// BeamParticle.cc is a part of the PYTHIA event generator.
// Copyright (C) 2017 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL version 2, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.

// Function definitions (not found in the header) for the
// BeamParticle class.

#include "Pythia8/BeamParticle.h"

namespace Pythia8 {

//==========================================================================

// The ColState class.

//--------------------------------------------------------------------------

// Small storage class needed to find the full colour state
// of all the scattered partons.

class ColState {
public:
  ColState() : nTotal(0.) {}
  vector< pair<int,int> > lastSteps;
  // nTotal is integer but can become extremely big.
  double nTotal;
};

//==========================================================================

// The BeamParticle class.

//--------------------------------------------------------------------------

// Constants: could be changed here if desired, but normally should not.
// These are of technical nature, as described for each.

// A lepton that takes (almost) the full beam energy does not leave a remnant.
const double BeamParticle::XMINUNRESOLVED = 1. - 1e-10;

// Fictitious Pomeron mass to leave some room for beam remnant.
const double BeamParticle::POMERONMASS = 1.;

// Avoid numerical instability in the x -> 1 limit for companion quark.
const double BeamParticle::XMAXCOMPANION = 0.99;

// Maximum number of tries to find a suitable colour.
const int BeamParticle::NMAX = 1000;

// Maximum number of tries to randomly assign colours to beam remnant.
// After this number is reached, a systematic approach is used.
const int BeamParticle::NRANDOMTRIES = 1000;

//--------------------------------------------------------------------------

// Initialize data on a beam particle and save pointers.

void BeamParticle::init( int idIn, double pzIn, double eIn, double mIn,
  Info* infoPtrIn, Settings& settings, ParticleData* particleDataPtrIn,
  Rndm* rndmPtrIn, PDF* pdfInPtr, PDF* pdfHardInPtr, bool isUnresolvedIn,
  StringFlav* flavSelPtrIn) {

  // Store input pointers (and one bool) for future use.
  infoPtr           = infoPtrIn;
  particleDataPtr   = particleDataPtrIn;
  rndmPtr           = rndmPtrIn;
  pdfBeamPtr        = pdfInPtr;
  pdfHardBeamPtr    = pdfHardInPtr;
  isUnresolvedBeam  = isUnresolvedIn;
  flavSelPtr        = flavSelPtrIn;

  // Save the usual PDF pointers as the normal ones may be overwritten
  // with unresolved PDFs when mixing different photoproduction modes.
  pdfBeamPtrSave     = pdfInPtr;
  pdfHardBeamPtrSave = pdfHardInPtr;

  // Check whether beam has a supplementary photon beam.
  bool beamHasGamma  = settings.flag("PDF:lepton2gamma");

  // To be set process by process but start with this.
  hasResGammaInBeam  = beamHasGamma;

  // Maximum quark kind in allowed incoming beam hadrons.
  maxValQuark       = settings.mode("BeamRemnants:maxValQuark");

  // Power of (1-x)^power/sqrt(x) for remnant valence quark distribution.
  valencePowerMeson = settings.parm("BeamRemnants:valencePowerMeson");
  valencePowerUinP  = settings.parm("BeamRemnants:valencePowerUinP");
  valencePowerDinP  = settings.parm("BeamRemnants:valencePowerDinP");

  // Enhancement factor of x of diquark.
  valenceDiqEnhance = settings.parm("BeamRemnants:valenceDiqEnhance");

  // Assume g(x) ~ (1-x)^power/x to constrain companion to sea quark.
  companionPower    = settings.mode("BeamRemnants:companionPower");

  // Assume g(x) ~ (1-x)^power/x with a cut-off for low x.
  gluonPower        = settings.parm("BeamRemnants:gluonPower");
  xGluonCutoff      = settings.parm("BeamRemnants:xGluonCutoff");

  // Allow or not more than one valence quark to be kicked out.
  allowJunction     = settings.flag("BeamRemnants:allowJunction");

  // Choose whether to form a di-quark or
  // a junction with new colur reconnection scheme.
  beamJunction       = settings.flag("beamRemnants:beamJunction");

  // Allow junctions in the outgoing colour state.
  allowBeamJunctions = settings.flag("beamRemnants:allowBeamJunction");

  // For low-mass diffractive system kick out q/g = norm / mass^power.
  pickQuarkNorm     = settings.parm("Diffraction:pickQuarkNorm");
  pickQuarkPower    = settings.parm("Diffraction:pickQuarkPower");

  // Controls the amount of saturation in the new model.
  beamSat           = settings.parm("BeamRemnants:saturation");

  // Width of primordial kT distribution in low-mass diffractive systems.
  diffPrimKTwidth   = settings.parm("Diffraction:primKTwidth");

  // Suppress large masses of beam remnant in low-mass diffractive systems.
  diffLargeMassSuppress = settings.parm("Diffraction:largeMassSuppress");

  // Check if ISR for photon collisions is applied and set pTmin.
  doND              = settings.flag("SoftQCD:nonDiffractive");
  doISR             = settings.flag("PartonLevel:ISR");
  doMPI             = settings.flag("PartonLevel:MPI");
  pTminISR          = settings.parm("SpaceShower:pTmin");

  // Store info on the incoming beam.
  idBeam            = idIn;
  initBeamKind();
  pBeam             = Vec4( 0., 0., pzIn, eIn);
  mBeam             = mIn;

  // Initialize parameters related to photon beams.
  resetGamma();
  resetGammaInLepton();

  clear();

}

//--------------------------------------------------------------------------

// Initialize kind and valence flavour content of incoming beam.
// For recognized hadrons one can generate multiparton interactions.
// Dynamic choice of meson valence flavours in newValenceContent below.

void BeamParticle::initBeamKind() {

  // Reset.
  idBeamAbs         = abs(idBeam);
  isLeptonBeam      = false;
  isHadronBeam      = false;
  isMesonBeam       = false;
  isBaryonBeam      = false;
  isGammaBeam       = false;
  nValKinds         = 0;

  // To be modified according to the process.
  gammaMode         = 0;
  isResUnres        = false;

  // Check for leptons or DM beams.
  if ( (idBeamAbs > 10 && idBeamAbs < 17)
    || (idBeamAbs > 50 && idBeamAbs < 60) ) {
    nValKinds = 1;
    nVal[0]   = 1;
    idVal[0]  = idBeam;
    isLeptonBeam = true;
  }

  // Valence content for photons.
  if (idBeamAbs == 22) {
    isGammaBeam = true;
    nValKinds = 2;
    nVal[0]   = 1;
    nVal[1]   = 1;
    newValenceContent();
    iPosVal   = -1;
  }

  //  Done if cannot be lowest-lying hadron state.
  if (idBeamAbs < 101 || idBeamAbs > 9999) return;

  // Resolve valence content for assumed Pomeron.
  if (idBeamAbs == 990) {
    isMesonBeam = true;
    nValKinds = 2;
    nVal[0]   = 1 ;
    nVal[1]   = 1 ;
    newValenceContent();

  // Resolve valence content for assumed meson. Flunk unallowed codes.
  } else if (idBeamAbs < 1000) {
    int id1 = idBeamAbs/100;
    int id2 = (idBeamAbs/10)%10;
    if ( id1 < 1 || id1 > maxValQuark
      || id2 < 1 || id2 > maxValQuark ) return;
    isMesonBeam = true;

    // Store valence content of a confirmed meson.
    nValKinds = 2;
    nVal[0]   = 1 ;
    nVal[1]   = 1;
    if (id1%2 == 0) {
      idVal[0] = id1;
      idVal[1] = -id2;
    } else {
      idVal[0] = id2;
      idVal[1] = -id1;
    }
    newValenceContent();

  // Resolve valence content for assumed baryon. Flunk unallowed codes.
  } else {
    int id1 = idBeamAbs/1000;
    int id2 = (idBeamAbs/100)%10;
    int id3 = (idBeamAbs/10)%10;
    if ( id1 < 1 || id1 > maxValQuark || id2 < 1 || id2 > maxValQuark
      || id3 < 1 || id3 > maxValQuark) return;
    if (id2 > id1 || id3 > id1) return;
    isBaryonBeam = true;

