[ML] Improve boosted tree training initialisation

docs/CHANGELOG.asciidoc

@@ -36,6 +36,8 @@
 For large data sets this change was observed to give a 10% to 20% decrease in
 train time. (See {ml-pull}622[#622].)
 * Upgrade Boost libraries to version 1.71. (See {ml-pull}638[#638].)
+* Improve initialisation of boosted tree training. This generally enables us to
+find lower loss models faster. (See {ml-pull}686[#686].)
 
 == {es} version 7.4.0
 

include/api/CDataFrameBoostedTreeRunner.h

@@ -47,6 +47,7 @@ class API_EXPORT CDataFrameBoostedTreeRunner final : public CDataFrameAnalysisRu
 private:
     using TBoostedTreeUPtr = std::unique_ptr<maths::CBoostedTree>;
     using TBoostedTreeFactoryUPtr = std::unique_ptr<maths::CBoostedTreeFactory>;
+    using TDataSearcherUPtr = CDataFrameAnalysisSpecification::TDataSearcherUPtr;
     using TMemoryEstimator = std::function<void(std::int64_t)>;
 
 private:
@@ -58,7 +59,8 @@ class API_EXPORT CDataFrameBoostedTreeRunner final : public CDataFrameAnalysisRu
     TMemoryEstimator memoryEstimator();
 
     bool restoreBoostedTree(core::CDataFrame& frame,
-                            CDataFrameAnalysisSpecification::TDataSearcherUPtr& restoreSearcher);
+                            std::size_t dependentVariableColumn,
+                            TDataSearcherUPtr& restoreSearcher);
 
 private:
     // Note custom config is written directly to the factory object.

include/core/CLoopProgress.h

@@ -14,6 +14,8 @@
 
 namespace ml {
 namespace core {
+class CStatePersistInserter;
+class CStateRestoreTraverser;
 
 //! \brief Manages recording the progress of a loop.
 //!
@@ -46,14 +48,35 @@ class CORE_EXPORT CLoopProgress {
     using TProgressCallback = std::function<void(double)>;
 
 public:
+    CLoopProgress();
     template<typename ITR>
-    CLoopProgress(ITR begin, ITR end, const TProgressCallback& recordProgress, double scale = 1.0)
+    CLoopProgress(ITR begin, ITR end, const TProgressCallback& recordProgress = noop, double scale = 1.0)
         : CLoopProgress(std::distance(begin, end), recordProgress, scale) {}
-    CLoopProgress(std::size_t size, const TProgressCallback& recordProgress, double scale = 1.0);
+    CLoopProgress(std::size_t size,
+                  const TProgressCallback& recordProgress = noop,
+                  double scale = 1.0);
+
+    //! Attach a new progress monitor callback.
+    void progressCallback(const TProgressCallback& recordProgress);
 
     //! Increment the progress by \p i.
     void increment(std::size_t i = 1);
 
+    //! Resume progress monitoring which was restored.
+    void resumeRestored();
+
+    //! Get a checksum for this object.
+    std::uint64_t checksum() const;
+
+    //! Persist by passing information to \p inserter.
+    void acceptPersistInserter(CStatePersistInserter& inserter) const;
+
+    //! Populate the object from serialized data.
+    bool acceptRestoreTraverser(CStateRestoreTraverser& traverser);
+
+private:
+    static void noop(double);
+
 private:
     std::size_t m_Size;
     std::size_t m_Steps;

include/maths/CBoostedTreeFactory.h

@@ -10,6 +10,7 @@
 #include <core/CDataFrame.h>
 
 #include <maths/CBoostedTree.h>
+#include <maths/CLinearAlgebra.h>
 #include <maths/ImportExport.h>
 
 #include <boost/optional.hpp>
@@ -43,12 +44,9 @@ class MATHS_EXPORT CBoostedTreeFactory final {
                                                        TLossFunctionUPtr loss);
 
     //! Construct a boosted tree object from its serialized version.
-    static TBoostedTreeUPtr
-    constructFromString(std::istream& jsonStringStream,
-                        core::CDataFrame& frame,
-                        TProgressCallback recordProgress = noopRecordProgress,
-                        TMemoryUsageCallback recordMemoryUsage = noopRecordMemoryUsage,
-                        TTrainingStateCallback recordTrainingState = noopRecordTrainingState);
+    //!
+    //! \warning Throws runtime error on fail to restore.
+    static CBoostedTreeFactory constructFromString(std::istream& jsonStringStream);
 
