Added a simple lcov writer

src/Draco.Compiler/Internal/Utilities/DotGraphBuilder.cs

@@ -175,10 +175,10 @@ public string ToDot()
         return new StreamReader(stream).ReadToEnd();
     }
 
-    public void WriteTo(Stream stream) => this.WriteTo(new StreamWriter(stream));
-
-    public void WriteTo(StreamWriter writer)
+    public void WriteTo(Stream stream)
     {
+        var writer = new StreamWriter(stream);
+
         // Header
         writer.Write(this.isDirected ? "digraph" : "graph");
         writer.Write(' ');

src/Draco.Coverage/CoverageResult.cs

@@ -1,5 +1,8 @@
 using System;
+using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.IO;
+using System.Linq;
 
 namespace Draco.Coverage;
 
@@ -33,4 +36,51 @@ public CoverageResult(ImmutableArray<SequencePoint> sequencePoints, ImmutableArr
         this.SequencePoints = sequencePoints;
         this.Hits = hits;
     }
+
+    /// <summary>
+    /// Writes the coverage result in LCOV format.
+    /// </summary>
+    /// <param name="stream">The stream to write to.</param>
+    public void WriteLcov(Stream stream)
+    {
+        using var writer = new StreamWriter(stream);
+
+        // We need to emit the data by file
+        var dataByFile = this.SequencePoints
+            .Zip(this.Hits, (sp, hits) => (SequencePoint: sp, Hits: hits))
+            .GroupBy(x => x.SequencePoint.FileName);
+        foreach (var group in dataByFile) WriteSequencePointsForFile(group.Key, group);
+
+        void WriteSequencePointsForFile(string fileName, IEnumerable<(SequencePoint SequencePoint, int Hits)> data)
+        {
+            writer.WriteLine($"SF:{fileName}");
+
+            // For each group, sequence points are grouped and sorted by start line
+            var dataByLine = data
+                .GroupBy(x => x.SequencePoint.StartLine)
+                .OrderBy(x => x.Key);
+
+            // Keep track of how many entries we have and how many are hit
+            var entries = 0;
+            var entriesHit = 0;
+
+            foreach (var lineData in dataByLine)
+            {
+                var line = lineData.Key;
+                // NOTE: To simplify things, if there are multiple sequence points on the same line,
+                // we sum their hits together
+                var hits = lineData.Sum(x => x.Hits);
+                writer.WriteLine($"DA:{line},{hits}");
+                ++entries;
+                if (hits != 0) ++entriesHit;
+            }
+
+            // Write entry count
+            writer.WriteLine($"LF:{entries}");
+            writer.WriteLine($"LH:{entriesHit}");
+
+            // End of file
+            writer.WriteLine("end_of_record");
+        }
+    }
 }