    // Store valence content of a confirmed baryon.
    nValKinds = 1; idVal[0] = id1; nVal[0] = 1;
    if (id2 == id1) ++nVal[0];
    else {
      nValKinds = 2;
      idVal[1]  = id2;
      nVal[1]   = 1;
    }
    if (id3 == id1) ++nVal[0];
    else if (id3 == id2) ++nVal[1];
    else {
      idVal[nValKinds] = id3;
      nVal[nValKinds]  = 1;
      ++nValKinds;
    }
  }

  // Flip flavours for antimeson or antibaryon, and then done.
  if (idBeam < 0) for (int i = 0; i < nValKinds; ++i) idVal[i] = -idVal[i];
  isHadronBeam = true;
  Q2ValFracSav = -1.;

}

//--------------------------------------------------------------------------

// Initialize the photon beam with additional unresolved PDF pointer.

void BeamParticle::initUnres(PDF* pdfUnresInPtr) {

  // Set the pointer and check that pointer exists.
  pdfUnresBeamPtr = pdfUnresInPtr;
  isResUnres      = (pdfUnresBeamPtr != 0 ) ? true : false;

}

//--------------------------------------------------------------------------

// Dynamic choice of meson valence flavours for pi0, K0S, K0L, Pomeron.

void BeamParticle::newValenceContent() {

  // A pi0 oscillates between d dbar and u ubar.
  if (idBeam == 111) {
    idVal[0] = (rndmPtr->flat() < 0.5) ? 1 : 2;
    idVal[1] = -idVal[0];

  // A K0S or K0L oscillates between d sbar and s dbar.
  } else if (idBeam == 130 || idBeam == 310) {
    idVal[0] = (rndmPtr->flat() < 0.5) ?  1 :  3;
    idVal[1] = (idVal[0] == 1)      ? -3 : -1;

  // For a Pomeron split gluon remnant into d dbar or u ubar.
  } else if (idBeam == 990) {
    idVal[0] = (rndmPtr->flat() < 0.5) ? 1 : 2;
    idVal[1] = -idVal[0];

  // For photons set an usused code to indicate that the flavour is not
  // chosen yet but will be done later.
  } else if (idBeam == 22) {
    idVal[0] = 10;
    idVal[1] = -idVal[0];

  // Other hadrons so far do not require any event-by-event change.
  } else return;

  // Propagate change to PDF routine(s).
  pdfBeamPtr->newValenceContent( idVal[0], idVal[1]);
  if (pdfHardBeamPtr != pdfBeamPtr && pdfHardBeamPtr != 0)
    pdfHardBeamPtr->newValenceContent( idVal[0], idVal[1]);

}

//--------------------------------------------------------------------------

double BeamParticle::xMax(int iSkip) {

  // Minimum requirement on remaining energy > nominal mass for hadron.
  double xLeft = 1.;
  if (idBeam == 990)   xLeft -= POMERONMASS / e();
  else if (isHadron()) xLeft -= m() / e();
  if (size() == 0) return xLeft;

  // Subtract what was carried away by initiators (to date).
  for (int i = 0; i < size(); ++i)
    if (i != iSkip && resolved[i].isFromBeam()) xLeft -= resolved[i].x();
  return xLeft;

}

//--------------------------------------------------------------------------

// Parton distributions, reshaped to take into account previous
// multiparton interactions. By picking a non-negative iSkip value,
// one particular interaction is skipped, as needed for ISR

double BeamParticle::xfModified(int iSkip, int idIn, double x, double Q2) {

  // Initial values.
  idSave    = idIn;
  iSkipSave = iSkip;
  xqVal     = 0.;
  xqgSea    = 0.;
  xqCompSum = 0.;

  // Fast procedure for first interaction.
  if (size() == 0) {
    if (x >= 1.) return 0.;
    bool canBeVal = false;
    for (int i = 0; i < nValKinds; ++i)
      if (idIn == idVal[i]) canBeVal = true;
    if (canBeVal) {
      xqVal     = xfVal( idIn, x, Q2);
      xqgSea    = xfSea( idIn, x, Q2);
    }
    else xqgSea = xf( idIn, x, Q2);

  // More complicated procedure for non-first interaction.
  } else {

    // Sum up the x already removed, and check that remaining x is enough.
    double xUsed = 0.;
    for (int i = 0; i < size(); ++i)
      if (i != iSkip) xUsed += resolved[i].x();
    double xLeft = 1. - xUsed;
    if (x >= xLeft) return 0.;
    double xRescaled = x / xLeft;

    // Calculate total and remaining amount of x carried by valence quarks.
    double xValTot = 0.;
    double xValLeft = 0.;
    for (int i = 0; i < nValKinds; ++i) {
      nValLeft[i] = nVal[i];
      for (int j = 0; j < size(); ++j)
      if (j != iSkip && resolved[j].isValence()
        && resolved[j].id() == idVal[i]) --nValLeft[i];
      double xValNow =  xValFrac(i, Q2);
      xValTot += nVal[i] * xValNow;
      xValLeft += nValLeft[i] * xValNow;
    }

    // Calculate total amount of x carried by unmatched companion quarks.
    double xCompAdded = 0.;
    for (int i = 0; i < size(); ++i)
    if (i != iSkip && resolved[i].isUnmatched()) xCompAdded
      += xCompFrac( resolved[i].x() / (xLeft + resolved[i].x()) )
      // Typo warning: extrafactor missing in Skands&Sjostrand article;
      // <x> for companion refers to fraction of x left INCLUDING sea quark.
      // To be modified further??
      * (1. + resolved[i].x() / xLeft);

    // Calculate total rescaling factor and pdf for sea and gluon.
    double rescaleGS = max( 0., (1. - xValLeft - xCompAdded)
      / (1. - xValTot) );
    xqgSea = rescaleGS * xfSea( idIn, xRescaled, Q2);

    // Find valence part and rescale it to remaining number of quarks.
    for (int i = 0; i < nValKinds; ++i)
    if (idIn == idVal[i] && nValLeft[i] > 0)
      xqVal = xfVal( idIn, xRescaled, Q2)
      * double(nValLeft[i]) / double(nVal[i]);

    // Find companion part, by adding all companion contributions.
    for (int i = 0; i < size(); ++i)
    if (i != iSkip && resolved[i].id() == -idIn
      && resolved[i].isUnmatched()) {
      double xsRescaled = resolved[i].x() / (xLeft + resolved[i].x());
      double xcRescaled = x / (xLeft + resolved[i].x());
      double xqCompNow = xCompDist( xcRescaled, xsRescaled);
      // Normalize the companion quark PDF to the total momentum carried
      // by the partons in case of photon beam at given scale Q^2.
      if ( isGamma() ) xqCompNow *= xIntegratedPDFs(Q2);
      resolved[i].xqCompanion( xqCompNow);
      xqCompSum += xqCompNow;
    }
  }

  // Add total, but only return relevant part for ISR. More cases??
  // Watch out, e.g. g can come from either kind of quark.??
  xqgTot = xqVal + xqgSea + xqCompSum;

  // If ISR with photon beams no distinction between valence and sea.
  if (isGammaBeam && doISR) return xqgTot;

  if (iSkip >= 0) {
    if (resolved[iSkip].isValence()) return xqVal;
    if (resolved[iSkip].isUnmatched()) return xqgSea + xqCompSum;
  }
  return xqgTot;

}

//--------------------------------------------------------------------------

// Decide whether a quark extracted from the beam is of valence, sea or
// companion kind; in the latter case also pick its companion.
// Assumes xfModified has already been called.

int BeamParticle::pickValSeaComp() {

  // If parton already has a companion than reset code for this.
  int oldCompanion = resolved[iSkipSave].companion();
  if (oldCompanion >= 0) resolved[oldCompanion].companion(-2);

  // Default assignment is sea.
  int vsc = -2;

  // For gluons or photons no sense of valence or sea.
  if (idSave == 21 || idSave == 22) vsc = -1;

  // For lepton beam assume same-kind lepton inside is valence.
  else if (isLeptonBeam && idSave == idBeam) vsc = -3;

  // Decide if valence or sea quark.
  // For photons, consider all partons as sea until valence content fixed.
  else {
    double xqRndm = xqgTot * rndmPtr->flat();
    if (xqRndm < xqVal && !isGammaBeam ) vsc = -3;
    else if (xqRndm < xqVal + xqgSea) vsc = -2;

