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

* implement `TargetIdGenerator`

* address changes

1 files changed, 62 insertions, 0 deletions

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