Add `ToString()` to `LevelDbTransaction` (#946)

* Initial plumbing for leveldb local parts

* Add model::BatchId

* Port leveldb_key.{h,cc}

* Add string StartsWith

* Add leveldb_key_test.cc to the project

* Revert back to using leveldb::Slice for read/describe

These operations universally operate on keys obtained from leveldb so
it's actually unhelpful to force all the callers to make
absl::string_views from them.

* Start work on ToString for transactions

* Add ToString() method to LevelDbTransaction

* Style

* lint

* Revert addition of util::StartsWith

3 files changed, 49 insertions, 0 deletions

diff --git a/Firestore/Example/Tests/Local/FSTLevelDBTransactionTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBTransactionTests.mm
index 704f8c6..2a1efb0 100644
--- a/Firestore/Example/Tests/Local/FSTLevelDBTransactionTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLevelDBTransactionTests.mm
@@ -20,7 +20,9 @@
 #include <absl/strings/string_view.h>
 #include <leveldb/db.h>
 #import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
+#import "Firestore/Protos/objc/firestore/local/Mutation.pbobjc.h"
 #import "Firestore/Protos/objc/firestore/local/Target.pbobjc.h"
+#include "Firestore/core/src/firebase/firestore/local/leveldb_key.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -29,6 +31,7 @@ using leveldb::Options;
 using leveldb::ReadOptions;
 using leveldb::WriteOptions;
 using leveldb::Status;
+using firebase::firestore::local::LevelDbMutationKey;
 using firebase::firestore::local::LevelDbTransaction;
 
 @interface FSTLevelDBTransactionTests : XCTestCase
@@ -269,6 +272,30 @@ using firebase::firestore::local::LevelDbTransaction;
   XCTAssertFalse(it->Valid());
 }
 
+- (void)testToString {
+  std::string key = LevelDbMutationKey::Key("user1", 42);
+  FSTPBWriteBatch *message = [FSTPBWriteBatch message];
+  message.batchId = 42;
+
+  LevelDbTransaction transaction(_db.get());
+  std::string description = transaction.ToString();
+  XCTAssertEqual(description, "<LevelDbTransaction: 0 changes (0 bytes):>");
+
+  transaction.Put(key, message);
+  description = transaction.ToString();
+  XCTAssertEqual(description,
+                 "<LevelDbTransaction: 1 changes (2 bytes):\n"
+                 "  - Put [mutation: user_id=user1 batch_id=42] (2 bytes)>");
+
+  std::string key2 = LevelDbMutationKey::Key("user1", 43);
+  transaction.Delete(key2);
+  description = transaction.ToString();
+  XCTAssertEqual(description,
+                 "<LevelDbTransaction: 2 changes (2 bytes):\n"
+                 "  - Delete [mutation: user_id=user1 batch_id=43]\n"
+                 "  - Put [mutation: user_id=user1 batch_id=42] (2 bytes)>");
+}
+
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/Firestore/core/src/firebase/firestore/local/leveldb_transaction.cc b/Firestore/core/src/firebase/firestore/local/leveldb_transaction.cc
index af72716..4e4a313 100644
--- a/Firestore/core/src/firebase/firestore/local/leveldb_transaction.cc
+++ b/Firestore/core/src/firebase/firestore/local/leveldb_transaction.cc
@@ -18,6 +18,7 @@
 
 #include <leveldb/write_batch.h>
 
+#include "Firestore/core/src/firebase/firestore/local/leveldb_key.h"
 #include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
 
 using leveldb::DB;
@@ -206,6 +207,25 @@ void LevelDbTransaction::Commit() {
                           status.ToString().c_str());
 }
 
+std::string LevelDbTransaction::ToString() {
+  std::string dest("<LevelDbTransaction: ");
+  int64_t changes = deletions_.size() + mutations_.size();
+  int64_t bytes = 0;  // accumulator for size of individual mutations.
+  dest += std::to_string(changes) + " changes ";
+  std::string items;  // accumulator for individual changes.
+  for (auto it = deletions_.begin(); it != deletions_.end(); it++) {
+    items += "\n  - Delete " + Describe(*it);
+  }
+  for (auto it = mutations_.begin(); it != mutations_.end(); it++) {
+    int64_t change_bytes = it->second.length();
+    bytes += change_bytes;
+    items += "\n  - Put " + Describe(it->first) + " (" +
+             std::to_string(change_bytes) + " bytes)";
+  }
+  dest += "(" + std::to_string(bytes) + " bytes):" + items + ">";
+  return dest;
+}
+
 }  // namespace local
 }  // namespace firestore
 }  // namespace firebase
diff --git a/Firestore/core/src/firebase/firestore/local/leveldb_transaction.h b/Firestore/core/src/firebase/firestore/local/leveldb_transaction.h
index 015a6cb..b219a69 100644
--- a/Firestore/core/src/firebase/firestore/local/leveldb_transaction.h
+++ b/Firestore/core/src/firebase/firestore/local/leveldb_transaction.h
@@ -188,6 +188,8 @@ class LevelDbTransaction {
    */
   void Commit();
 
+  std::string ToString();
+
  private:
   leveldb::DB* db_;
   Mutations mutations_;