Add NodeMerger for merging sorted logs

Following
https://docs.google.com/document/d/1RZ6ZlADRUHmwiFOOmXA87FHPLFuN-7mS7tbFCwguZDE/edit#
further, the next step is to sort all the log files together for a
single node, deduplicating here.  This implements and tests that.

Change-Id: I506ae65b350d83e41ba011521c0860627ac662df
diff --git a/aos/events/logging/logfile_utils_test.cc b/aos/events/logging/logfile_utils_test.cc
index d1025fa..94ae25b 100644
--- a/aos/events/logging/logfile_utils_test.cc
+++ b/aos/events/logging/logfile_utils_test.cc
@@ -417,6 +417,7 @@
 
 using LogPartsSorterTest = SortingElementTest;
 using LogPartsSorterDeathTest = LogPartsSorterTest;
+using NodeMergerTest = SortingElementTest;
 
 // Tests that we can pull messages out of a log sorted in order.
 TEST_F(LogPartsSorterTest, Pull) {
@@ -507,6 +508,93 @@
   EXPECT_DEATH({ parts_sorter.Front(); }, "Max out of order exceeded.");
 }
 
+// Tests that we can merge data from 2 separate files, including duplicate data.
+TEST_F(NodeMergerTest, TwoFileMerger) {
+  const aos::monotonic_clock::time_point e = monotonic_clock::epoch();
+  {
+    DetachedBufferWriter writer0(logfile0_, std::make_unique<DummyEncoder>());
+    writer0.QueueSpan(config0_.span());
+    DetachedBufferWriter writer1(logfile1_, std::make_unique<DummyEncoder>());
+    writer1.QueueSpan(config1_.span());
+
+    writer0.QueueSizedFlatbuffer(
+        MakeLogMessage(e + chrono::milliseconds(1000), 0, 0x005));
+    writer1.QueueSizedFlatbuffer(
+        MakeLogMessage(e + chrono::milliseconds(1001), 1, 0x105));
+
+    writer0.QueueSizedFlatbuffer(
+        MakeLogMessage(e + chrono::milliseconds(2000), 0, 0x006));
+    writer1.QueueSizedFlatbuffer(
+        MakeLogMessage(e + chrono::milliseconds(1002), 1, 0x106));
+
+    // Make a duplicate!
+    SizePrefixedFlatbufferDetachedBuffer<MessageHeader> msg(
+        MakeLogMessage(e + chrono::milliseconds(3000), 0, 0x007));
+    writer0.QueueSpan(msg.span());
+    writer1.QueueSpan(msg.span());
+
+    writer1.QueueSizedFlatbuffer(
+        MakeLogMessage(e + chrono::milliseconds(3002), 1, 0x107));
+  }
+
+  const std::vector<LogFile> parts = SortParts({logfile0_, logfile1_});
+  ASSERT_EQ(parts.size(), 2u);
+
+  std::vector<std::unique_ptr<LogPartsSorter>> parts_sorters;
+  parts_sorters.emplace_back(
+      std::make_unique<LogPartsSorter>(parts[0].parts[0]));
+  parts_sorters.emplace_back(
+      std::make_unique<LogPartsSorter>(parts[1].parts[0]));
+
+  NodeMerger merger(std::move(parts_sorters));
+
+  EXPECT_EQ(merger.sorted_until(), monotonic_clock::min_time);
+
+  std::deque<Message> output;
+
+  EXPECT_EQ(merger.sorted_until(), monotonic_clock::min_time);
+  ASSERT_TRUE(merger.Front() != nullptr);
+  EXPECT_EQ(merger.sorted_until(), e + chrono::milliseconds(1900));
+
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), e + chrono::milliseconds(1900));
+
+  ASSERT_TRUE(merger.Front() != nullptr);
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), e + chrono::milliseconds(2900));
+
+  ASSERT_TRUE(merger.Front() != nullptr);
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), e + chrono::milliseconds(2900));
+
+  ASSERT_TRUE(merger.Front() != nullptr);
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), e + chrono::milliseconds(2900));
+
+  ASSERT_TRUE(merger.Front() != nullptr);
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), monotonic_clock::max_time);
+
+  ASSERT_TRUE(merger.Front() != nullptr);
+  output.emplace_back(std::move(*merger.Front()));
+  merger.PopFront();
+  EXPECT_EQ(merger.sorted_until(), monotonic_clock::max_time);
+
+  ASSERT_TRUE(merger.Front() == nullptr);
+
+  EXPECT_EQ(output[0].timestamp, e + chrono::milliseconds(1000));
+  EXPECT_EQ(output[1].timestamp, e + chrono::milliseconds(1001));
+  EXPECT_EQ(output[2].timestamp, e + chrono::milliseconds(1002));
+  EXPECT_EQ(output[3].timestamp, e + chrono::milliseconds(2000));
+  EXPECT_EQ(output[4].timestamp, e + chrono::milliseconds(3000));
+  EXPECT_EQ(output[5].timestamp, e + chrono::milliseconds(3002));
+}
+
 }  // namespace testing
 }  // namespace logger
 }  // namespace aos