     ~CBoostedTreeFactory();
     CBoostedTreeFactory(CBoostedTreeFactory&) = delete;
@@ -93,17 +91,22 @@ class MATHS_EXPORT CBoostedTreeFactory final {
     TBoostedTreeUPtr buildFor(core::CDataFrame& frame, std::size_t dependentVariable);
 
 private:
+    using TDoubleDoublePr = std::pair<double, double>;
     using TOptionalDouble = boost::optional<double>;
     using TOptionalSize = boost::optional<std::size_t>;
+    using TVector = CVectorNx1<double, 3>;
+    using TOptionalVector = boost::optional<TVector>;
     using TPackedBitVectorVec = std::vector<core::CPackedBitVector>;
     using TBoostedTreeImplUPtr = std::unique_ptr<CBoostedTreeImpl>;
+    using TApplyRegularizerStep =
+        std::function<void(CBoostedTreeImpl&, double, std::size_t)>;
 
 private:
     static const double MINIMUM_ETA;
     static const std::size_t MAXIMUM_NUMBER_TREES;
 
 private:
-    CBoostedTreeFactory(std::size_t numberThreads, TLossFunctionUPtr loss);
+    CBoostedTreeFactory(bool restored, std::size_t numberThreads, TLossFunctionUPtr loss);
 
     //! Compute the row masks for the missing values for each feature.
     void initializeMissingFeatureMasks(const core::CDataFrame& frame) const;
@@ -121,25 +124,53 @@ class MATHS_EXPORT CBoostedTreeFactory final {
     //! Initialize the regressors sample distribution.
     bool initializeFeatureSampleDistribution() const;
 
-    //! Read overrides for hyperparameters and if necessary estimate the initial
-    //! values for \f$\lambda\f$ and \f$\gamma\f$ which match the gain from an
-    //! overfit tree.
-    void initializeHyperparameters(core::CDataFrame& frame) const;
+    //! Set the initial values for the various hyperparameters.
+    void initializeHyperparameters(core::CDataFrame& frame);
+
+    //! Estimate a good central value for the regularisation hyperparameters
+    //! search bounding box.
+    void initializeUnsetRegularizationHyperparameters(core::CDataFrame& frame);
+
+    //! Estimate the reduction in gain from a split and the total curvature of
+    //! the loss function at a split.
+    TDoubleDoublePr estimateTreeGainAndCurvature(core::CDataFrame& frame,
+                                                 const core::CPackedBitVector& trainingRowMask) const;
+
+    //! Perform a line search for the test loss w.r.t. a single regularization
+    //! hyperparameter and apply Newton's method to find the minimum. The plan
+    //! is to find a value near where the model starts to overfit.
+    //!
+    //! \return The interval to search during the main hyperparameter optimisation
+    //! loop or null if this couldn't be found.
+    TOptionalVector testLossNewtonLineSearch(core::CDataFrame& frame,
+                                             core::CPackedBitVector trainingRowMask,
+                                             const TApplyRegularizerStep& applyRegularizerStep,
+                                             double returnedIntervalLeftEndOffset,
+                                             double returnedIntervalRightEndOffset) const;
 
     //! Initialize the state for hyperparameter optimisation.
     void initializeHyperparameterOptimisation() const;
 
     //! Get the number of hyperparameter tuning rounds to use.
     std::size_t numberHyperparameterTuningRounds() const;
 
+    //! Setup monitoring for training progress.
+    void initializeTrainingProgressMonitoring();
+
+    //! Refresh progress monitoring after restoring from saved training state.
+    void resumeRestoredTrainingProgressMonitoring();
+
     static void noopRecordProgress(double);
     static void noopRecordMemoryUsage(std::int64_t);
     static void noopRecordTrainingState(CDataFrameRegressionModel::TPersistFunc);
 
 private:
     TOptionalDouble m_MinimumFrequencyToOneHotEncode;
     TOptionalSize m_BayesianOptimisationRestarts;
+    bool m_Restored = false;
     TBoostedTreeImplUPtr m_TreeImpl;
+    TVector m_LogGammaSearchInterval;
+    TVector m_LogLambdaSearchInterval;
     TProgressCallback m_RecordProgress = noopRecordProgress;
     TMemoryUsageCallback m_RecordMemoryUsage = noopRecordMemoryUsage;
     TTrainingStateCallback m_RecordTrainingState = noopRecordTrainingState;