sanitizeFileName — don't create stringBuilder/string if name is sanitized

platform/util/src/com/intellij/openapi/util/io/FileUtil.java

@@ -1053,21 +1053,40 @@ public class FileUtil extends FileUtilRt {
    */
   @NotNull
   public static String sanitizeFileName(@NotNull String name) {
-    StringBuilder result = new StringBuilder();
-
-    for (int i = 0; i < name.length(); i++) {
-      final char ch = name.charAt(i);
-
-      if (ch > 0 && ch < 255) {
-        if (Character.isLetterOrDigit(ch)) {
-          result.append(ch);
-        }
-        else {
-          result.append("_");
+    StringBuilder result = null;
+    int last = 0;
+    int length = name.length();
+    for (int i = 0; i < length; i++) {
+      char c = name.charAt(i);
+      boolean appendReplacement = true;
+      if (c > 0 && c < 255) {
+        if (Character.isLetterOrDigit(c) || c == '_') {
+          continue;
         }
       }
+      else {
+        appendReplacement = false;
+      }
+
+      if (result == null) {
+        result = new StringBuilder();
+      }
+      if (last < i) {
+        result.append(name, last, i);
+      }
+      if (appendReplacement) {
+        result.append('_');
+      }
+      last = i + 1;
     }
 
+    if (result == null) {
+      return name;
+    }
+
+    if (last < length) {
+      result.append(name, last, length);
+    }
     return result.toString();
   }
 

platform/util/testSrc/com/intellij/openapi/util/io/FileUtilLightTest.java

@@ -24,7 +24,6 @@ import com.intellij.util.containers.ContainerUtil;
 import com.intellij.util.containers.Convertor;
 import org.junit.Test;
 
-import java.io.File;
 import java.io.IOException;
 import java.io.StringReader;
 import java.util.Arrays;
@@ -32,6 +31,8 @@ import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.sameInstance;
 import static org.junit.Assert.*;
 
 public class FileUtilLightTest {
@@ -198,4 +199,18 @@ public class FileUtilLightTest {
     String expected = SystemInfo.isWindows ? "~\\relative" : "~/relative";
     assertEquals(expected, FileUtil.getLocationRelativeToUserHome(SystemProperties.getUserHome() + "/relative", false));
   }
+
+  @Test
+  public void sanitizeFileName() {
+    String newS = "tmp";
+    assertThat(FileUtil.sanitizeFileName(newS), sameInstance(newS));
+    assertThat(FileUtil.sanitizeFileName("_test"), sameInstance("_test"));
+
+    assertThat(FileUtil.sanitizeFileName(" "), equalTo("_"));
+    assertThat(FileUtil.sanitizeFileName("\u2026"), equalTo(""));
+    assertThat(FileUtil.sanitizeFileName("q_test"), sameInstance("q_test"));
+    assertThat(FileUtil.sanitizeFileName("12_"), sameInstance("12_"));
+    assertThat(FileUtil.sanitizeFileName("12_  123"), equalTo("12___123"));
+    assertThat(FileUtil.sanitizeFileName(" 12\u2026123"), equalTo("_12123"));
+  }
 }