    // If not either, loop over all possible companion quarks.
    else {
      xqRndm -= xqVal + xqgSea;
      for (int i = 0; i < size(); ++i)
      if (i != iSkipSave && resolved[i].id() == -idSave
        && resolved[i].isUnmatched()) {
        xqRndm -= resolved[i].xqCompanion();
        if (xqRndm < 0.) vsc = i;
        break;
      }
    }
  }

  // Bookkeep assignment; for sea--companion pair both ways.
  resolved[iSkipSave].companion(vsc);
  if (vsc >= 0) resolved[vsc].companion(iSkipSave);

  // Done; return code for choice (to distinguish valence/sea in Info).
  return vsc;

}

//--------------------------------------------------------------------------

// Fraction of hadron momentum sitting in a valence quark distribution.
// Based on hardcoded parametrizations of CTEQ 5L numbers.

double BeamParticle::xValFrac(int j, double Q2) {

  // Only recalculate when required.
  if (Q2 != Q2ValFracSav) {
    Q2ValFracSav = Q2;

    // Q2-dependence of log-log form; assume fixed Lambda = 0.2.
    double llQ2 = log( log( max( 1., Q2) / 0.04 ));

    // Fractions carried by u and d in proton.
    uValInt =  0.48 / (1. + 1.56 * llQ2);
    dValInt = 0.385 / (1. + 1.60 * llQ2);
  }

  // Baryon with three different quark kinds: (2 * u + d) / 3 of proton.
  if (isBaryonBeam && nValKinds == 3) return (2. * uValInt + dValInt) / 3.;

  // Baryon with one or two identical: like d or u of proton.
  if (isBaryonBeam && nVal[j] == 1) return dValInt;
  if (isBaryonBeam && nVal[j] == 2) return uValInt;

  // Meson: (2 * u + d) / 2 of proton so same total valence quark fraction.
  return 0.5 * (2. * uValInt + dValInt);

}

//--------------------------------------------------------------------------

// The momentum integral of a companion quark, with its partner at x_s,
// using an approximate gluon density like (1 - x_g)^power / x_g.
// The value corresponds to an unrescaled range between 0 and 1 - x_s.

double BeamParticle::xCompFrac(double xs) {

  // Tiny answer for xs -> 1 is numerically unstable, so set = 0.
  if (xs > XMAXCOMPANION) return 0.;

  // Select case by power of gluon (1-x_g) shape.
  switch (companionPower) {

    case 0:
       return xs * ( 5. + xs * (-9. - 2. * xs * (-3. + xs)) + 3. * log(xs) )
         / ( (-1. + xs) * (2. + xs * (-1. + 2. * xs)) );

    case 1:
       return -1. -3. * xs + ( 2. * pow2(-1. + xs) * (1. + xs + xs*xs))
         / ( 2. + xs*xs * (xs - 3.) + 3. * xs * log(xs) );

    case 2:
       return xs * ( (1. - xs) * (19. + xs * (43. + 4. * xs))
         + 6. * log(xs) * (1. + 6. * xs + 4.*xs*xs) ) /
        ( 4. * ( (xs - 1.) * (1. + xs * (4. + xs) )
        - 3. * xs * log(xs) * (1 + xs) ) );

    case 3:
      return 3. * xs * ( (xs - 1.) * (7. + xs * (28. + 13. * xs))
        - 2. * log(xs) * (1. + xs * (9. + 2. * xs * (6. + xs))) )
        / ( 4. + 27. * xs - 31. * pow3(xs)
        + 6. * xs * log(xs) * (3. + 2. * xs * (3.+xs)) );

    default:
      return ( -9. * xs * (xs*xs - 1.) * (5. + xs * (24. + xs)) + 12. * xs
        * log(xs) * (1. + 2. * xs) * (1. + 2. * xs * (5. + 2. * xs)) )
        / ( 8. * (1. + 2. * xs) * ((xs - 1.) * (1. + xs * (10. + xs))
        - 6. * xs * log(xs) * (1. + xs)) );

  }
}

//--------------------------------------------------------------------------

// The x*f pdf of a companion quark at x_c, with its sea partner at x_s,
// using an approximate gluon density like (1 - x_g)^power / x_g.
// The value corresponds to an unrescaled range between 0 and 1 - x_s.

double BeamParticle::xCompDist(double xc, double xs) {

  // Tiny answer for xs -> 1 is numerically unstable, so set = 0.
  if (xs > XMAXCOMPANION) return 0.;

  // Mother gluon momentum fraction. Check physical limit.
  double xg = xc + xs;
  if (xg > 1.) return 0.;

  // Common factor, including splitting kernel and part of gluon density
  // (and that it is x_c * f that is coded).
  double fac = 3. * xc * xs * (xc*xc + xs*xs) / pow4(xg);

  // Select case by power of gluon (1-x_g) shape.
  switch (companionPower) {

    case 0:
      return fac / ( 2. - xs * (3. - xs * (3. - 2. * xs)) );

    case 1:
      return fac * (1. - xg) / ( 2. + xs*xs * (-3. + xs) + 3. * xs * log(xs) );

    case 2:
      return fac * pow2(1. - xg) / ( 2. * ((1. - xs) * (1. + xs * (4. + xs))
        + 3. * xs * (1. + xs) * log(xs)) );

    case 3:
      return fac * pow3(1. - xg) * 2. / ( 4. + 27. * xs - 31. * pow3(xs)
        + 6. * xs * log(xs) * (3. + 2. * xs * (3. + xs)) );

    default:
       return fac * pow4(1. - xg) / ( 2. * (1. + 2. * xs) * ((1. - xs)
         * (1. + xs * (10. + xs)) + 6. * xs * log(xs) * (1. + xs)) );

  }
}

//--------------------------------------------------------------------------

// Set the photon mode (none (0), resolved (1), unresolved (2)) of the beam.

void BeamParticle::setGammaMode(int gammaModeIn)  {

  // For hadrons mode always 0.
  if (isHadron()) {
    gammaMode         = 0;
    pdfBeamPtr        = pdfBeamPtrSave;
    pdfHardBeamPtr    = pdfHardBeamPtrSave;
    hasResGammaInBeam = false;
    isResolvedGamma   = false;
    return;
  }

  // Save the mode of the photon beam.
  gammaMode = gammaModeIn;

  // Set the beam and PDF pointers to unresolved mode.
  if (gammaMode == 2 && isResUnres) {
    pdfBeamPtr        = pdfUnresBeamPtr;
    pdfHardBeamPtr    = pdfUnresBeamPtr;
    isResolvedGamma   = false;
    hasResGammaInBeam = false;

    // Only a photon beam can be unresolved with gammaMode == 2.
    if ( isGamma()) isUnresolvedBeam = true;

  // Set the beam and PDF pointers to resolved mode.
  } else {
    pdfBeamPtr        = pdfBeamPtrSave;
    pdfHardBeamPtr    = pdfHardBeamPtrSave;
    isUnresolvedBeam  = false;
    if ( isGamma()) isResolvedGamma = true;
    else            isResolvedGamma = false;
    if ( isLepton() && gammaMode == 1 ) hasResGammaInBeam = true;
    else                                hasResGammaInBeam = false;
  }

}

//--------------------------------------------------------------------------

// Check whether parton iResolved with given Q^2 is a valence quark.

bool BeamParticle::gammaInitiatorIsVal(int iResolved, double Q2) {
  return gammaInitiatorIsVal( iResolved, resolved[iResolved].id(),
    resolved[iResolved].x(), Q2 );
}

//--------------------------------------------------------------------------

// Check whether initiator parton is a valence quark using the PDFs.
// Set the position of the valence quark to iGamVal.

bool BeamParticle::gammaInitiatorIsVal(int iResolved, int idInit,
  double x, double Q2) {

  // Reset the valence quark position.
  iPosVal = -1;

  // Gluon is not a valence parton. Sample content accordingly.
  if ( idInit == 0 || abs(idInit) == 21 ) {
    idVal[0] = pdfBeamPtr->sampleGammaValFlavor(Q2);
    idVal[1] = -idVal[0];
    return false;
  } else {

    // Set the valence content to match with the hard process to get the
    // correct PDFs and to store the choice. Changed by sampleGammaValFlavor.
    idVal[0] =  idInit;
    idVal[1] = -idInit;
    pdfBeamPtr->newValenceContent( idVal[0], idVal[1]);

