Extract logic for created header controlled resource bundles

service/src/main/java/org/signal/i18n/HeaderControlledResourceBundleLookup.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.i18n;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.List;
+import java.util.Locale;
+import java.util.Objects;
+import java.util.ResourceBundle;
+import java.util.ResourceBundle.Control;
+import java.util.stream.Collectors;
+import javax.annotation.Nonnull;
+
+public class HeaderControlledResourceBundleLookup {
+
+  private static final int MAX_LOCALES = 15;
+
+  private final ResourceBundleFactory resourceBundleFactory;
+
+  public HeaderControlledResourceBundleLookup() {
+    this(ResourceBundle::getBundle);
+  }
+
+  @VisibleForTesting
+  public HeaderControlledResourceBundleLookup(
+      @Nonnull final ResourceBundleFactory resourceBundleFactory) {
+    this.resourceBundleFactory = Objects.requireNonNull(resourceBundleFactory);
+  }
+
+  @Nonnull
+  private List<Locale> getAcceptableLocales(final List<Locale> acceptableLanguages) {
+    return acceptableLanguages.stream().limit(MAX_LOCALES).distinct().collect(Collectors.toList());
+  }
+
+  @Nonnull
+  public ResourceBundle getResourceBundle(final String baseName, final List<Locale> acceptableLocales) {
+    final List<Locale> deduplicatedLocales = getAcceptableLocales(acceptableLocales);
+    final Locale desiredLocale = deduplicatedLocales.isEmpty() ? Locale.getDefault() : deduplicatedLocales.get(0);
+    // define a control with a fallback order as specified in the header
+    Control control = new Control() {
+      @Override
+      public List<String> getFormats(final String baseName) {
+        Objects.requireNonNull(baseName);
+        return Control.FORMAT_PROPERTIES;
+      }
+
+      @Override
+      public Locale getFallbackLocale(final String baseName, final Locale locale) {
+        Objects.requireNonNull(baseName);
+        if (locale.equals(Locale.getDefault())) {
+          return null;
+        }
+        final int localeIndex = deduplicatedLocales.indexOf(locale);
+        if (localeIndex < 0 || localeIndex >= deduplicatedLocales.size() - 1) {
+          return Locale.getDefault();
+        }
+        // [0, deduplicatedLocales.size() - 2] is now the possible range for localeIndex
+        return deduplicatedLocales.get(localeIndex + 1);
+      }
+    };
+
+    return resourceBundleFactory.createBundle(baseName, desiredLocale, control);
+  }
+}

service/src/main/java/org/signal/i18n/ResourceBundleFactory.java

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.i18n;
+
+import java.util.Locale;
+import java.util.ResourceBundle;
+
+public interface ResourceBundleFactory {
+  ResourceBundle createBundle(String baseName, Locale locale, ResourceBundle.Control control);
+}