6 @@ public final class DocumentUtil {
     if (result + lineOffset > document.getTextLength()) {
       result--;
     }
-    return Math.max(result, 0);
+    return document.getCharsSequence().subSequence(lineOffset, lineOffset + Math.max(result, 0));
   }
 }

platform/platform-tests/testSrc/com/intellij/openapi/editor/DocumentUtilTest.java

@@ -0,0 +1,17 @@
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+package com.intellij.openapi.editor;
+
+import com.intellij.openapi.editor.impl.DocumentImpl;
+import com.intellij.util.DocumentUtil;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class DocumentUtilTest {
+  @Test
+  public void testGetIndent() {
+    final Document doc = new DocumentImpl("line1\n" +
+                                          " \t line2");
+    assertEquals(" \t ", DocumentUtil.getIndent(doc, doc.getTextLength() - 1).toString());
+  }
+}