Geant4EventSeed.h

Go to the documentation of this file.

//==========================================================================
//  AIDA Detector description implementation 
//--------------------------------------------------------------------------
// Copyright (C) Organisation europeenne pour la Recherche nucleaire (CERN)
// All rights reserved.
//
// For the licensing terms see $DD4hepINSTALL/LICENSE.
// For the list of contributors see $DD4hepINSTALL/doc/CREDITS.
//
// Author     : A.Sailer
//
//==========================================================================
#ifndef DDG4_PLUGINS_GEANT4EVENTSEED_H
#define DDG4_PLUGINS_GEANT4EVENTSEED_H
 
// Framework include files
#include <DDG4/Geant4RunAction.h>
 
// Forward declarations
class G4StackManager;
 
 
 
namespace dd4hep {
 
  namespace sim {
 
    class Geant4EventSeed: public Geant4RunAction {
 
    protected:
      unsigned int m_initialSeed;
      unsigned int m_runID;
      std::string m_type;
      bool m_initialised;
    public:
      Geant4EventSeed(Geant4Context*, const std::string& );
      virtual ~Geant4EventSeed();
      void begin(const G4Run*);
      void beginEvent(const G4Event*);
      void prepareEvent(G4StackManager*);
    };
 
    /*
 
      Hashing for random seed as used in Marlin EventSeeder processor
 
      Original source by Bob Jenkins
 
      http://www.burtleburtle.net/bob/hash/doobs.html
 
      Hash a variable-length key into a 32-bit value
 
    */
 
#define hashsize(n) ( 1U << (n) )
#define hashmask(n) ( hashsize ( n ) - 1 )
 
 
    /*
      --------------------------------------------------------------------
      mix -- mix 3 32-bit values reversibly.
      For every delta with one or two bits set, and the deltas of all three
      high bits or all three low bits, whether the original value of a,b,c
      is almost all zero or is uniformly distributed,
      * If mix() is run forward or backward, at least 32 bits in a,b,c
      have at least 1/4 probability of changing.
      * If mix() is run forward, every bit of c will change between 1/3 and
      2/3 of the time.  (Well, 22/100 and 78/100 for some 2-bit deltas.)
      mix() was built out of 36 single-cycle latency instructions in a
      structure that could supported 2x parallelism, like so:
      a -= b;
      a -= c; x = (c>>13);
      b -= c; a ^= x;
      b -= a; x = (a<<8);
      c -= a; b ^= x;
      c -= b; x = (b>>13);
      ...
      Unfortunately, superscalar Pentiums and Sparcs can't take advantage
      of that parallelism.  They've also turned some of those single-cycle
      latency instructions into multi-cycle latency instructions.  Still,
      this is the fastest good hash I could find.  There were about 2^^68
      to choose from.  I only looked at a billion or so.
      --------------------------------------------------------------------
    */
#define mix(a,b,c)                              \
    {                                           \
      a -= b; a -= c; a ^= (c>>13);             \
      b -= c; b -= a; b ^= (a<<8);              \
      c -= a; c -= b; c ^= (b>>13);             \
      a -= b; a -= c; a ^= (c>>12);             \
      b -= c; b -= a; b ^= (a<<16);             \
      c -= a; c -= b; c ^= (b>>5);              \
      a -= b; a -= c; a ^= (c>>3);              \
      b -= c; b -= a; b ^= (a<<10);             \
      c -= a; c -= b; c ^= (b>>15);             \
    }
 
    /*
      --------------------------------------------------------------------
      jenkins_hash() -- hash a variable-length key into a 32-bit value
      k       : the key (the unaligned variable-length array of bytes)
      len     : the length of the key, counting by bytes
      initval : can be any 4-byte value
      Returns a 32-bit value.  Every bit of the key affects every bit of
      the return value.  Every 1-bit and 2-bit delta achieves avalanche.
      About 6*len+35 instructions.
 
      The best hash table sizes are powers of 2.  There is no need to do
      mod a prime (mod is sooo slow!).  If you need less than 32 bits,
      use a bitmask.  For example, if you need only 10 bits, do
      h = (h & hashmask(10));
      In which case, the hash table should have hashsize(10) elements.
 
      If you are hashing n strings (ub1 **)k, do it like this:
      for (i=0, h=0; i<n; ++i) h = hash( k[i], len[i], h);
 
      By Bob Jenkins, 1996.  bob_jenkins@burtleburtle.net.  You may use this
      code any way you wish, private, educational, or commercial.  It's free.
 
      See http://burtleburtle.net/bob/hash/evahash.html
      Use for hash table lookup, or anything where one collision in 2^^32 is
      acceptable.  Do NOT use for cryptographic purposes.
      --------------------------------------------------------------------
    */
    unsigned jenkins_hash ( unsigned char *k, unsigned length, unsigned initval )
    {
      unsigned a, b;
      unsigned c = initval;
      unsigned len = length;
 
      a = b = 0x9e3779b9;
 
      while ( len >= 12 ) {
        a += ( k[0] + ( (unsigned)k[1] << 8 )
               + ( (unsigned)k[2] << 16 )
               + ( (unsigned)k[3] << 24 ) );
        b += ( k[4] + ( (unsigned)k[5] << 8 )
               + ( (unsigned)k[6] << 16 )
               + ( (unsigned)k[7] << 24 ) );
        c += ( k[8] + ( (unsigned)k[9] << 8 )
               + ( (unsigned)k[10] << 16 )
               + ( (unsigned)k[11] << 24 ) );
 
        mix ( a, b, c );
 
        k += 12;
        len -= 12;
      }
 
      c += length;
 
      switch ( len ) {
      case 11: c += ( (unsigned)k[10] << 24 ); [[fallthrough]];
      case 10: c += ( (unsigned)k[9] << 16 ); [[fallthrough]];
      case 9 : c += ( (unsigned)k[8] << 8 ); [[fallthrough]];
        /* First byte of c reserved for length */
      case 8 : b += ( (unsigned)k[7] << 24 ); [[fallthrough]];
      case 7 : b += ( (unsigned)k[6] << 16 ); [[fallthrough]];
      case 6 : b += ( (unsigned)k[5] << 8 ); [[fallthrough]];
      case 5 : b += k[4]; [[fallthrough]];
      case 4 : a += ( (unsigned)k[3] << 24 ); [[fallthrough]];
      case 3 : a += ( (unsigned)k[2] << 16 ); [[fallthrough]];
      case 2 : a += ( (unsigned)k[1] << 8 ); [[fallthrough]];
      case 1 : a += k[0];
      }
 
      mix ( a, b, c );
 
      return c;
    }
 
    unsigned int hash( unsigned int initialSeed, unsigned int eventNumber, unsigned int runNumber ){
      unsigned int seed = 0;
      unsigned char * c = (unsigned char *) &eventNumber ;
      seed = jenkins_hash( c, sizeof eventNumber, seed) ;
 
      c = (unsigned char *) &runNumber ;
      seed = jenkins_hash( c, sizeof runNumber, seed) ;
 
      c = (unsigned char *) &initialSeed ;
      seed = jenkins_hash( c, sizeof initialSeed, seed) ;
 
      return seed;
    }
 
  }    // End namespace sim
}      // End namespace dd4hep
 
#endif // DDG4_PLUGINS_GEANT4EVENTSEED_H