implement `TargetIdGenerator` in C++ for Firestore (#701)

* implement `TargetIdGenerator`

* address changes

4 files changed, 194 insertions, 0 deletions

diff --git a/Firestore/core/src/firebase/firestore/core/CMakeLists.txt b/Firestore/core/src/firebase/firestore/core/CMakeLists.txt
new file mode 100644
index 0000000..a62985c
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/core/CMakeLists.txt
@@ -0,0 +1,20 @@
+# Copyright 2018 Google
+#
+# 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.
+
+cc_library(
+  firebase_firestore_core
+  SOURCES
+    target_id_generator.cc
+    target_id_generator.h
+)
diff --git a/Firestore/core/src/firebase/firestore/core/target_id_generator.cc b/Firestore/core/src/firebase/firestore/core/target_id_generator.cc
new file mode 100644
index 0000000..6d23d64
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/core/target_id_generator.cc
@@ -0,0 +1,62 @@
+/*
+ * Copyright 2018 Google
+ *
+ * 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.
+ */
+
+#include "Firestore/core/src/firebase/firestore/core/target_id_generator.h"
+
+namespace firebase {
+namespace firestore {
+namespace core {
+
+TargetIdGenerator::TargetIdGenerator(const TargetIdGenerator& value)
+    : generator_id_(value.generator_id_), previous_id_(value.previous_id_) {
+}
+
+TargetIdGenerator::TargetIdGenerator(TargetIdGeneratorId generator_id,
+                                     TargetId after)
+    : generator_id_(generator_id) {
+  const TargetId after_without_generator = (after >> kReservedBits)
+                                           << kReservedBits;
+  const TargetId after_generator = after - after_without_generator;
+  const TargetId generator = static_cast<TargetId>(generator_id);
+  if (after_generator >= generator) {
+    // For example, if:
+    //   self.generatorID = 0b0000
+    //   after = 0b1011
+    //   afterGenerator = 0b0001
+    // Then:
+    //   previous = 0b1010
+    //   next = 0b1100
+    previous_id_ = after_without_generator | generator;
+  } else {
+    // For example, if:
+    //   self.generatorID = 0b0001
+    //   after = 0b1010
+    //   afterGenerator = 0b0000
+    // Then:
+    //   previous = 0b1001
+    //   next = 0b1011
+    previous_id_ = (after_without_generator | generator) - (1 << kReservedBits);
+  }
+}
+
+TargetId TargetIdGenerator::NextId() {
+  previous_id_ += 1 << kReservedBits;
+  return previous_id_;
+}
+
+}  // namespace core
+}  // namespace firestore
+}  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/core/target_id_generator.h b/Firestore/core/src/firebase/firestore/core/target_id_generator.h
new file mode 100644
index 0000000..345f141
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/core/target_id_generator.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright 2018 Google
+ *
+ * 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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TARGET_ID_GENERATOR_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TARGET_ID_GENERATOR_H_
+
+#include "Firestore/core/src/firebase/firestore/core/types.h"
+
+namespace firebase {
+namespace firestore {
+namespace core {
+
+/** The set of all valid generators. */
+enum class TargetIdGeneratorId { LocalStore = 0, SyncEngine = 1 };
+
+/**
+ * Generates monotonically increasing integer IDs. There are separate generators
+ * for different scopes. While these generators will operate independently of
+ * each other, they are scoped, such that no two generators will ever produce
+ * the same ID. This is useful, because sometimes the backend may group IDs from
+ * separate parts of the client into the same ID space.
+ *
+ * Not thread-safe.
+ */
+class TargetIdGenerator {
+ public:
+  TargetIdGenerator(const TargetIdGenerator& value);
+
+  /**
+   * Creates and returns the TargetIdGenerator for the local store.
+   *
+   * @param after An ID to start at. Every call to NextId returns a larger id.
+   * @return An instance of TargetIdGenerator.
+   */
+  static TargetIdGenerator LocalStoreTargetIdGenerator(TargetId after) {
+    return TargetIdGenerator(TargetIdGeneratorId::LocalStore, after);
+  }
+
+  /**
+   * Creates and returns the TargetIdGenerator for the sync engine.
+   *
+   * @param after An ID to start at. Every call to NextId returns a larger id.
+   * @return An instance of TargetIdGenerator.
+   */
+  static TargetIdGenerator SyncEngineTargetIdGenerator(TargetId after) {
+    return TargetIdGenerator(TargetIdGeneratorId::SyncEngine, after);
+  }
+
+  TargetIdGeneratorId generator_id() {
+    return generator_id_;
+  }
+
+  TargetId NextId();
+
+ private:
+  TargetIdGenerator(TargetIdGeneratorId generator_id, TargetId after);
+  TargetIdGeneratorId generator_id_;
+  TargetId previous_id_;
+
+  static const int kReservedBits = 1;
+};
+
+}  // namespace core
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TARGET_ID_GENERATOR_H_
diff --git a/Firestore/core/src/firebase/firestore/core/types.h b/Firestore/core/src/firebase/firestore/core/types.h
new file mode 100644
index 0000000..65c2b8c
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/core/types.h
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2018 Google
+ *
+ * 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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TYPES_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TYPES_H_
+
+#include <stdint.h>
+
+namespace firebase {
+namespace firestore {
+namespace core {
+
+typedef int32_t TargetId;
+
+}  // namespace core
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_TYPES_H_