Fix Histogram.AddMetric losing values after single-bucket to array transition

src/TraceEvent/Stacks/Histogram.cs

@@ -49,20 +49,26 @@ public void AddMetric(float metric, int bucket)
         {
             Debug.Assert(0 <= bucket && bucket < Count);
 
-            if (m_singleBucketNum < 0)
-            {
-                m_singleBucketNum = bucket;
-            }
-
-            if (m_singleBucketNum == bucket)
-            {
-                m_singleBucketValue += metric;
-                return;
-            }
             if (m_buckets == null)
             {
+                // We're in single-bucket mode
+                if (m_singleBucketNum < 0)
+                {
+                    m_singleBucketNum = bucket;
+                }
+
+                if (m_singleBucketNum == bucket)
+                {
+                    m_singleBucketValue += metric;
+                    return;
+                }
+
+                // Need to transition to array mode
                 m_buckets = new float[Count];
                 m_buckets[m_singleBucketNum] = m_singleBucketValue;
+                // Clear the single bucket tracking since we're now using the array
+                m_singleBucketNum = -1;
+                m_singleBucketValue = 0;
             }
             m_buckets[bucket] += metric;
         }

src/TraceEvent/TraceEvent.Tests/Stacks/HistogramTests.cs

@@ -0,0 +1,124 @@
+using Microsoft.Diagnostics.Tracing.Stacks;
+using Xunit;
+
+namespace TraceEventTests.Stacks
+{
+    public class HistogramTests
+    {
+        /// <summary>
+        /// Test for the bug where AddMetric doesn't properly transition from single-bucket to array mode.
+        /// After creating the array, subsequent calls to AddMetric with the original bucket should update
+        /// the array, not the m_singleBucketValue.
+        /// </summary>
+        [Fact]
+        public void AddMetric_TransitionToArrayMode_OriginalBucketUpdatesArray()
+        {
+            // Create a simple histogram controller for testing
+            var stackSource = new CopyStackSource();
+            var callTree = new CallTree(ScalingPolicyKind.TimeMetric)
+            {
+                StackSource = stackSource
+            };
+            var controller = new TimeHistogramController(callTree, 0, 100);
+            var histogram = new Histogram(controller);
+
+            // Step 1: Add metric to bucket 0 (single bucket mode)
+            histogram.AddMetric(1.0f, 0);
+            Assert.Equal(1.0f, histogram[0]);
+
+            // Step 2: Add metric to bucket 1 (should trigger array creation)
+            histogram.AddMetric(2.0f, 1);
+            Assert.Equal(1.0f, histogram[0]); // Bucket 0 should still have 1.0
+            Assert.Equal(2.0f, histogram[1]); // Bucket 1 should have 2.0
+
+            // Step 3: Add metric to bucket 0 again (this is where the bug manifests)
+            // The value should be added to the array, not to m_singleBucketValue
+            histogram.AddMetric(3.0f, 0);
+            Assert.Equal(4.0f, histogram[0]); // Bucket 0 should now have 1.0 + 3.0 = 4.0
+            Assert.Equal(2.0f, histogram[1]); // Bucket 1 should still have 2.0
+        }
+
+        /// <summary>
+        /// Test that multiple buckets work correctly after transitioning to array mode.
+        /// </summary>
+        [Fact]
+        public void AddMetric_MultipleBuckets_AllValuesCorrect()
+        {
+            var stackSource = new CopyStackSource();
+            var callTree = new CallTree(ScalingPolicyKind.TimeMetric)
+            {
+                StackSource = stackSource
+            };
+            var controller = new TimeHistogramController(callTree, 0, 100);
+            var histogram = new Histogram(controller);
+
+            // Add to multiple buckets in various order
+            histogram.AddMetric(10.0f, 5);
+            histogram.AddMetric(20.0f, 10);
+            histogram.AddMetric(30.0f, 5);  // Add more to bucket 5
+            histogram.AddMetric(40.0f, 15);
+            histogram.AddMetric(50.0f, 10); // Add more to bucket 10
+
+            // Verify all values are correct
+            Assert.Equal(0.0f, histogram[0]);
+            Assert.Equal(40.0f, histogram[5]);  // 10 + 30
+            Assert.Equal(70.0f, histogram[10]); // 20 + 50
+            Assert.Equal(40.0f, histogram[15]);
+        }
+
+        /// <summary>
+        /// Test single bucket mode (when only one bucket is ever used).
+        /// </summary>
+        [Fact]
+        public void AddMetric_SingleBucketOnly_ValuesCorrect()
+        {
+            var stackSource = new CopyStackSource();
+            var callTree = new CallTree(ScalingPolicyKind.TimeMetric)
+            {
+                StackSource = stackSource
+            };
+            var controller = new TimeHistogramController(callTree, 0, 100);
+            var histogram = new Histogram(controller);
+
+            // Add multiple values to the same bucket
+            histogram.AddMetric(1.0f, 7);
+            histogram.AddMetric(2.0f, 7);
+            histogram.AddMetric(3.0f, 7);
+
+            // Verify the value accumulated correctly
+            Assert.Equal(6.0f, histogram[7]);
+            Assert.Equal(0.0f, histogram[0]);
+            Assert.Equal(0.0f, histogram[10]);
+        }
+
+        /// <summary>
+        /// Test AddScaled works correctly after histogram transition to array mode.
+        /// </summary>
+        [Fact]
+        public void AddScaled_AfterArrayTransition_ValuesCorrect()
+        {
+            var stackSource = new CopyStackSource();
+            var callTree = new CallTree(ScalingPolicyKind.TimeMetric)
+            {
+                StackSource = stackSource
+            };
+            var controller = new TimeHistogramController(callTree, 0, 100);
+
+            // Create source histogram with multiple buckets
+            var sourceHistogram = new Histogram(controller);
+            sourceHistogram.AddMetric(10.0f, 5);
+            sourceHistogram.AddMetric(20.0f, 10);
+
+            // Create target histogram that starts in single-bucket mode
+            var targetHistogram = new Histogram(controller);
+            targetHistogram.AddMetric(5.0f, 5);
+
+            // Add scaled source to target - this should transition target to array mode
+            targetHistogram.AddScaled(sourceHistogram);
+
+            // Verify values
+            Assert.Equal(15.0f, targetHistogram[5]);  // 5 + 10
+            Assert.Equal(20.0f, targetHistogram[10]); // 0 + 20
+        }
+    }
+}