Allow PasswordGenerator to use different alphabets

Per mcilwain's suggestion in the LRP design doc, LRP tokens should use a Base58 alphabet. I'll move PasswordGenerator out of the tools package and into a utils class in a future CL, as we'll want to use this generator in the LrpToken class itself rather than relegate the token definition to a tool.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132358363

javatests/google/registry/tools/DeterministicStringGenerator.java

@@ -0,0 +1,55 @@
+// 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.tools;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.collect.Lists.charactersOf;
+
+import com.google.common.collect.Iterators;
+import java.util.Iterator;
+
+import javax.inject.Named;
+
+/**
+ * A string generator that produces strings using sequential characters in its alphabet. This is
+ * most useful in tests as a "fake" password generator (which would otherwise use
+ * {@link RandomStringGenerator}.
+ * 
+ * <p>Note that consecutive calls to createString will continue where the last call left off in
+ * the alphabet.
+ */
+class DeterministicStringGenerator extends StringGenerator {
+
+  private Iterator<Character> iterator;
+
+  /**
+   * Generates a string using sequential characters in the generator's alphabet, cycling back to the
+   * beginning of the alphabet if necessary.
+   */
+  @Override
+  public String createString(int length) {
+    checkArgument(length > 0, "String length must be positive.");
+    String password = "";
+    for (int i = 0; i < length; i++) {
+      password += iterator.next();
+    }
+    return password;
+  }
+
+  public DeterministicStringGenerator(@Named("alphabet") String alphabet) {
+    super(alphabet);
+    iterator = Iterators.cycle(charactersOf(alphabet));
+  }
+}