    // If initiator from gamma->qqbar splitting then it is a valence quark.
    if ( iResolved == iGamVal ) {
      iPosVal = iResolved;
      return true;

    // If Q^2 is smaller than mass of quark set to valence.
    } else if ( Q2 < pdfBeamPtr->gammaPDFRefScale(idInit) ) {
      iPosVal = iResolved;
      return true;

    // Use PDFs to decide if valence parton.
    } else {
      double xVal = xfVal( idInit, x, Q2);
      double xSea = xfSea( idInit, x, Q2);
      if ( rndmPtr->flat() < xVal/( xVal + xSea ) ) {
        iPosVal = iResolved;
        return true;

      // If the initiator not valence sample the flavour.
      } else {
        idVal[0] = pdfBeamPtr->sampleGammaValFlavor(Q2);
        idVal[1] = -idVal[0];
        return false;
      }
    }
  }
}

//--------------------------------------------------------------------------

// Return the type of the hard parton from a photon beam.

int BeamParticle::gammaValSeaComp(int iResolved) {

  // Default choice a sea quark.
  int vsc = -2;

  // Gluons and photons -1.
  if ( resolved[iResolved].id() == 21 ||
       resolved[iResolved].id() == 22 ) vsc = -1;

  // Quarks are valence partons if decided so earlier.
  else if (iResolved == iPosVal) vsc = -3;
  resolved[iResolved].companion(vsc);

  return vsc;
}

//--------------------------------------------------------------------------

// Add required extra remnant flavour content. Also initial colours.

bool BeamParticle::remnantFlavours(Event& event, bool isDIS) {

  // Elastically scattered beam, e.g. for coherent photon from proton.
  if (isHadronBeam && isUnresolvedBeam) {
    append( 0, idBeam, 0., -1);
    resolved[1].m( particleDataPtr->m0( idBeamAbs ) );
    return true;
  }

  // A baryon will have a junction, unless a diquark is formed later.
  hasJunctionBeam = (isBaryon());

  // Store how many hard-scattering partons were removed from beam.
  nInit = size();
  if (isDIS && nInit != 1) return false;

  // Decide the valence content of photon beam here if ISR is applied.
  if ( isGammaBeam ) {

    // For direct-resolved processes no need for remnants on direct side.
    if (resolved[0].id() == 22) return true;

    // If ISR but no MPIs check only for the one initiator.
    if ( doISR && !doMPI ) {

      // If remnants are constructed fix the valence content using the
      // scale where ISR have stopped.
      if ( isResolvedGamma ) {
        gammaInitiatorIsVal(0, pow2(pTminISR));
      }

      // Set the initiator companion code after the valence content is fixed.
      gammaValSeaComp(0);

    } else if ( doMPI || doND ) {

      // If ISR is applied, use the min. scale of evolution for the valence
      // parton decision. Otherwise use the pT of latest MPI (set in scatter).
      double pTmin = doISR ? pTminISR : pTminMPI;

      // If a quark from gamma->qqbar exists, this must be the valence so
      // set it to valence and the possible companion to other valence.
      if ( iGamVal >= 0 ) {
        gammaInitiatorIsVal(iGamVal, pow2(pTmin));
        int valComp = resolved[iGamVal].companion();
        if ( valComp >= 0 ) resolved[valComp].companion(-3);
        gammaValSeaComp(iGamVal);
      } else {

        // Loop through initiators starting from parton from the hardest
        // interaction and check whether valence. When found, no need to
        // check further.
        for ( int i = 0; i < size(); ++i) {
          bool isValence = gammaInitiatorIsVal(i, pow2(pTminISR));
          int origComp = resolved[i].companion();
          gammaValSeaComp(i);
          if (isValence) {
            // If the chosen valence parton has a companion, set this to be
            // the other valence parton.
            if ( origComp >= 0 ) resolved[origComp].companion(-3);
            break;
          }
        }
      }
    }
  }

  // No need for remnants with unresolved photon from leptons.
  else if ( isLeptonBeam && gammaMode == 2) return true;

  // Find remaining valence quarks.
  for (int i = 0; i < nValKinds; ++i) {
    nValLeft[i] = nVal[i];
    for (int j = 0; j < nInit; ++j) if (resolved[j].isValence()
      && resolved[j].id() == idVal[i]) --nValLeft[i];
      // No valence quarks if ISR find the original beam photon.
    if ( isGammaBeam && doISR && !isResolvedGamma && !doMPI ) nValLeft[i] = 0;
    // Add remaining valence quarks to record. Partly temporary values.
    for (int k = 0; k < nValLeft[i]; ++k) append(0, idVal[i], 0., -3);
  }

  // If at least two valence quarks left in baryon remnant then form diquark.
  int nInitPlusVal = size();
  if (isBaryon() && nInitPlusVal - nInit >= 2) {

    // If three, pick two at random to form diquark, else trivial.
    int iQ1 = nInit;
    int iQ2 = nInit + 1;
    if (nInitPlusVal - nInit == 3) {
      double pickDq = 3. * rndmPtr->flat();
      if (pickDq > 1.) iQ2 = nInit + 2;
      if (pickDq > 2.) iQ1 = nInit + 1;
    }

    // Pick spin 0 or 1 according to SU(6) wave function factors.
    int idDq = flavSelPtr->makeDiquark( resolved[iQ1].id(),
      resolved[iQ2].id(), idBeam);

    // Overwrite with diquark flavour and remove one slot. No more junction.
    resolved[iQ1].id(idDq);
    if (nInitPlusVal - nInit == 3 && iQ2 == nInit + 1)
      resolved[nInit + 1].id( resolved[nInit + 2].id() );
    resolved.pop_back();
    hasJunctionBeam = false;
  }

  // Find companion quarks to unmatched sea quarks.
  for (int i = 0; i < nInit; ++i)
  if (resolved[i].isUnmatched()) {

    // Add companion quark to record; and bookkeep both ways.
    append(0, -resolved[i].id(), 0., i);
    resolved[i].companion(size() - 1);
  }

  // If no other remnants found, add a gluon or photon to carry momentum.
  // Add partons for photons only if remnants needed.
  if ( size() == nInit && !isUnresolvedBeam &&
       (!isGammaBeam || isResolvedGamma || doMPI) ) {
    int idRemnant = (isHadronBeam || isGammaBeam) ? 21 : 22;
    append(0, idRemnant, 0., -1);
  }

  // For DIS allow collapse to one colour singlet hadron.
  if (isHadronBeam && isDIS && size() > 2 && resolved[0].id() != 21) {
    if (size() != 4) {
      infoPtr->errorMsg("Error in BeamParticle::remnantFlavours: "
        "unexpected number of beam remnants for DIS");
      return false;
    }

    // Companion last; find parton with matching colour.
    int colTypeComp = particleDataPtr->colType( resolved[3].id() );
    int colType1    = particleDataPtr->colType( resolved[1].id() );
    int i12         = (colType1 == -colTypeComp) ? 1 : 2;

    // Combine to new hadron flavour.
    int idHad = flavSelPtr->combineId( resolved[i12].id(), resolved[3].id() );
    if (idHad == 0) {
      infoPtr->errorMsg("Error in BeamParticle::remnantFlavours: "
        "failed to combine hadron for DIS");
      return false;
    }

    // Overwrite with hadron flavour and remove companion.
    resolved[i12].id(idHad);
    resolved.pop_back();
    resolved[0].companion(-3);
  }

  // Set initiator and remnant masses.
  for (int i = 0; i < size(); ++i) {
    if (i < nInit) resolved[i].m(0.);
    else resolved[i].m( particleDataPtr->m0( resolved[i].id() ) );
  }

  // For debug purposes: reject beams with resolved junction topology.
  if (hasJunctionBeam && !allowJunction) return false;

  // Pick initial colours for remnants.
  for (int i = nInit; i < size(); ++i) {
    int colType = particleDataPtr->colType( resolved[i].id() );
    int col = (colType == 1 || colType == 2) ? event.nextColTag() : 0;
    int acol = (colType == -1 || colType == 2) ? event.nextColTag() : 0;
    resolved[i].cols( col, acol);
  }

  // Done.
  return true;

}

//--------------------------------------------------------------------------

// Correlate all initiators and remnants to make a colour singlet.

