clarify lowercasing implementation

apollo-api/src/main/java/com/spotify/apollo/HeadersValue.java

@@ -25,7 +25,6 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Optional;
-import java.util.function.Function;
 
 import javax.annotation.Nonnull;
 
@@ -44,28 +43,38 @@ public Optional<String> get(@Nonnull String name) {
   }
 
   static Headers create(Map<String, String> headers) {
-    Map<String, String> allLowerCaseKeys = headers.keySet().stream()
-        .filter(HeadersValue::hasNonLowerCase)
-        .findAny()
-        .map(lowerCaseKeys(headers))
-        .orElse(headers);
+    return new AutoValue_HeadersValue(convertKeysToLowerCase(headers));
+  }
 
-    return new AutoValue_HeadersValue(allLowerCaseKeys);
+  private static Map<String, String> convertKeysToLowerCase(Map<String, String> headers) {
+    // we expect the most common case to be that all header names are already lower-case,
+    // since most requests we handle are internal, originating from other Apollo services.
+    // hence, attempt to save some allocation through reusing the original map in that case.
+    if (keysAlreadyLowerCase(headers)) {
+      return headers;
+    }
+
+    return toLowerCaseKeys(headers);
   }
 
-  private static Function<String, Map<String, String>> lowerCaseKeys(Map<String, String> headers) {
-    return s -> {
-      Map<String, String> result = new LinkedHashMap<>(headers.size());
+  private static boolean keysAlreadyLowerCase(Map<String, String> headers) {
+    return !headers.keySet().stream()
+        .filter(HeadersValue::hasUpperCase)
+        .findAny()
+        .isPresent();
+  }
 
-      for (Map.Entry<String, String> entry : headers.entrySet()) {
-        result.put(entry.getKey().toLowerCase(), entry.getValue());
-      }
+  private static Map<String, String> toLowerCaseKeys(Map<String, String> headers) {
+    Map<String, String> result = new LinkedHashMap<>(headers.size());
+
+    for (Map.Entry<String, String> entry : headers.entrySet()) {
+      result.put(entry.getKey().toLowerCase(), entry.getValue());
+    }
 
-      return result;
-    };
+    return result;
   }
 
-  private static boolean hasNonLowerCase(String s) {
+  private static boolean hasUpperCase(String s) {
     for (char c : s.toCharArray()) {
       if (ASCII_UPPER_CASE.matches(c)) {
         return true;

apollo-api/src/test/java/com/spotify/apollo/HeadersValueTest.java

@@ -0,0 +1,50 @@
+/*
+ * -\-\-
+ * Spotify Apollo API Interfaces
+ * --
+ * Copyright (C) 2013 - 2016 Spotify AB
+ * --
+ * 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.spotify.apollo;
+
+import com.google.common.collect.ImmutableMap;
+
+import org.junit.Test;
+
+import java.util.Map;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.CoreMatchers.sameInstance;
+import static org.junit.Assert.assertThat;
+
+public class HeadersValueTest {
+  private Map<String, String> headers;
+
+  @Test
+  public void shouldReuseMapIfAlreadyLowerCase() throws Exception {
+    headers = ImmutableMap.of("key", "value", "another-key", "val");
+
+    assertThat(HeadersValue.create(headers).asMap(), is(sameInstance(headers)));
+  }
+
+  @Test
+  public void shouldConvertKeysToLowerCase() throws Exception {
+    headers = ImmutableMap.of("KEY", "value", "Another-Key", "val");
+
+    assertThat(HeadersValue.create(headers).asMap(),
+               equalTo(ImmutableMap.of("key", "value", "another-key", "val")));
+  }
+}