source: branches/3022-FastFunctionExtraction/FFX/BFUtils.cs @ 17738

using HeuristicLab.Algorithms.DataAnalysis.Glmnet;
using HeuristicLab.Data;
using HeuristicLab.Problems.DataAnalysis;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;

[assembly: InternalsVisibleTo("UnitTests")]
namespace HeuristicLab.Algorithms.DataAnalysis.FastFunctionExtraction {
    // utility functions for creating Basis Functions
    internal static class BFUtils {
        public static IEnumerable<IBasisFunction> CreateBasisFunctions(IRegressionProblemData data, Approach approach) {
            var exponents = approach.AllowExp ? approach.Exponents : new HashSet<double> { 1 };
            var funcs = approach.AllowNonlinFuncs ? approach.NonlinFuncs : new HashSet<NonlinearOperator> { NonlinearOperator.None };
            var simpleBasisFuncs = CreateSimpleBases(data, exponents, funcs);

            if (approach.AllowHinge) {
                // only allow hinge functions for features with exponent = 1 (deemed too complex otherwise)
                var linearSimpleBasisFuncs = simpleBasisFuncs.Where(simpleBf => simpleBf.Exponent == 1);
                simpleBasisFuncs = simpleBasisFuncs.Concat(CreateHingeBases(data, linearSimpleBasisFuncs, approach.MinHingeThr, approach.MaxHingeThr, approach.NumHingeThrs));
            }

            IEnumerable<IBasisFunction> functions = simpleBasisFuncs;

            if (approach.AllowInter) {
                var multivariateBases = CreateMultivariateBases(data, simpleBasisFuncs.ToArray());
                functions = functions.Concat(multivariateBases);
            }

            if (approach.AllowDenom) {
                var denominatorBases = CreateDenominatorBases(functions);
                functions = functions.Concat(denominatorBases);
            }
            return functions;
        }

        public static IEnumerable<ISimpleBasisFunction> CreateSimpleBases(IRegressionProblemData problemData, HashSet<double> exponents, HashSet<NonlinearOperator> nonlinFuncs) {
            var simpleBasisFunctions = new List<ISimpleBasisFunction>();
            foreach (var variableName in problemData.AllowedInputVariables) {
                var vals = problemData.Dataset.GetDoubleValues(variableName).ToArray();
                var min = vals.Min();
                foreach (var exp in exponents) {
                    var simpleBase = new SimpleBasisFunction(variableName, exp, NonlinearOperator.None);
                    // if the basis function is not valid without any operator, then it won't be valid in combination with any nonlinear operator -> skip
                    if (!Ok(simpleBase.Simulate(problemData))) continue;

                    foreach (NonlinearOperator op in nonlinFuncs) {
                        // ignore cases where op has no effect
                        if (op.Equals(NonlinearOperator.Abs) && new[] { -2.0, 2.0 }.Contains(exp) && nonlinFuncs.Contains(NonlinearOperator.None)) continue;
                        if (op.Equals(NonlinearOperator.Abs) && min >= 0) continue;
                        var nonsimpleBase = (SimpleBasisFunction)simpleBase.DeepCopy();
                        nonsimpleBase.Operator = op;
                        if (!Ok(nonsimpleBase.Simulate(problemData))) continue;
                        simpleBasisFunctions.Add(nonsimpleBase);
                    }
                }
            }
            return simpleBasisFunctions;
        }

        public static IEnumerable<IBasisFunction> CreateMultivariateBases(IRegressionProblemData data, IList<ISimpleBasisFunction> univariateBases) {
            var orderedFuncs = OrderBasisFuncsByImportance(data, univariateBases).ToArray();
            var multivariateBases = new List<IBasisFunction>();
            int maxSize = 2 * orderedFuncs.Length;
            foreach (var bf in orderedFuncs) {
                // disallow bases with exponents
                if (bf.Exponent != 1) continue;
                multivariateBases.Add(new ProductBaseFunction(bf, bf, true));
            }

            for (int i = 0; i < orderedFuncs.Count(); i++) {
                var b_i = orderedFuncs.ElementAt(i);
                for (int j = 0; j < i; j++) {
                    var b_j = orderedFuncs.ElementAt(j);
                    if (b_j.Operator != NonlinearOperator.None) continue; // disallow op() * op(); deemed to complex
                    var b_inter = new ProductBaseFunction(b_i, b_j, true);
                    if (!Ok(b_inter.Simulate(data))) continue;
                    multivariateBases.Add(b_inter);
                    if (multivariateBases.Count() >= maxSize)
                        return multivariateBases;
                }
            }
            return multivariateBases;
        }