bool BeamParticle::remnantColours(Event& event, vector<int>& colFrom,
  vector<int>& colTo) {

  // No colours in lepton beams so no need to do anything.
  if (isLeptonBeam) return true;

  // Copy initiator colour info from the event record to the beam.
  for (int i = 0; i < size(); ++i) {
    int j =  resolved[i].iPos();
    resolved[i].cols( event[j].col(), event[j].acol());
  }

  // Find number and position of valence quarks, of gluons, and
  // of sea-companion pairs (counted as gluons) in the beam remnants.
  // Skip gluons with same colour as anticolour and rescattering partons.
  vector<int> iVal;
  vector<int> iGlu;
  for (int i = 0; i < size(); ++i)
  if (resolved[i].isFromBeam()) {
    if ( resolved[i].isValence() ) iVal.push_back(i);
    else if ( resolved[i].isCompanion() && resolved[i].companion() > i )
      iGlu.push_back(i);
    else if ( resolved[i].id() == 21
      && resolved[i].col() != resolved[i].acol() ) iGlu.push_back(i);
  }

  // Pick a valence quark to which gluons are attached.
  // Do not resolve quarks in diquark. (More sophisticated??)
  int iValSel = 0;
  if (iVal.size() != 0) {
    iValSel = iVal[0];
    if (iVal.size() == 2) {
      if ( abs(resolved[iValSel].id()) > 10 ) iValSel = iVal[1];
    } else {
      double rndmValSel = 3. * rndmPtr->flat();
      if (rndmValSel > 1.) iValSel= iVal[1];
      if (rndmValSel > 2.) iValSel= iVal[2];
    }
  }

  // This valence quark defines initial (anti)colour.
  int iBeg = iValSel;
  bool hasCol = (resolved[iBeg].col() > 0);
  int begCol = (hasCol) ? resolved[iBeg].col() : resolved[iBeg].acol();

  // Do random stepping through gluon/(sea+companion) list.
  vector<int> iGluRndm;
  for (int i = 0; i < int(iGlu.size()); ++i)
    iGluRndm.push_back( iGlu[i] );
  for (int iOrder = 0; iOrder < int(iGlu.size()); ++iOrder) {
    int iRndm = int( double(iGluRndm.size()) * rndmPtr->flat());
    int iGluSel = iGluRndm[iRndm];
    iGluRndm[iRndm] = iGluRndm[iGluRndm.size() - 1];
    iGluRndm.pop_back();

    // Find matching anticolour/colour to current colour/anticolour.
    int iEnd = iGluSel;
    int endCol = (hasCol) ? resolved[iEnd].acol() : resolved[iEnd].col();
    // Not gluon but sea+companion pair: go to other.
    if (endCol == 0) {
      iEnd = resolved[iEnd].companion();
      endCol = (hasCol) ? resolved[iEnd].acol() : resolved[iEnd].col();
    }

    // Collapse this colour-anticolour pair to the lowest one.
    if (begCol < endCol) {
      if (hasCol) resolved[iEnd].acol(begCol);
      else resolved[iEnd].col(begCol);
      colFrom.push_back(endCol);
      colTo.push_back(begCol);
    } else {
      if (hasCol) resolved[iBeg].col(endCol);
      else resolved[iBeg].acol(endCol);
      colFrom.push_back(begCol);
      colTo.push_back(endCol);
    }

    // Pick up the other colour of the recent gluon and repeat.
    iBeg = iEnd;
    begCol = (hasCol) ? resolved[iBeg].col() : resolved[iBeg].acol();
    // Not gluon but sea+companion pair: go to other.
    if (begCol == 0) {
      iBeg = resolved[iBeg].companion();
      begCol = (hasCol) ? resolved[iBeg].col() : resolved[iBeg].acol();
    }

  // At end of gluon/(sea+companion) list.
  }

  // Now begin checks, and also finding junction information.
  // Loop through remnant partons; isolate all colours and anticolours.
  vector<int> colList;
  vector<int> acolList;
  for (int i = 0; i < size(); ++i)
  if ( resolved[i].isFromBeam() )
  if ( resolved[i].col() != resolved[i].acol() ) {
    if (resolved[i].col() > 0) colList.push_back( resolved[i].col() );
    if (resolved[i].acol() > 0) acolList.push_back( resolved[i].acol() );
  }

  // Remove all matching colour-anticolour pairs.
  bool foundPair = true;
  while (foundPair && colList.size() > 0 && acolList.size() > 0) {
    foundPair = false;
    for (int iCol = 0; iCol < int(colList.size()); ++iCol) {
      for (int iAcol = 0; iAcol < int(acolList.size()); ++iAcol) {
        if (acolList[iAcol] == colList[iCol]) {
          colList[iCol] = colList.back();
          colList.pop_back();
          acolList[iAcol] = acolList.back();
          acolList.pop_back();
          foundPair = true;
          break;
        }
      } if (foundPair) break;
    }
  }

  // Usually one unmatched pair left to collapse.
  if (colList.size() == 1 && acolList.size() == 1) {
    int finalFrom = max( colList[0], acolList[0]);
    int finalTo   = min( colList[0], acolList[0]);
    for (int i = 0; i < size(); ++i)
    if ( resolved[i].isFromBeam() ) {
      if (resolved[i].col()  == finalFrom) resolved[i].col(finalTo);
      if (resolved[i].acol() == finalFrom) resolved[i].acol(finalTo);
    }
    colFrom.push_back(finalFrom);
    colTo.push_back(finalTo);

  // Store an (anti)junction when three (anti)coloured daughters.
  } else if (hasJunctionBeam && colList.size() == 3
    && acolList.size() == 0) {
    event.appendJunction( 1, colList[0], colList[1], colList[2]);
    junCol[0] = colList[0];
    junCol[1] = colList[1];
    junCol[2] = colList[2];
  } else if (hasJunctionBeam && acolList.size() == 3
    && colList.size() == 0) {
    event.appendJunction( 2, acolList[0], acolList[1], acolList[2]);
    junCol[0] = acolList[0];
    junCol[1] = acolList[1];
    junCol[2] = acolList[2];

  // Any other nonvanishing values indicate failure.
  } else if (colList.size() > 0 || acolList.size() > 0) {
    infoPtr->errorMsg("Error in BeamParticle::remnantColours: "
      "leftover unmatched colours");
    return false;
  }

  // Store colour assignment of beam particles.
  for (int i = nInit; i < size(); ++i)
    event[resolved[i].iPos()].cols( resolved[i].col(), resolved[i].acol() );

  // Done.
  return true;
}


//--------------------------------------------------------------------------

// Pick unrescaled x values for beam remnant sharing.

double BeamParticle::xRemnant( int i) {

  double x = 0.;

  // Hadrons (only used for DIS) rather primitive for now (probably OK).
  int idAbs = abs(resolved[i].id());
  if (idAbs > 100 && (idAbs/10)%10 != 0) {
    x = 1.;

  // Calculation of x of valence quark or diquark, for latter as sum.
  } else if (resolved[i].isValence()) {

    // Resolve diquark into sum of two quarks.
    int id1 = resolved[i].id();
    int id2 = 0;
    if (abs(id1) > 10) {
      id2 = (id1 > 0) ? (id1/100)%10 : -(((-id1)/100)%10);
      id1 = (id1 > 0) ? id1/1000 : -((-id1)/1000);
    }

    // Loop over (up to) two quarks; add their contributions.
    for (int iId = 0; iId < 2; ++iId) {
      int idNow = (iId == 0) ? id1 : id2;
      if (idNow == 0) break;
      double xPart = 0.;

      // Assume form (1-x)^a / sqrt(x).
      double xPow = valencePowerMeson;
      if (isBaryonBeam) {
        if (nValKinds == 3 || nValKinds == 1)
          xPow = (3. * rndmPtr->flat() < 2.)
            ? valencePowerUinP : valencePowerDinP ;
        else if (nValence(idNow) == 2) xPow = valencePowerUinP;
        else xPow = valencePowerDinP;
      }
      do xPart = pow2( rndmPtr->flat() );
      while ( pow(1. - xPart, xPow) < rndmPtr->flat() );

      // End loop over (up to) two quarks. Possibly enhancement for diquarks.
      x += xPart;
    }
   if (id2 != 0) x *= valenceDiqEnhance;

