Cache the indentation strings of the output lines (google-java-format #1466)

open-java-format/src/main/java/com/palantir/javaformat/java/JavaOutput.java

@@ -56,6 +56,25 @@ public final class JavaOutput extends Output {
     // never ends in whitespace: trailing tabs inside a comment were kept before, and only a second run removed them.
     private StringBuilder spacesPending = new StringBuilder();
 
+    private static final int MAX_CACHED_SPACES = 100;
+    private static final String[] SPACES = new String[MAX_CACHED_SPACES + 1];
+
+    /** Returns an indentation string of the given number of spaces. */
+    static String spaces(int indent) {
+        if (indent <= 0) {
+            return "";
+        }
+        if (indent <= MAX_CACHED_SPACES) {
+            String result = SPACES[indent];
+            if (result == null) {
+                result = " ".repeat(indent);
+                SPACES[indent] = result;
+            }
+            return result;
+        }
+        return " ".repeat(indent);
+    }
+
     /**
      * {@code JavaOutput} constructor.
      *
@@ -157,7 +176,7 @@ public void append(State state, String text, Range<Integer> range) {
 
     @Override
     public void indent(int indent) {
-        spacesPending.append(" ".repeat(indent));
+        spacesPending.append(spaces(indent));
     }
 
     /** Flush any incomplete last line, then add the EOF token into our data structures. */

open-java-format/src/test/java/com/palantir/javaformat/java/JavaOutputTest.java

@@ -0,0 +1,43 @@
+/*
+ * (c) Copyright 2026 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.javaformat.java;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+final class JavaOutputTest {
+
+    @Test
+    void cachesTheIndentationStrings() {
+        // Every output line used to get a fresh `" ".repeat(indent)`; the usual widths now come from a cache.
+        assertThat(JavaOutput.spaces(4)).isEqualTo("    ");
+        assertThat(JavaOutput.spaces(4)).isSameAs(JavaOutput.spaces(4));
+        assertThat(JavaOutput.spaces(100)).isSameAs(JavaOutput.spaces(100));
+    }
+
+    @Test
+    void returnsNoSpacesForZeroOrLess() {
+        assertThat(JavaOutput.spaces(0)).isEmpty();
+        assertThat(JavaOutput.spaces(-1)).isEmpty();
+    }
+
+    @Test
+    void buildsWidthsBeyondTheCache() {
+        assertThat(JavaOutput.spaces(104)).isEqualTo(" ".repeat(104));
+    }
+}