MonteCarloAbsorption.cpp

//------------------------------------------------------------------------------
// Includes
//------------------------------------------------------------------------------
#include "MantidAlgorithms/MonteCarloAbsorption.h"
#include "MantidAPI/InstrumentValidator.h"
#include "MantidAPI/SampleEnvironment.h"
#include "MantidAPI/WorkspaceProperty.h"
#include "MantidAPI/WorkspaceUnitValidator.h"
#include "MantidGeometry/IDetector.h"
#include "MantidGeometry/Objects/Track.h"
#include "MantidKernel/BoundedValidator.h"
#include "MantidKernel/CompositeValidator.h"
#include "MantidKernel/Material.h"
#include "MantidKernel/VectorHelper.h"

#include <boost/random/uniform_int.hpp>
#include <boost/random/uniform_real.hpp>
#include <boost/random/variate_generator.hpp>

/// @cond
namespace {
/// Number of attempts to choose a random point within the object before it
/// gives up
const int MaxRandPointAttempts(500);

/// Element size in mm
const double ELEMENT_SIZE = 1.0;
}
/// @endcond

namespace Mantid {
namespace Algorithms {

DECLARE_ALGORITHM(MonteCarloAbsorption)

using namespace API;
using namespace Geometry;
using namespace Kernel;

//------------------------------------------------------------------------------
// Public methods
//------------------------------------------------------------------------------

/**
 * Constructor
 */
MonteCarloAbsorption::MonteCarloAbsorption()
    : m_samplePos(), m_sourcePos(), m_numVolumeElements(0), m_blocks(),
      m_blkHalfX(0.0), m_blkHalfY(0.0), m_blkHalfZ(0.0), m_rngs(0), m_inputWS(),
      m_sampleShape(nullptr), m_sampleMaterial(nullptr), m_container(nullptr),
      m_numberOfPoints(0), m_xStepSize(0), m_numberOfEvents(300) {}

/**
 * Destructor
 */
MonteCarloAbsorption::~MonteCarloAbsorption() {}

//------------------------------------------------------------------------------
// Private methods
//------------------------------------------------------------------------------

/**
 * Initialize the algorithm
 */
void MonteCarloAbsorption::init() {
  // The input workspace must have an instrument and units of wavelength
  auto wsValidator = boost::make_shared<CompositeValidator>();
  wsValidator->add<WorkspaceUnitValidator>("Wavelength");
  wsValidator->add<InstrumentValidator>();

  declareProperty(new WorkspaceProperty<>("InputWorkspace", "",
                                          Direction::Input, wsValidator),
                  "The name of the input workspace.  The input workspace must "
                  "have X units of wavelength.");
  declareProperty(
      new WorkspaceProperty<>("OutputWorkspace", "", Direction::Output),
      "The name to use for the output workspace.");
  auto positiveInt = boost::make_shared<Kernel::BoundedValidator<int>>();
  positiveInt->setLower(1);
  declareProperty("NumberOfWavelengthPoints", EMPTY_INT(), positiveInt,
                  "The number of wavelength points for which a simulation is "
                  "atttempted (default: all points)");
  declareProperty(
      "EventsPerPoint", m_numberOfEvents, positiveInt,
      "The number of \"neutron\" events to generate per simulated point");
  declareProperty("SeedValue", 123456789, positiveInt,
                  "Seed the random number generator with this value");
}

/**
 * Execution code
 */
void MonteCarloAbsorption::exec() {
  retrieveInput();
  initCaches();

  g_log.debug() << "Creating output workspace\n";
  MatrixWorkspace_sptr correctionFactors =
      WorkspaceFactory::Instance().create(m_inputWS);
  correctionFactors->isDistribution(
      true);                       // The output of this is a distribution
  correctionFactors->setYUnit(""); // Need to explicitly set YUnit to nothing
  correctionFactors->setYUnitLabel("Attenuation factor");

  const bool isHistogram = m_inputWS->isHistogramData();
  const int numHists = static_cast<int>(m_inputWS->getNumberHistograms());
  const int numBins = static_cast<int>(m_inputWS->blocksize());