  // Calculation of x of sea quark, based on companion association.
  } else if (resolved[i].isCompanion()) {

    // Find rescaled x value of companion.
    double xLeft = 1.;
    for (int iInit = 0; iInit < nInit; ++iInit)
      if (resolved[iInit].isFromBeam()) xLeft -= resolved[iInit].x();
    double xCompanion = resolved[ resolved[i].companion() ].x();
    xCompanion /= (xLeft + xCompanion);

    // Now use ansatz q(x; x_c) < N/(x +x_c) to pick x.
    do x = pow( xCompanion, rndmPtr->flat()) - xCompanion;
    while ( pow( (1. - x - xCompanion) / (1. - xCompanion), companionPower)
      * (pow2(x) + pow2(xCompanion)) / pow2(x + xCompanion)
      < rndmPtr->flat() );

  // Else a gluon remnant.
  // Rarely it is a single gluon remnant, for that case value does not matter.
  } else {
    do x = pow(xGluonCutoff, 1 - rndmPtr->flat());
    while ( pow(1. - x, gluonPower) < rndmPtr->flat() );
  }

  return x;

}

//--------------------------------------------------------------------------

// Approximate the remnant mass according to the initiator.

double BeamParticle::remnantMass(int idIn) {

  int idLight = 2;

  // Hadrons: remove valence flavour masses from the hadron mass, add others.
  if ( isHadron() ) {
    double mRem = particleDataPtr->m0( id());
    int valSign1 = (nValence(idIn) > 0) ? -1 : 1;
    mRem += valSign1 * particleDataPtr->m0(idIn);
    return mRem;

  // Photons: For gluons, add two light partons to act as valence, otherwise
  // add mass of companion. No remnants for unresolved photons.
  } else if ( isGamma() ) {
    if ( isUnresolved() ) return 0.;
    double mRem = (idIn == 21) ? 2*( particleDataPtr->m0( idLight ) ) :
      particleDataPtr->m0( idIn );
    return mRem;

  } else return 0.;

}

//--------------------------------------------------------------------------

// Check whether room for a one remnant system.

bool BeamParticle::roomFor1Remnant(double eCM) {

  // If no remnants for the beam return true.
  if (!isResolvedGamma) return true;

  // Else check whether room with given kinematics.
  double x1 = resolved[0].x();
  int id1   = resolved[0].id();
  return roomFor1Remnant(id1, x1, eCM);
}

//--------------------------------------------------------------------------

// Check whether room for a one remnant system.

bool BeamParticle::roomFor1Remnant(int id1, double x1, double eCM) {

  // Use u-quark mass as a lower limit for the remnant mass.
  int idLight = 2;

  // For gluons minimum requirement two light quarks.
  // For quarks need room for one quark of same flavor.
  double mRemnant = (id1 == 21) ? 2*( particleDataPtr->m0( idLight ) ) :
    particleDataPtr->m0( id1 );
  return ( mRemnant < eCM*( 1 - sqrt(x1) ) );
}

//--------------------------------------------------------------------------

// Check whether room for two remnants in the event.

bool BeamParticle::roomFor2Remnants(int id1, double x1, double eCM) {

  // Use u-quark mass as a lower limit for the remnant mass.
  int idLight = 2;
  double id2  = resolved[0].id();
  double x2   = resolved[0].x();

  // For gluons minimum requirement two light quarks.
  // For quarks need room for one quark of same flavor.
  double m1 = (id1 == 21) ? 2*( particleDataPtr->m0( idLight ) ) :
    particleDataPtr->m0( id1 );
  double m2 = (id2 == 21) ? 2*( particleDataPtr->m0( idLight ) ) :
    particleDataPtr->m0( id2 );
  return ( (m1 + m2) < eCM*sqrt( (1.0 - x1)*(1.0 - x2) ) );
}


//--------------------------------------------------------------------------

// Check whether room for two remnants in the event. This used by MPI.

bool BeamParticle::roomForRemnants(BeamParticle beamOther) {

  // Calculate the invariant mass remaining after MPIs.
  double xLeftA   = this->xMax(-1);
  double xLeftB   = beamOther.xMax(-1);
  double eCM      = infoPtr->eCM();
  double Wleft    = eCM*sqrt(xLeftA*xLeftB);
  double mRemA    = 0;
  double mRemB    = 0;
  bool allGluonsA = true;
  bool allGluonsB = true;

  // Calculate the total mass of each beam remnant.
  for (int i = 0; i < this->size(); ++i) {
    if ( resolved[i].id() != 21 ) {
      allGluonsA = false;
      // If initiator a valence, no need for a companion remnant.
      if ( resolved[i].companion() < 0  && resolved[i].companion() != -3 )
        mRemA += particleDataPtr->m0( resolved[i].id() );
    }
  }
  for (int i = 0; i < beamOther.size(); ++i) {
    if ( beamOther[i].id() != 21 )
      allGluonsB = false;
    if ( beamOther[i].companion() < 0 && beamOther[i].companion() != -3 )
      mRemB += particleDataPtr->m0( beamOther[i].id() );
  }

  // If all initiators are gluons leave room for two light quarks.
  // In case of hadrons the mass is taken into account already in xMax().
  if ( allGluonsA) mRemA = this->isGamma()     ? 2*particleDataPtr->m0(2) : 0.;
  if ( allGluonsB) mRemB = beamOther.isGamma() ? 2*particleDataPtr->m0(2) : 0.;

  // If not enough invariant mass left for remnants reject scattering.
  if ( Wleft < mRemA + mRemB ) return false;
  else return true;
}

//--------------------------------------------------------------------------

// Print the list of resolved partons in a beam.

void BeamParticle::list() const {

  // Header.
  cout << "\n --------  PYTHIA Partons resolved in beam  -----------------"
       << "-------------------------------------------------------------\n"
       << "\n    i  iPos      id       x    comp   xqcomp    pTfact      "
       << "colours      p_x        p_y        p_z         e          m \n";

  // Loop over list of removed partons and print it.
  double xSum  = 0.;
  Vec4   pSum;
  for (int i = 0; i < size(); ++i) {
    ResolvedParton res = resolved[i];
    cout << fixed << setprecision(6) << setw(5) << i << setw(6) << res.iPos()
         << setw(8) << res.id() << setw(10) << res.x() << setw(6)
         << res.companion() << setw(10) << res.xqCompanion() << setw(10)
         << res.pTfactor() << setprecision(3) << setw(6) << res.col()
         << setw(6) << res.acol() << setw(11) << res.px() << setw(11)
         << res.py() << setw(11) << res.pz() << setw(11) << res.e()
         << setw(11) << res.m() << "\n";

    // Also find sum of x and p values.
    if (res.companion() != -10) {
      xSum  += res.x();
      pSum  += res.p();
    }
  }

  // Print sum and endline.
  cout << setprecision(6) << "             x sum:" << setw(10) << xSum
       << setprecision(3) << "                                p sum:"
       << setw(11) << pSum.px() << setw(11) << pSum.py() << setw(11)
       << pSum.pz() << setw(11) << pSum.e()
       << "\n\n --------  End PYTHIA Partons resolved in beam  -----------"
       << "---------------------------------------------------------------"
       << endl;
}

//--------------------------------------------------------------------------

// Test whether a lepton is to be considered as unresolved.

bool BeamParticle::isUnresolvedLepton() {

  // Require record to consist of lepton with full energy plus a photon.
  if (!isLeptonBeam || resolved.size() > 2 || resolved[1].id() != 22
    || resolved[0].x() < XMINUNRESOLVED) return false;
  return true;

}

//--------------------------------------------------------------------------

// For a diffractive system, decide whether to kick out gluon or quark.

bool BeamParticle::pickGluon(double mDiff) {

  // Relative weight to pick a quark, assumed falling with energy.
  double probPickQuark = pickQuarkNorm / pow( mDiff, pickQuarkPower);
  return  ( (1. + probPickQuark) * rndmPtr->flat() < 1. );

}

//--------------------------------------------------------------------------

// Pick a valence quark at random. (Used for diffractive systems.)

int BeamParticle::pickValence() {

  // Pick one valence quark at random.
  int nTotVal = (isBaryonBeam) ? 3 : 2;
  double rnVal = rndmPtr->flat() * nTotVal;
  int iVal = (rnVal < 1.) ? 1 : ( (rnVal < 2.) ? 2 : 3 );

