SimpleHistogramGenerator.cc

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#include "FastSimulation/Utilities/interface/SimpleHistogramGenerator.h"
#include "FastSimulation/Utilities/interface/RandomEngineAndDistribution.h"

#include <cmath>
#include "TH1.h"
// #include <iostream>

SimpleHistogramGenerator::SimpleHistogramGenerator(TH1* histo)
    :  //myHisto(histo),
      //theXaxis(histo->GetXaxis()),
      nBins(histo->GetXaxis()->GetNbins()),
      xMin(histo->GetXaxis()->GetXmin()),
      xMax(histo->GetXaxis()->GetXmax()),
      binWidth((xMax - xMin) / (float)nBins) {
  integral.reserve(nBins + 2);
  integral.push_back(0.);
  for (int i = 1; i <= nBins; ++i)
    integral.push_back(integral[i - 1] + histo->GetBinContent(i));
  integral.push_back(integral[nBins]);
  nEntries = integral[nBins + 1];
  for (int i = 1; i <= nBins; ++i)
    integral[i] /= nEntries;
}

double SimpleHistogramGenerator::generate(RandomEngineAndDistribution const* random) const {
  // return a random number distributed according the histogram bin contents.
  // NB Only valid for 1-d histograms, with fixed bin width.

  double r1 = random->flatShoot();
  int ibin = binarySearch(nBins, integral, r1);
  double x = xMin + (double)(ibin)*binWidth;
  if (r1 > integral[ibin])
    x += binWidth * (r1 - integral[ibin]) / (integral[ibin + 1] - integral[ibin]);
  return x;
}

int SimpleHistogramGenerator::binarySearch(const int& n, const std::vector<float>& array, const double& value) const {
  // Binary search in an array of n values to locate value.
  //
  // Array is supposed  to be sorted prior to this call.
  // If match is found, function returns position of element.
  // If no match found, function gives nearest element smaller than value.

  int nabove, nbelow, middle;
  nabove = n + 1;
  nbelow = 0;
  while (nabove - nbelow > 1) {
    middle = (nabove + nbelow) / 2;
    if (value == array[middle - 1])
      return middle - 1;
    if (value < array[middle - 1])
      nabove = middle;
    else
      nbelow = middle;
  }
  return nbelow - 1;
}