  // Compute the step size
  m_xStepSize = numBins / m_numberOfPoints;

  g_log.information() << "Simulation performed every " << m_xStepSize
                      << " wavelength points" << std::endl;

  Progress prog(this, 0.0, 1.0, numHists * numBins / m_xStepSize);
  PARALLEL_FOR1(correctionFactors)
  for (int i = 0; i < numHists; ++i) {
    PARALLEL_START_INTERUPT_REGION

    // Copy over the X-values
    const MantidVec &xValues = m_inputWS->readX(i);
    correctionFactors->dataX(i) = xValues;
    // Final detector position
    IDetector_const_sptr detector;
    try {
      detector = m_inputWS->getDetector(i);
    } catch (Kernel::Exception::NotFoundError &) {
      // intel compiler hangs with continue statements inside a catch
      // block that is within an omp loop...
    }
    if (!detector)
      continue;

    MantidVec &yValues = correctionFactors->dataY(i);
    MantidVec &eValues = correctionFactors->dataE(i);
    // Simulation for each requested wavelength point
    for (int bin = 0; bin < numBins; bin += m_xStepSize) {
      prog.report("Computing corrections for bin " +
                  boost::lexical_cast<std::string>(bin));
      const double lambda = isHistogram
                                ? (0.5 * (xValues[bin] + xValues[bin + 1]))
                                : xValues[bin];
      doSimulation(detector.get(), lambda, yValues[bin], eValues[bin]);
      // Ensure we have the last point for the interpolation
      if (m_xStepSize > 1 && bin + m_xStepSize >= numBins &&
          bin + 1 != numBins) {
        bin = numBins - m_xStepSize - 1;
      }
    }

    // Interpolate through points not simulated
    if (m_xStepSize > 1) {
      prog.report("Interpolating unsimulated points");
      Kernel::VectorHelper::linearlyInterpolateY(xValues, yValues, m_xStepSize);
    }

    PARALLEL_END_INTERUPT_REGION
  }
  PARALLEL_CHECK_INTERUPT_REGION

  // Save the results
  setProperty("OutputWorkspace", correctionFactors);
}

/**
 * Perform the simulation
 * @param detector :: A pointer to the current detector
 * @param lambda :: The chosen wavelength
 * @param attenFactor :: [Output] The calculated attenuation factor for this
 * wavelength
 * @param error :: [Output] The value of the error on the factor
 */
void MonteCarloAbsorption::doSimulation(const IDetector *const detector,
                                        const double lambda,
                                        double &attenFactor, double &error) {
  /**
   Currently, assuming square beam profile to pick start position then randomly
   selecting
   a point within the sample using it's bounding box.
   This point defines the single scattering point and hence the attenuation path
   lengths and final
   directional vector to the detector
   */
  // Absolute detector position
  const V3D detectorPos(detector->getPos());

  int numDetected(0);
  attenFactor = 0.0;
  error = 0.0;
  while (numDetected < m_numberOfEvents) {
    V3D startPos = sampleBeamProfile();
    V3D scatterPoint = selectScatterPoint();
    double eventFactor(0.0);
    if (attenuationFactor(startPos, scatterPoint, detectorPos, lambda,
                          eventFactor)) {
      attenFactor += eventFactor;
      ++numDetected;
    }
  }

  if (0 == numDetected)
    throw std::runtime_error(
        "Unexpected inconsistency found while running simulation: the number "
        "of events detected is 0.");

  // Attenuation factor is simply the average value
  attenFactor /= numDetected;
  // Error is 1/sqrt(nevents)
  error = 1. / sqrt(static_cast<double>(numDetected));
}

/**
 * Sample the beam profile for a random start location, assigning a weight
 * to the given point
 * @returns The starting location
 */
V3D MonteCarloAbsorption::sampleBeamProfile() const { return m_sourcePos; }