  // This valence in slot 1, the rest thereafter.
  idVal1 = 0;
  idVal2 = 0;
  idVal3 = 0;
  int iNow = 0;
  for (int i = 0; i < nValKinds; ++i)
  for (int j = 0; j < nVal[i]; ++j) {
    ++iNow;
    if (iNow == iVal) idVal1 = idVal[i];
    else if ( idVal2 == 0) idVal2 = idVal[i];
    else idVal3 = idVal[i];
  }

  // Construct diquark if baryon.
  if (idVal3 != 0) idVal2 = flavSelPtr->makeDiquark( idVal2, idVal3, idBeam);

  // Done.
  return idVal1;

}

//--------------------------------------------------------------------------

// Share lightcone momentum between two remnants in a diffractive system.

double BeamParticle::zShare( double mDiff, double m1, double m2) {

  // Set up as valence in normal beam so can use xRemnant code.
  append(0, idVal1, 0., -3);
  append(0, idVal2, 0., -3);
  double m2Diff = mDiff*mDiff;

  // Begin to generate z and pT until acceptable solution.
  double wtAcc = 0.;
  do {
    double x1 = xRemnant(0);
    double x2 = xRemnant(0);
    zRel = x1 / (x1 + x2);
    pair<double, double> gauss2 = rndmPtr->gauss2();
    pxRel = diffPrimKTwidth * gauss2.first;
    pyRel = diffPrimKTwidth * gauss2.second;

    // Suppress large invariant masses of remnant system.
    double mTS1 = m1*m1 + pxRel*pxRel + pyRel*pyRel;
    double mTS2 = m2*m2 + pxRel*pxRel + pyRel*pyRel;
    double m2Sys = mTS1 / zRel + mTS2 / (1. - zRel);
    wtAcc = (m2Sys < m2Diff)
      ? pow( 1. - m2Sys / m2Diff, diffLargeMassSuppress) : 0.;
  } while (wtAcc < rndmPtr->flat());

  // Done.
  return zRel;

}

// -------------------------------------------------------------------------

// Find the colour configuration of the scattered partons
// and update the colour tags to match this configuration.

void BeamParticle::findColSetup(Event& event) {

  // Reset current colour setup;
  colSetup = make_pair(0,0);
  cols.clear();
  acols.clear();
  colUpdates.clear();
  nJuncs = 0;
  nAjuncs = 0;

  // Setup colour states.
  vector<vector <vector <ColState> > > colStates;
  colStates.resize(size() + 1);
  for (int i = 0; i < size() + 1; ++i) {
    colStates[i].resize(2*(i + 1));
    for (int j = 0; j < 2*(i+1); ++j)
      colStates[i][j].resize(2*(i+1));
  }
  colStates[0][0][0].nTotal = 1.;

  bool noColouredParticles = true;
  // Find all possible multiplets and their degeneracies.
  for (int i = 0; i < size(); ++i) {
    for (int j = 0; j < int(colStates[i].size()); ++j) {
      for (int k = 0; k < int(colStates[i][j].size()); ++k) {
        if (colStates[i][j][k].nTotal < 0.5) continue;
        int idParton = resolved[i].id();

        // If particle is a quark.
        if (idParton > 0 && idParton < 9) {
          colStates[i+1][j+1][k].lastSteps.push_back(make_pair(j,k));
          colStates[i+1][j+1][k].nTotal += colStates[i][j][k].nTotal;
          if (k > 0) {
            colStates[i+1][j][k -1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j][k -1].nTotal += colStates[i][j][k].nTotal;
          }

          // Junction combination.
          if (j > 0 && allowBeamJunctions) {
            colStates[i+1][j - 1][k + 1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j - 1][k + 1].nTotal += colStates[i][j][k].nTotal;
          }
        }

        // If particle is an anti quark.
        if (idParton < 0 && idParton > -9) {
          colStates[i+1][j][k + 1].lastSteps.push_back(make_pair(j,k));
          colStates[i+1][j][k + 1].nTotal += colStates[i][j][k].nTotal;
          if (j > 0) {
            colStates[i+1][j - 1][k].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j - 1][k].nTotal += colStates[i][j][k].nTotal;
          }

          // Junction combination.
          if (k > 0 && allowBeamJunctions) {
            colStates[i+1][j + 1][k - 1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j + 1][k - 1].nTotal += colStates[i][j][k].nTotal;
          }
        }

        // If particle is a gluon.
        if (idParton == 21) {
          colStates[i+1][j + 1][k + 1].lastSteps.push_back(make_pair(j,k));
          colStates[i+1][j + 1][k + 1].nTotal += colStates[i][j][k].nTotal;
          if (j > 0) {
            colStates[i+1][j][k].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j][k].nTotal += colStates[i][j][k].nTotal;
          }
          if (k > 0) {
            colStates[i+1][j][k].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j][k].nTotal += colStates[i][j][k].nTotal;
          }
          if (j > 0 && k > 0) {
            colStates[i+1][j - 1][k - 1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j - 1][k - 1].nTotal += colStates[i][j][k].nTotal;
          }

          // Junction combinations.
          if (k > 0 && allowBeamJunctions) {
            colStates[i+1][j + 2][k - 1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j + 2][k - 1].nTotal += colStates[i][j][k].nTotal;
          }
          if (j > 0 && allowBeamJunctions) {
            colStates[i+1][j - 1][k + 2].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j - 1][k + 2].nTotal += colStates[i][j][k].nTotal;
          }
          if (j > 1 && allowBeamJunctions) {
            colStates[i+1][j - 2][k + 1].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j - 2][k + 1].nTotal += colStates[i][j][k].nTotal;
          }
          if (k > 1 && allowBeamJunctions) {
            colStates[i+1][j + 1][k - 2].lastSteps.push_back(make_pair(j,k));
            colStates[i+1][j + 1][k - 2].nTotal += colStates[i][j][k].nTotal;
          }
        }
        // If the parton is not a quark or a gluon.
        if (idParton != 21 && abs(idParton) > 9) {
          colStates[i+1][j][k].lastSteps.push_back(make_pair(j,k));
          colStates[i+1][j][k].nTotal += colStates[i][j][k].nTotal;
        } else
          noColouredParticles = false;
      }
    }
  }

  // Pick a specific multiplet depending on colour weight and saturation.
  // Start by calculating the sum of all weights.
  double totalSize = 0;
  for (int i = 0;i < int(colStates[size()].size()); ++i) {
    for (int j = 0;j < int(colStates[size()][i].size()); ++j) {
      // Do not allow colour singlet states, since this will overlap
      // with diffractive events described elsewhere in PYTHIA.
      if (i == 0 && j == 0 && !noColouredParticles) continue;

      double multipletSize = (i + 1) * (j + 1) * (i + j + 2) / 2.;
      totalSize += colStates[size()][i][j].nTotal *
        multipletSize * exp(-multipletSize / beamSat);
    }
  }

  // Choose one colour configuration.
  double curSize = 0;
  double chosenSize = rndmPtr->flat() * totalSize;
  for (int i = 0;i < int(colStates[size()].size()); ++i) {
    for (int j = 0;j < int(colStates[size()][i].size()); ++j) {

      // Do not allow singlets.
      if (i == 0 && j == 0 && !noColouredParticles) continue;

      // Reweight according to multiplet size.
      double multipletSize = (i + 1) * (j + 1) * (i + j + 2) / 2.;
      curSize += colStates[size()][i][j].nTotal *
        multipletSize * exp(-multipletSize / beamSat);
      if (curSize > chosenSize) {
        colSetup.first = i;
        colSetup.second = j;
        break;
      }
    }
    if (curSize > chosenSize) break;
  }

  // Find the whole colour flow chain.
  vector<pair<int, int> >  colSetupChain;
  colSetupChain.resize(size() + 1);
  pair<int,int> curColSetup = make_pair(colSetup.first, colSetup.second);
  for (int i = size(); i > 0; --i) {
    colSetupChain[i] = curColSetup;
    int curColSize = colStates[i][curColSetup.first][curColSetup.second].
      lastSteps.size();
    int iCurCol = int(rndmPtr->flat() * curColSize);
    curColSetup = colStates[i][curColSetup.first][curColSetup.second].
      lastSteps[iCurCol];
  }
  colSetupChain[0] = curColSetup;

