Create GenerateAllocationTokens nomulus tool command

This creates a specified number of tokens of a given schema, with a dryrun option
to not persist them.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181403775

java/google/registry/tools/GenerateAllocationTokensCommand.java

@@ -0,0 +1,126 @@
+// Copyright 2017 The Nomulus 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.collect.ImmutableSet.toImmutableSet;
+import static com.google.common.collect.Sets.difference;
+import static google.registry.model.ofy.ObjectifyService.ofy;
+
+import com.beust.jcommander.Parameter;
+import com.beust.jcommander.Parameters;
+import com.google.appengine.tools.remoteapi.RemoteApiException;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableSet;
+import com.googlecode.objectify.Key;
+import google.registry.model.domain.AllocationToken;
+import google.registry.tools.Command.RemoteApiCommand;
+import google.registry.util.NonFinalForTesting;
+import google.registry.util.Retrier;
+import google.registry.util.StringGenerator;
+import java.util.Collection;
+import javax.inject.Inject;
+import javax.inject.Named;
+
+/** Command to generate and persist {@link AllocationToken}s. */
+@NonFinalForTesting
+@Parameters(
+  separators = " =",
+  commandDescription =
+      "Generates and persists the given number of AllocationTokens, printing each token to stdout."
+)
+public class GenerateAllocationTokensCommand implements RemoteApiCommand {
+
+  @Parameter(
+    names = {"-p", "--prefix"},
+    description = "Allocation token prefix; defaults to blank"
+  )
+  private String prefix = "";
+
+  @Parameter(
+    names = {"-n", "--number"},
+    description = "The number of tokens to generate",
+    required = true
+  )
+  private long numTokens;
+
+  @Parameter(
+    names = {"-l", "--length"},
+    description = "The length of each token, exclusive of the prefix (if specified); defaults to 12"
+  )
+  private int tokenLength = 12;
+
+  @Parameter(
+      names = {"-d", "--dry_run"},
+      description = "Do not actually persist the tokens; defaults to false")
+  boolean dryRun;
+
+  @Inject @Named("base58StringGenerator") StringGenerator stringGenerator;
+  @Inject Retrier retrier;
+
+  private static final int BATCH_SIZE = 20;
+
+  @Override
+  public void run() throws Exception {
+    int tokensSaved = 0;
+    do {
+      ImmutableSet<AllocationToken> tokens =
+          generateTokens(BATCH_SIZE)
+              .stream()
+              .limit(numTokens - tokensSaved)
+              .map(t -> new AllocationToken.Builder().setToken(t).build())
+              .collect(toImmutableSet());
+      // Wrap in a retrier to deal with transient 404 errors (thrown as RemoteApiExceptions).
+      tokensSaved += retrier.callWithRetry(() -> saveTokens(tokens), RemoteApiException.class);
+    } while (tokensSaved < numTokens);
+  }
+
+  @VisibleForTesting
+  int saveTokens(final ImmutableSet<AllocationToken> tokens) {
+    Collection<AllocationToken> savedTokens =
+        dryRun ? tokens : ofy().transact(() -> ofy().save().entities(tokens).now().values());
+    savedTokens.stream().map(AllocationToken::getToken).forEach(System.out::println);
+    return savedTokens.size();
+  }
+
+  /**
+   * This function generates at MOST {@code count} tokens, filtering out already-existing token
+   * strings.
+   *
+   * <p>Note that in the incredibly rare case that all generated tokens already exist, this function
+   * may return an empty set.
+   */
+  private ImmutableSet<String> generateTokens(int count) {
+    ImmutableSet<String> candidates =
+        stringGenerator
+            .createStrings(tokenLength, count)
+            .stream()
+            .map(s -> prefix + s)
+            .collect(toImmutableSet());
+    ImmutableSet<Key<AllocationToken>> existingTokenKeys =
+        candidates
+            .stream()
+            .map(input -> Key.create(AllocationToken.class, input))
+            .collect(toImmutableSet());
+    ImmutableSet<String> existingTokenStrings =
+        ofy()
+            .load()
+            .keys(existingTokenKeys)
+            .values()
+            .stream()
+            .map(AllocationToken::getToken)
+            .collect(toImmutableSet());
+    return ImmutableSet.copyOf(difference(candidates, existingTokenStrings));
+  }
+}