Adds common header

Tests/FrameworkTests/OverwatchTranscript/TranscriptTests.cs

@@ -50,6 +50,8 @@ namespace FrameworkTests.OverwatchTranscript
                 EventData = EventData1
             });
 
+            if (File.Exists(TranscriptFilename)) File.Delete(TranscriptFilename);
+
             writer.Write(TranscriptFilename);
         }
 
@@ -59,6 +61,9 @@ namespace FrameworkTests.OverwatchTranscript
 
             var header = reader.GetHeader<TestHeader>(HeaderKey);
             Assert.That(header.HeaderData, Is.EqualTo(HeaderData));
+            Assert.That(reader.Header.NumberOfEvents, Is.EqualTo(3));
+            Assert.That(reader.Header.EarliestUct, Is.EqualTo(t0));
+            Assert.That(reader.Header.LatestUtc, Is.EqualTo(t2));
 
             var events = new List<MyEvent>();
             reader.AddHandler<MyEvent>((utc, e) =>

Tools/OverwatchTranscript/Model.cs

@@ -10,9 +10,18 @@
     [Serializable]
     public class OverwatchHeader
     {
+        public OverwatchCommonHeader Common { get; set; } = new();
         public OverwatchHeaderEntry[] Entries { get; set; } = Array.Empty<OverwatchHeaderEntry>();
     }
 
+    [Serializable]
+    public class OverwatchCommonHeader
+    {
+        public long NumberOfEvents { get; set; }
+        public DateTime EarliestUct { get; set; }
+        public DateTime LatestUtc { get; set; }
+    }
+
     [Serializable]
     public class OverwatchHeaderEntry
     {

Tools/OverwatchTranscript/TranscriptReader.cs

@@ -34,6 +34,15 @@ namespace OverwatchTranscript
             LoadModel(inputFilename);
         }
 
+        public OverwatchCommonHeader Header
+        {
+            get
+            {
+                CheckClosed(); 
+                return model.Header.Common;
+            }
+        }
+
         public T GetHeader<T>(string key)
         {
             CheckClosed();

Tools/OverwatchTranscript/TranscriptWriter.cs

@@ -77,6 +77,8 @@ namespace OverwatchTranscript
 
         public void Write(string outputFilename)
         {
+            if (!buffer.Any()) throw new Exception("No entries added.");
+
             CheckClosed();
             closed = true;
 
@@ -97,6 +99,7 @@ namespace OverwatchTranscript
                 {
                     Header = new OverwatchHeader
                     {
+                        Common = CreateCommonHeader(),
                         Entries = header.Select(h =>
                         {
                             return new OverwatchHeaderEntry
@@ -123,6 +126,16 @@ namespace OverwatchTranscript
             }
         }
 
+        private OverwatchCommonHeader CreateCommonHeader()
+        {
+            return new OverwatchCommonHeader
+            {
+                NumberOfEvents = buffer.Sum(e => e.Value.Count),
+                EarliestUct = buffer.Min(e => e.Key),
+                LatestUtc = buffer.Max(e => e.Key)
+            };
+        }
+
         private void CheckClosed()
         {
             if (closed) throw new Exception("Transcript has already been written. Cannot modify or write again.");