/**
 * Selects a random location within the sample + container environment. The
 * bounding box is
 * used as an approximation to generate a point and this is then tested for its
 * validity within
 * the shape.
 * @returns Selected position as V3D object
 */
V3D MonteCarloAbsorption::selectScatterPoint() const {
  // Randomly select a block from the subdivided set and then randomly select a
  // point
  // within that block and test if it inside the sample/container. If yes then
  // accept, else
  // keep trying.
  boost::uniform_int<size_t> uniIntDist(0, m_blocks.size() - 1);
  boost::variate_generator<boost::mt19937 &, boost::uniform_int<size_t>> uniInt(
      rgen(), uniIntDist);
  boost::uniform_real<> uniRealDist(0, 1.0);
  boost::variate_generator<boost::mt19937 &, boost::uniform_real<>> uniReal(
      rgen(), uniRealDist);

  V3D scatterPoint;
  int nattempts(0);
  while (nattempts < MaxRandPointAttempts) {
    size_t index = uniInt();
    const auto &block = m_blocks[index];
    const double x = m_blkHalfX * (2.0 * uniReal() - 1.0) + block.xMin();
    const double y = m_blkHalfY * (2.0 * uniReal() - 1.0) + block.yMin();
    const double z = m_blkHalfZ * (2.0 * uniReal() - 1.0) + block.zMin();
    scatterPoint(x, y, z);
    ++nattempts;
    if (ptIntersectsSample(scatterPoint)) {
      scatterPoint += m_samplePos;
      return scatterPoint;
    }
  }
  // If we got here then the shape is too strange for the bounding box to be of
  // any use.
  g_log.error() << "Attempts to generate a random point with the sample/can "
                << "have exceeded the allowed number of tries.\n";
  throw std::runtime_error(
      "Attempts to produce random scatter point failed. Check sample shape.");
}

/**
 * Return the attenuation factor for the given track
 * @param startPos :: The origin of the track
 * @param scatterPoint :: The point of scatter
 * @param finalPos :: The end point of the track
 * @param lambda :: The wavelength of the neutron
 * @param factor :: Output parameter storing the attenuation factor
 * @returns True if the track was valid, false otherwise
 */
bool MonteCarloAbsorption::attenuationFactor(const V3D &startPos,
                                             const V3D &scatterPoint,
                                             const V3D &finalPos,
                                             const double lambda,
                                             double &factor) {
  // Start at one
  factor = 1.0;
  // Define two tracks, before and after scatter, and trace check their
  // intersections with the the environment and sample
  Track beforeScatter(scatterPoint, (startPos - scatterPoint));
  Track afterScatter(scatterPoint, (finalPos - scatterPoint));
  // Theoretically this should never happen as there should always be an
  // intersection
  // but do to precision limitations points very close to the surface give
  // zero intersection, so just reject
  if (m_sampleShape->interceptSurface(beforeScatter) == 0 ||
      m_sampleShape->interceptSurface(afterScatter) == 0) {
    return false;
  }

  double length = beforeScatter.cbegin()->distInsideObject;
  factor *= attenuation(length, *m_sampleMaterial, lambda);

  beforeScatter.clearIntersectionResults();
  if (m_container) {
    m_container->interceptSurfaces(beforeScatter);
  }
  // Attenuation factor is product of factor for each material
  for (const auto &citr : beforeScatter) {
    length = citr.distInsideObject;
    factor *= attenuation(length, citr.object->material(), lambda);
  }

  length = afterScatter.cbegin()->distInsideObject;
  factor *= attenuation(length, *m_sampleMaterial, lambda);

  afterScatter.clearIntersectionResults();
  if (m_container) {
    m_container->interceptSurfaces(afterScatter);
  }
  // Attenuation factor is product of factor for each material
  for (const auto &citr : afterScatter) {
    length = citr.distInsideObject;
    factor *= attenuation(length, citr.object->material(), lambda);
  }

  return true;
}

/**
 * Calculate the attenuation for a given length, material and wavelength
 * @param length :: Distance through the material
 * @param material :: A reference to the Material
 * @param lambda :: The wavelength
 * @returns The attenuation factor
 */
double MonteCarloAbsorption::attenuation(const double length,
                                         const Kernel::Material &material,
                                         const double lambda) const {
  const double rho = material.numberDensity() * 100.0;
  const double sigma_s = material.totalScatterXSection(lambda);
  const double sigma_t = sigma_s + material.absorbXSection(lambda);
  return exp(-rho * sigma_t * length);
}

