[ML] Randomize train/test cluster boundary assignment in RDataLoader

tree/ml/inc/ROOT/ML/RClusterLoader.hxx

@@ -227,18 +227,27 @@ public:
          // --- Shuffled path
          // Every cluster contributes a prefix to training and a suffix to validation.
          // Cost: Each cluster is read twice per epoch, only when validation split is more than 0.
-         // TODO(staider) Swicth between prefix or suffix for validation randomly per cluster
+         // We generate a random boolean value to decide whether the training set gets the prefix
+         // or suffix of each cluster to ensure better shuffling across runs when splitting.
+         std::mt19937 g(fSetSeed);
+         std::uniform_int_distribution<int> coin(0, 1);
+
          for (const RClusterRange &c : fAllClusters) {
             const std::size_t sz = c.GetNumEntries();
             const std::size_t trainSz = static_cast<std::size_t>((1.0f - fValidationSplit) * sz);
             const std::size_t valSz = sz - trainSz;
 
+            // Randomly assign prefix or suffix to training
+            const uint64_t trainIsPrefix = coin(g);
+            const uint64_t trainStart = trainIsPrefix ? c.start : c.start + static_cast<std::uint64_t>(valSz);
+            const uint64_t valStart = trainIsPrefix ? c.start + static_cast<std::uint64_t>(trainSz) : c.start;
+
             if (trainSz > 0) {
-               fTrainingClusters.push_back({c.rdfIdx, c.start, c.start + static_cast<std::uint64_t>(trainSz)});
+               fTrainingClusters.push_back({c.rdfIdx, trainStart, trainStart + static_cast<std::uint64_t>(trainSz)});
                fNumTrainingEntries += trainSz;
             }
             if (valSz > 0) {
-               fValidationClusters.push_back({c.rdfIdx, c.start + static_cast<std::uint64_t>(trainSz), c.end});
+               fValidationClusters.push_back({c.rdfIdx, valStart, valStart + static_cast<std::uint64_t>(valSz)});
                fNumValidationEntries += valSz;
             }
          }
@@ -392,14 +401,26 @@ public:
             std::min(static_cast<std::size_t>(totalFiltered * (1.0f - fValidationSplit)), trainRemaining);
          const std::size_t valCount = totalFiltered - trainCount;
 
+         // We generate a random boolean value to decide whether the training set gets the prefix
+         // or suffix of each cluster to ensure better shuffling across runs when splitting.
+         std::mt19937 g(fSetSeed + fAccumulatedFilteredForTrain); // vary per cluster
+         std::uniform_int_distribution<int> coin(0, 1);
+         const uint64_t trainIsPrefix = coin(g);
+
          // The boundary is the raw entry index of the first entry assigned to validation.
          // Stable across epochs since the same filter always produces the same ordered entries.
-         const std::uint64_t boundary = (valCount > 0) ? rdfEntries[trainCount] : endRow;
+         const std::uint64_t trainBoundaryEntry = trainIsPrefix ? rdfEntries[trainCount] : rdfEntries[valCount];
+         const std::uint64_t boundary = (valCount > 0) ? trainBoundaryEntry : endRow;
+
+         const std::uint64_t trainStart = trainIsPrefix ? startRow : boundary;
+         const std::uint64_t trainEnd = trainIsPrefix ? boundary : endRow;
+         const std::uint64_t valStart = trainIsPrefix ? boundary : startRow;
+         const std::uint64_t valEnd = trainIsPrefix ? endRow : boundary;
 
          if (trainCount > 0)
-            fTrainingClusters.push_back({rdfIdx, startRow, boundary, trainCount});
+            fTrainingClusters.push_back({rdfIdx, trainStart, trainEnd, trainCount});
          if (valCount > 0)
-            fValidationClusters.push_back({rdfIdx, boundary, endRow, valCount});
+            fValidationClusters.push_back({rdfIdx, valStart, valEnd, valCount});
 
          fAccumulatedFilteredForTrain += trainCount;
          return trainCount;