        // order basis functions by importance (decr)
        // the importance of a basis function is measured by the absolute value of its coefficient when optimized on the data
        public static IEnumerable<ISimpleBasisFunction> OrderBasisFuncsByImportance(IRegressionProblemData data, IList<ISimpleBasisFunction> candidateFunctions) {
            var elnetData = PrepareData(Normalize(data), candidateFunctions);
            var coeff = ElasticNetLinearRegression.CalculateModelCoefficients(elnetData, 0, 0, out var trainNMSE, out var testNMSE); // LS-fit
            var intercept = coeff.Last();
            coeff = coeff.Take(coeff.Length - 1).ToArray();
            var order = Utils.Argsort(coeff);
            Array.Reverse(order);
            return order.Select(idx => candidateFunctions[idx]);
        }

        public static IList<ISimpleBasisFunction> CreateHingeBases(IRegressionProblemData data, IEnumerable<ISimpleBasisFunction> simple_bfs, double relative_start_thr = 0.0, double relative_end_thr = 1.0, int num_thrs = 3, IntRange trainingPartition = null) {
            var hingeBases = new List<ISimpleBasisFunction>();

            foreach (var simple_bf in simple_bfs) {
                hingeBases.AddRange(CreateHingeBases(data, simple_bf, relative_start_thr, relative_end_thr, num_thrs, trainingPartition));
            }
            return hingeBases;
        }

        private static IEnumerable<ISimpleBasisFunction> CreateHingeBases(IRegressionProblemData data, ISimpleBasisFunction simple_bf, double relative_start_thr, double relative_end_thr, int num_thrs, IntRange trainingPartition) {
            if (relative_start_thr >= relative_end_thr) throw new ArgumentException($"{nameof(relative_start_thr)} must be smaller than {nameof(relative_end_thr)}.");
            var ans = new List<ISimpleBasisFunction>();

            var vals = simple_bf.Simulate(data);
            var temp = trainingPartition ?? data.TrainingPartition;
            double min = Double.MaxValue;
            double max = Double.MinValue;
            for (int i = temp.Start; i < temp.End; i++) {
                min = Math.Min(min, vals[i]);
                max = Math.Max(max, vals[i]);
            }
            if (max - min == 0) return ans;
            var full_range = max - min;
            var start_thr = min + relative_start_thr * full_range;
            var end_thr = min + relative_end_thr * full_range;
            var thresholds = Utils.Linspace(start_thr, end_thr, num_thrs);

            foreach (var thr in thresholds) {
                ans.Add(new SimpleBasisFunction(simple_bf.Feature, 1, NonlinearOperator.Gth, true, thr));
                ans.Add(new SimpleBasisFunction(simple_bf.Feature, 1, NonlinearOperator.Lth, true, thr));
            }
            return ans;
        }

        public static IEnumerable<IBasisFunction> CreateDenominatorBases(IEnumerable<IBasisFunction> basisFunctions) {
            List<IBasisFunction> ans = new List<IBasisFunction>();
            foreach (var bf in basisFunctions) {
                if (!bf.IsNominator) continue;
                var denomFunc = bf.DeepCopy();
                denomFunc.IsNominator = false;
                ans.Add(denomFunc);
            }
            return ans;
        }

        public static IRegressionProblemData PrepareData(IRegressionProblemData problemData, IEnumerable<IBasisFunction> basisFunctions) {
            int numRows = problemData.Dataset.Rows;
            int numCols = basisFunctions.Count();
            HashSet<string> allowedInputVars = new HashSet<string>();
            double[,] variableValues = new double[numRows, numCols + 1]; // +1 for target var

            int col = 0;
            foreach (var basisFunc in basisFunctions) {
                allowedInputVars.Add(basisFunc.ToString() + (!basisFunc.IsNominator ? " * " + problemData.TargetVariable : ""));
                var vals = basisFunc.Simulate(problemData);
                for (int i = 0; i < numRows; i++) {
                    variableValues[i, col] = vals[i];
                }
                col++;
            }

            // add the unmodified target variable to the dataset
            var allVariables = new HashSet<string>(allowedInputVars);
            allVariables.Add(problemData.TargetVariable);

            var targetVals = problemData.TargetVariableValues.ToArray();
            for (int i = 0; i < numRows; i++) {
                variableValues[i, col] = targetVals[i];
            }

            var temp = new Dataset(allVariables, variableValues);

            IRegressionProblemData rpd = new RegressionProblemData(temp, allowedInputVars, problemData.TargetVariable);
            rpd.TrainingPartition.Start = problemData.TrainingPartition.Start;
            rpd.TrainingPartition.End = problemData.TrainingPartition.End;
            rpd.TestPartition.Start = problemData.TestPartition.Start;
            rpd.TestPartition.End = problemData.TestPartition.End;
            return rpd;
        }

        private static IRegressionProblemData Normalize(IRegressionProblemData data)
            => new RegressionProblemData(Normalize(data.Dataset), data.AllowedInputVariables, data.TargetVariable);

        // return a normalized version of IDataset ds
        private static IDataset Normalize(IDataset ds) {
            var doubleNames = ds.DoubleVariables.ToArray();
            if (ds.VariableNames.Count() != doubleNames.Length) throw new ArgumentException(nameof(ds));
            var variableVals = new List<List<double>>();
            foreach (var name in doubleNames) {
                var vals = Utils.Normalize(ds.GetDoubleValues(name).ToArray());
                variableVals.Add(vals.ToList());
            }
            return new Dataset(doubleNames, variableVals);
        }

        private static bool Ok(IEnumerable<double> data) => data.All(x => !double.IsNaN(x) && !double.IsInfinity(x));
    }
}