/**
 * Gather the input values and check validity
 * @throw std::invalid_argument If the input is invalid. Currently if there is
 * no defined sample shape
 */
void MonteCarloAbsorption::retrieveInput() {
  m_inputWS = getProperty("InputWorkspace");

  m_sampleShape = &(m_inputWS->sample().getShape());
  m_sampleMaterial = &(m_inputWS->sample().getMaterial());
  if (!m_sampleShape->hasValidShape()) {
    g_log.debug() << "Invalid shape defined on workspace. TopRule = "
                  << m_sampleShape->topRule() << ", No. of surfaces: "
                  << m_sampleShape->getSurfacePtr().size() << "\n";
    throw std::invalid_argument("Input workspace has an invalid sample shape.");
  }

  if (m_sampleMaterial->totalScatterXSection(1.0) == 0.0) {
    g_log.warning() << "The sample material appears to have zero scattering "
                       "cross section.\n"
                    << "Result will most likely be nonsensical.\n";
  }

  try {
    m_container = &(m_inputWS->sample().getEnvironment());
  } catch (std::runtime_error &) {
    m_container = nullptr;
    g_log.information()
        << "No environment has been defined, continuing with only sample.\n";
  }

  m_numberOfPoints = getProperty("NumberOfWavelengthPoints");
  if (isEmpty(m_numberOfPoints) ||
      m_numberOfPoints > static_cast<int>(m_inputWS->blocksize())) {
    m_numberOfPoints = static_cast<int>(m_inputWS->blocksize());
    if (!isEmpty(m_numberOfPoints)) {
      g_log.warning() << "The requested number of wavelength points is larger "
                         "than the spectra size. "
                      << "Defaulting to spectra size.\n";
    }
  }

  m_numberOfEvents = getProperty("EventsPerPoint");
}

/**
 * Initialise the caches used here including setting up the random
 * number generator
 */
void MonteCarloAbsorption::initCaches() {
  g_log.debug() << "Caching input\n";
  // Setup random number generators for parallel execution
  initRNG();

  m_samplePos = m_inputWS->getInstrument()->getSample()->getPos();
  m_sourcePos = m_inputWS->getInstrument()->getSource()->getPos();
  BoundingBox box(m_sampleShape->getBoundingBox());
  if (m_container) {
    box.grow(m_container->boundingBox());
  }

  // Chop the bounding box up into a set of small boxes. This will be used
  // as a first guess for generating a random scatter point
  const double cubeSide = ELEMENT_SIZE * 1e-3;
  const double xLength = box.width().X();
  const double yLength = box.width().Y();
  const double zLength = box.width().Z();
  const int numXSlices = static_cast<int>(xLength / cubeSide);
  const int numYSlices = static_cast<int>(yLength / cubeSide);
  const int numZSlices = static_cast<int>(zLength / cubeSide);
  const double xThick = xLength / numXSlices;
  const double yThick = yLength / numYSlices;
  const double zThick = zLength / numZSlices;
  m_blkHalfX = 0.5 * xThick;
  m_blkHalfY = 0.5 * yThick;
  m_blkHalfZ = 0.5 * zThick;

  const size_t numPossibleVolElements = numXSlices * numYSlices * numZSlices;
  g_log.debug() << "Attempting to divide sample + container into "
                << numPossibleVolElements << " blocks.\n";

  try {
    m_blocks.clear();
    m_blocks.reserve(numPossibleVolElements / 2);
  } catch (std::exception &) {
    // Typically get here if the number of volume elements is too large
    // Provide a bit more information
    g_log.error("Too many volume elements requested - try increasing the value "
                "of the ElementSize property.");
    throw;
  }

