Centralize token generation and formats

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135313726

java/google/registry/util/StringGenerator.java

@@ -0,0 +1,56 @@
+// Copyright 2016 The Domain Registry Authors. 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 google.registry.util;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Strings.isNullOrEmpty;
+
+import com.google.common.collect.ImmutableList;
+import java.util.Collection;
+
+/** String generator. */
+public abstract class StringGenerator {
+
+  /** A class containing different alphabets used to generate strings. */
+  public static class Alphabets {
+
+    /** A URL-safe Base64 alphabet (alphanumeric, hyphen, underscore). */
+    public static final String BASE_64 =
+        "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-";
+
+    /** An alphanumeric alphabet that omits visually similar characters. */
+    public static final String BASE_58 =
+        "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
+  }
+
+  protected String alphabet;
+
+  protected StringGenerator(String alphabet) {
+    checkArgument(!isNullOrEmpty(alphabet), "Alphabet cannot be null or empty.");
+    this.alphabet = alphabet;
+  }
+
+  /** Generates a string of a specified length. */
+  public abstract String createString(int length);
+
+  /** Batch-generates an {@link ImmutableList} of strings of a specified length. */
+  public Collection<String> createStrings(int length, int count) {
+    ImmutableList.Builder<String> listBuilder = new ImmutableList.Builder<>();
+    for (int i = 0; i < count; i++) {
+      listBuilder.add(createString(length));
+    }
+    return listBuilder.build();
+  }
+}