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// .NAME CosmoHaloFinder - find halos within a cosmology data file
// .SECTION Description
// CosmoHaloFinder is a filter object that operates on the unstructured 
// grid created when a CosmoReader reads a .cosmo data file. 
// It operates by finding clusters of neighbors.
//
// .SECTION Note
// This halo finder implements a recursive algorithm using a k-d tree.
// Linked lists are used to connect halos found during the recursive merge. 
// Bounding boxes are calculated for each particle for pruning the merge tree.
//
// The halo finder doesn't actually build a tree that can be walked but
// rather reorganizes the particles into the k-d tree using recursion, such that
// when myFOF is walked in the same way, the data will match. This is stored
// in the seq[] array.
//
// First step is Reorder().  When it is called the first time it divides all
// the particles on the X axis such that the particle at the halfway mark in
// the array is correctly positioned, and all particles in the array below it 
// have an X value less than it and all particles in the array above it have
// an X value higher.  Reorder() calls nth_element() which is a partial sort but
// faster.  So the division does not physically divide the space in half
// but rather divides the number of particles in half on a dimension.
// 
// Next step is the first level of recursion.  Each of the halves from above
// are divided on the Y axis, again such that the number of particles is the
// same in each half although the physical space is not divided.  Partial
// ordering is done again by resequencing the seq array.  Each of these now
// four pieces is divided on the Z axis next, and this continues until there
// is one particle at the bottom of the tree.
// 
// Next step in the halo finder is to call ComputeLU() which computes a 
// lower and upper bound for each particle based on the k-d tree of the next
// axis positioning.  This is used in pruning the merge tree during myFOF().
// This means that if there is a branch of the k-d tree with some
// halos in it, but that the next jump to a particle is too far away, then
// that entire branch is ignored.
// 
// Finally myFOF() is called and its recursion mimics that done by Reorder()
// so that it is looking at the k-d tree resequence correctly.  myFOF()
// recurses down to the bottom of the tree going to the left first.  When it
// gets to the bottom it calls Merge() to see if those particles at the
// bottom are close enough to each other.  Remembering that at each stage
// of the k-d tree the two halves are divided on the next axis by count and
// not by physical space, you can see that the Merge() must be done on those
// four parts as follows.
// 
// Merge(A,C) Merge(A,D) Merge(B,C) Merge(B,D).  
//
// This is because it is unknown if A shares a boundary with C and D or 
// B shares that boundary.  As particles are found to be close to each other, 
// if they are already a part of a halo, the two halos must unite.  
// While all this is going on, we also prune which means we stop the recursion.
// As Merge() and myFOF() walk through the recursion chains of halos are
// created and joined where they have a particle withing the required distance.
// When myFOF() ends it has a chain of first particle in a halo and nextp 
// pointing on down until -1 is reached.  Also the halo tag field for each
// particle is constantly altered so that each particle knows what halo it
// is part of, and that halo tag is the id of the lowest particle in the halo.
//

#ifndef CosmoHaloFinder_h
#define CosmoHaloFinder_h

#include <string>

#include "Definition.h"

#define numDataDims 3
#define dataX 0
#define dataY 1
#define dataZ 2

using namespace std;

/****************************************************************************/
typedef POSVEL_T* floatptr;

//
// Particle information for reordering the particles according to position
// Value is either the X, Y or Z position depending on the recursion
// Id in the standalone serial version is the particle tag
// Id in the parallel version is the index of that particle on a
// particular processor which is why it can be int and not ID_T
//
struct ValueIdPair {
  POSVEL_T value;
  int id;
};

class ValueIdPairLT {
public:
  bool operator() (const ValueIdPair& p, const ValueIdPair& q) const
  {
  return p.value < q.value;
  }
};

/****************************************************************************/

class CosmoHaloFinder
{
public:
  // create a finder
  CosmoHaloFinder();
  ~CosmoHaloFinder();

  void Finding();

  // Read alive particles
#ifndef USE_VTK_COSMO
  void Reading();
  void Writing();

  // execute the finder
  void Execute();
#endif

  void setInFile(string inFile)         { infile = inFile.c_str(); }
  void setOutFile(string outFile)       { outfile = outFile.c_str(); }

  void setParticleLocations(POSVEL_T** d) { data = d; }
  void setNumberOfParticles(int n)      { npart = n; }
  void setMyProc(int r)                 { myProc = r; }

  int* getHaloTag()                     { return ht; }

  POSVEL_T* getXLoc()                   { return xx; }
  POSVEL_T* getYLoc()                   { return yy; }
  POSVEL_T* getZLoc()                   { return zz; }
  POSVEL_T* getXVel()                   { return vx; }
  POSVEL_T* getYVel()                   { return vy; }
  POSVEL_T* getZVel()                   { return vz; }
  POSVEL_T* getMass()                   { return ms; }
  int*   getTag()                       { return pt; }
  
  // np.in
  int np;
  POSVEL_T rL;
  POSVEL_T bb;
  int pmin;
  bool periodic;
  const char *infile;
  const char *outfile;
  const char *textmode;

private:

  // input/output interface
  POSVEL_T *xx, *yy, *zz, *vx, *vy, *vz, *ms;
  int   *pt, *ht;

  // internal state
  int npart, nhalo, nhalopart;
  int myProc;

  // data[][] stores xx[], yy[], zz[].
  POSVEL_T **data;

  // scale factor
  POSVEL_T xscal, vscal;

  int *halo, *nextp, *hsize;

  // Creates a sequence array containing ids of particle rearranged into
  // a k-d tree.  Recursive method.
  ValueIdPair *v;
  int *seq;
  void Reorder
    (int first, 
     int last, 
     int flag);

  // Calculates a lower and upper bound for each particle so that the 
  // mergeing step can prune parts of the k-d tree
  POSVEL_T **lb, **ub;
  void ComputeLU(int, int);

  // Recurses through the k-d tree merging particles to create halos
  void myFOF(int, int, int);
  void Merge(int, int, int, int, int);
};

#endif