  const auto boxCentre = box.centrePoint();
  const double x0 = boxCentre.X() - 0.5 * xLength;
  const double y0 = boxCentre.Y() - 0.5 * yLength;
  const double z0 = boxCentre.Z() - 0.5 * zLength;
  // Store a chunk as a BoundingBox object.
  // Only cache blocks that have some intersection with the
  // sample or container.
  for (int i = 0; i < numZSlices; ++i) {
    const double zmin = z0 + i * zThick;
    const double zmax = zmin + zThick;
    for (int j = 0; j < numYSlices; ++j) {
      const double ymin = y0 + j * yThick;
      const double ymax = ymin + yThick;
      for (int k = 0; k < numXSlices; ++k) {
        const double xmin = x0 + k * xThick;
        const double xmax = xmin + xThick;
        if (boxIntersectsSample(xmax, ymax, zmax, xmin, ymin, zmin)) {
#if !(defined(__INTEL_COMPILER)) && !(defined(__clang__))
          m_blocks.emplace_back(xmax, ymax, zmax, xmin, ymin, zmin);
#else
          m_blocks.push_back(BoundingBox(xmax, ymax, zmax, xmin, ymin, zmin));
#endif
        }
      }
    }
  }

  m_numVolumeElements = m_blocks.size();
  g_log.debug() << "Sample + container divided into " << m_numVolumeElements
                << " blocks.\n";
  if (m_numVolumeElements == 0) {
    throw std::runtime_error("No intersection with sample + container.");
  }
  if (m_numVolumeElements != numPossibleVolElements)
    g_log.debug()
        << " Skipped " << (numPossibleVolElements - m_numVolumeElements)
        << " blocks that do not intersect with the sample + container\n";
}

/**
 */
void MonteCarloAbsorption::initRNG() {
  const int baseSeed = getProperty("SeedValue");
  // For parallel execution use a vector of RNG, each with a seed one higher
  // that the previous
  m_rngs.resize(PARALLEL_GET_MAX_THREADS);
  // Set the seeds
  for (int i = 0; i < static_cast<int>(m_rngs.size()); ++i) {
    m_rngs[i].seed(baseSeed + i);
  }
}

/**
 * @return A reference to a boost::random::mt19937 object
 */
boost::mt19937 &MonteCarloAbsorption::rgen() const {
  // Const from point of view of caller
  return const_cast<MonteCarloAbsorption *>(this)
      ->m_rngs[PARALLEL_THREAD_NUMBER];
}

/**
 * @param xmax max x-coordinate of cuboid point
 * @param ymax max y-coordinate of cuboid point
 * @param zmax max z-coordinate of cuboid point
 * @param xmin min z-coordinate of cuboid point
 * @param ymin min z-coordinate of cuboid point
 * @param zmin min z-coordinate of cuboid point
 * @return True if any of the vertices intersect the sample or container
 */
bool MonteCarloAbsorption::boxIntersectsSample(
    const double xmax, const double ymax, const double zmax, const double xmin,
    const double ymin, const double zmin) const {
  // Check all 8 corners for intersection
  if (ptIntersectsSample(V3D(xmax, ymin, zmin)) || // left-front-bottom
      ptIntersectsSample(V3D(xmax, ymax, zmin)) || // left-front-top
      ptIntersectsSample(V3D(xmin, ymax, zmin)) || // right-front-top
      ptIntersectsSample(V3D(xmin, ymin, zmin)) || // right-front-bottom
      ptIntersectsSample(V3D(xmax, ymin, zmax)) || // left-back-bottom
      ptIntersectsSample(V3D(xmax, ymax, zmax)) || // left-back-top
      ptIntersectsSample(V3D(xmin, ymax, zmax)) || // right-back-top
      ptIntersectsSample(V3D(xmin, ymin, zmax)))   // right-back-bottom
  {
    return true;
  } else
    return false;
}

/**
 *
 * @param pt A V3D giving a point to test
 * @return True if point is inside sample or container, false otherwise
 */
bool MonteCarloAbsorption::ptIntersectsSample(const V3D &pt) const {
  return m_sampleShape->isValid(pt) ||
         (m_container && m_container->isValid(pt));
}
}
}