[CODE HEALTH] Fix clang-tidy warnings in base2 exponential histogram aggregation (open-telemetry#3997)

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

.github/workflows/clang-tidy.yaml

@@ -17,9 +17,9 @@ jobs:
       matrix:
         include:
           - cmake_options: all-options-abiv1-preview
-            warning_limit: 42
+            warning_limit: 35
           - cmake_options: all-options-abiv2-preview
-            warning_limit: 44
+            warning_limit: 37
     env:
       CC: /usr/bin/clang-18
       CXX: /usr/bin/clang++-18

CHANGELOG.md

@@ -58,6 +58,9 @@ Increment the:
 * [CODE HEALTH] Fix misc clang-tidy warnings
   [#3993](https://github.com/open-telemetry/opentelemetry-cpp/pull/3993)
 
+* [CODE HEALTH] Fix clang-tidy warnings in base2 exponential histogram aggregation
+  [#3997](https://github.com/open-telemetry/opentelemetry-cpp/pull/3997)
+
 Important changes:
 
 * Enable WITH_OTLP_RETRY_PREVIEW by default

sdk/src/metrics/aggregation/base2_exponential_histogram_aggregation.cc

@@ -32,7 +32,7 @@ namespace
 uint32_t GetScaleReduction(int32_t start_index, int32_t end_index, size_t max_buckets) noexcept
 {
   uint32_t scale_reduction = 0;
-  while (static_cast<size_t>(end_index - start_index + 1) > max_buckets)
+  while (static_cast<int64_t>(end_index) - start_index + 1 > static_cast<int64_t>(max_buckets))
   {
     start_index >>= 1;
     end_index >>= 1;
@@ -212,7 +212,7 @@ void Base2ExponentialHistogramAggregation::Downscale(uint32_t by) noexcept
     DownscaleBuckets(point_data_.negative_buckets_, by);
   }
 
-  point_data_.scale_ -= by;
+  point_data_.scale_ -= static_cast<int32_t>(by);
   indexer_ = Base2ExponentialHistogramIndexer(point_data_.scale_);
 }
 
@@ -324,9 +324,9 @@ std::unique_ptr<Aggregation> Base2ExponentialHistogramAggregation::Merge(
       DownscaleBuckets(high_res.positive_buckets_, scale_reduction);
       DownscaleBuckets(low_res.negative_buckets_, scale_reduction);
       DownscaleBuckets(high_res.negative_buckets_, scale_reduction);
-      low_res.scale_ -= scale_reduction;
-      high_res.scale_ -= scale_reduction;
-      result_value.scale_ -= scale_reduction;
+      low_res.scale_ -= static_cast<int32_t>(scale_reduction);
+      high_res.scale_ -= static_cast<int32_t>(scale_reduction);
+      result_value.scale_ -= static_cast<int32_t>(scale_reduction);
     }
   }
 
@@ -409,8 +409,8 @@ std::unique_ptr<Aggregation> Base2ExponentialHistogramAggregation::Diff(
     {
       DownscaleBuckets(right.negative_buckets_, scale_reduction);
     }
-    left.scale_ -= scale_reduction;
-    right.scale_ -= scale_reduction;
+    left.scale_ -= static_cast<int32_t>(scale_reduction);
+    right.scale_ -= static_cast<int32_t>(scale_reduction);
   }
 
   Base2ExponentialHistogramPointData result_value;

sdk/test/metrics/aggregation_test.cc

@@ -608,3 +608,27 @@ TEST(Aggregation, Base2ExponentialHistogramAggregationMergeCountInvariant)
     verify_invariant(point, expected_count, "Cycle 6: indices 5-9");
   }
 }
+
+TEST(Aggregation, Base2ExponentialHistogramAggregationDiffDownscale)
+{
+  // Force the scale-reduction branch in Diff() by giving left and right
+  // bucket indices whose combined span exceeds max_buckets. Each side
+  // individually has only one bucket so neither triggers Aggregate's
+  // internal Downscale; the reduction must happen inside Diff().
+  Base2ExponentialHistogramAggregationConfig config;
+  config.max_scale_   = 0;
+  config.max_buckets_ = 5;
+
+  Base2ExponentialHistogramAggregation left(&config);
+  left.Aggregate(2.0, {});  // 2^1 -> index 0
+
+  Base2ExponentialHistogramAggregation right(&config);
+  right.Aggregate(128.0, {});  // 2^7 -> index 6
+
+  // Combined span 0..6 (7 buckets) > max_buckets=5, so Diff() must downscale.
+  // GetScaleReduction(0, 6, 5) returns 1, so the result scale drops from 0 to -1.
+  auto diffed       = left.Diff(right);
+  auto diffed_point = nostd::get<Base2ExponentialHistogramPointData>(diffed->ToPoint());
+
+  EXPECT_EQ(diffed_point.scale_, -1);
+}