  // Update scattered partons to reflect new colour configuration and
  // store still unconnected colours and anti-colours.
  for (int i = 0; i < size() ; ++i) {

    // If particle is a quark.
    if (resolved[i].id() > 0 && resolved[i].id() < 9) {
      // Add quark to list of colours.
      if (colSetupChain[i].first + 1 == colSetupChain[i + 1].first)
        cols.push_back(resolved[i].col());

      // Find anti colour that quark connects to and update event record.
      else if (colSetupChain[i].second - 1 == colSetupChain[i + 1].second) {
        int iAcol = int(acols.size() * rndmPtr->flat());
        int acol = acols[iAcol];
        acols.erase(acols.begin() + iAcol);
        updateSingleCol(acol, resolved[i].col());
      }

      // Else must be junction:
      else {
        int iCol = int(cols.size() * rndmPtr->flat());
        int juncCol = event.nextColTag();
        event.appendJunction(1, cols[iCol], resolved[i].col(), juncCol);
        event.saveJunctionSize();
        acols.push_back(juncCol);
        cols.erase(cols.begin() + iCol);
        nJuncs++;
      }
    }

    // If particle is an anti quark.
    if (resolved[i].id() < 0 && resolved[i].id() > -9) {
      // Add anti quark to list of anti colours
      if (colSetupChain[i].second + 1 == colSetupChain[i + 1].second)
        acols.push_back(resolved[i].acol());

      // Find colour that anti quark connects to and update event record.
      else if (colSetupChain[i].first - 1 == colSetupChain[i + 1].first) {
        int iCol = int(cols.size() * rndmPtr->flat());
        int col = cols[iCol];
        cols.erase(cols.begin() + iCol);
        updateSingleCol(col, resolved[i].acol());
      }

      // Else it has to be a junction configuration:
      else {
        int iAcol = int(acols.size() * rndmPtr->flat());
        int juncCol = event.nextColTag();
        event.appendJunction(2, acols[iAcol], resolved[i].acol(), juncCol);
        event.saveJunctionSize();
        cols.push_back(juncCol);
        acols.erase(acols.begin() + iAcol);
        nAjuncs++;
      }
    }

    // If particle is a gluon.
    if (resolved[i].id() == 21) {
      // Add gluon to list of colours.
      if (colSetupChain[i].first + 1 == colSetupChain[i + 1].first &&
          colSetupChain[i].second + 1 == colSetupChain[i + 1].second ) {
        acols.push_back(resolved[i].acol());
        cols.push_back(resolved[i].col());
      }

      // Remove colour and anti colour.
      if (colSetupChain[i].first - 1 == colSetupChain[i + 1].first &&
          colSetupChain[i].second - 1 == colSetupChain[i + 1].second ) {
        // First remove colour.
        int iCol = int(cols.size() * rndmPtr->flat());
        int col = cols[iCol];
        cols.erase(cols.begin() + iCol);
        updateSingleCol(col, resolved[i].acol());
        // Then remove anti colour.
        int iAcol = int(acols.size() * rndmPtr->flat());
        int acol = acols[iAcol];
        acols.erase(acols.begin() + iAcol);
        updateSingleCol(acol, resolved[i].col());
      }

      // If the gluon connects to a single end.
      // If it is possible to both go to a colour or anti colour pick randomly.
      if (colSetupChain[i].first == colSetupChain[i + 1].first &&
          colSetupChain[i].second == colSetupChain[i + 1].second ) {
        bool removeColour = true;
        if (cols.size() > 0 && acols.size() > 0)
          removeColour = (rndmPtr->flat() > 0.5);
        else if (acols.size() > 0)
          removeColour = false;
        // Remove colour and add new colour.
        if (removeColour) {
          int iCol = int(cols.size() * rndmPtr->flat());
          int col = cols[iCol];
          cols.erase(cols.begin() + iCol);
          cols.push_back(resolved[i].col());
          updateSingleCol(col, resolved[i].acol());
        }
        // Else remove anti colour and add new anti colour.
        else {
          int iAcol = int(acols.size() * rndmPtr->flat());
          int acol = acols[iAcol];
          acols.erase(acols.begin() + iAcol);
          acols.push_back(resolved[i].acol());
          updateSingleCol(acol, resolved[i].col());
        }
      }

      // Junction configuratons.

      // Gluon anti-junction case.
      if (colSetupChain[i].first + 2 == colSetupChain[i + 1].first &&
          colSetupChain[i].second - 1 == colSetupChain[i + 1].second ) {
        int iAcol = int(acols.size() * rndmPtr->flat());
        int acol = acols[iAcol];
        acols.erase(acols.begin() + iAcol);
        int acol3 = event.nextColTag();
        event.appendJunction(2,acol,resolved[i].acol(),acol3);
        cols.push_back(resolved[i].col());
        cols.push_back(acol3);
        nAjuncs++;
      }

      // Gluon junction case.
      if (colSetupChain[i].first - 1 == colSetupChain[i + 1].first &&
          colSetupChain[i].second + 2 == colSetupChain[i + 1].second ) {
        int iCol = int(cols.size() * rndmPtr->flat());
        int col = cols[iCol];
        cols.erase(cols.begin() + iCol);
        int col3 = event.nextColTag();
        event.appendJunction(1,col,resolved[i].col(),col3);
        acols.push_back(resolved[i].acol());
        acols.push_back(col3);
        nJuncs++;
      }

      // Gluon anti-junction case.
      if (colSetupChain[i].first + 1 == colSetupChain[i + 1].first &&
          colSetupChain[i].second - 2 == colSetupChain[i + 1].second ) {
        int iAcol = int(acols.size() * rndmPtr->flat());
        int acol = acols[iAcol];
        acols.erase(acols.begin() + iAcol);
        int iAcol2 = int(acols.size() * rndmPtr->flat());
        int acol2 = acols[iAcol2];
        acols.erase(acols.begin() + iAcol2);
        event.appendJunction(2,acol,resolved[i].acol(),acol2);
        cols.push_back(resolved[i].col());
        nAjuncs++;
      }

      // Gluon junction case.
      if (colSetupChain[i].first - 2 == colSetupChain[i + 1].first &&
          colSetupChain[i].second + 1 == colSetupChain[i + 1].second ) {
        int iCol = int(cols.size() * rndmPtr->flat());
        int col = cols[iCol];
        cols.erase(cols.begin() + iCol);
        int iCol2 = int(cols.size() * rndmPtr->flat());
        int col2 = cols[iCol2];
        cols.erase(cols.begin() + iCol2);
        event.appendJunction(1,col,resolved[i].col(),col2);
        acols.push_back(resolved[i].acol());
        nJuncs++;
      }
    }
  }

  // Done updating event.
}

// -------------------------------------------------------------------------

// Add required extra remnant flavour content. Also initial colours.

bool BeamParticle::remnantFlavoursNew(Event& event) {

  // A baryon will have a junction, unless a diquark is formed later.
  hasJunctionBeam = (isBaryon());

  // Store how many hard-scattering partons were removed from beam.
  nInit = size();

  // Find remaining valence quarks.
  for (int i = 0; i < nValKinds; ++i) {
    nValLeft[i] = nVal[i];
    for (int j = 0; j < nInit; ++j) if (resolved[j].isValence()
      && resolved[j].id() == idVal[i]) --nValLeft[i];
    // Add remaining valence quarks to record. Partly temporary values.
    for (int k = 0; k < nValLeft[i]; ++k) append(0, idVal[i], 0., -3);
  }
  int nInitPlusVal = size();

  // Find companion quarks to unmatched sea quarks.
  for (int i = 0; i < nInit; ++i)
  if (resolved[i].isUnmatched()) {

    // Add companion quark to record; and bookkeep both ways.
    append(0, -resolved[i].id(), 0., i);
    resolved[i].companion(size() - 1);
  }
  int beamJunc = 0;
  if (isBaryon()) beamJunc = 1;
  if (id() < 0)   beamJunc = -beamJunc;

  // Count the number of gluons that needs to be added.
  int nGluons =  (colSetup.first + colSetup.second - (size() - nInit)
                  +abs( (nJuncs - nAjuncs) - beamJunc)) / 2;
  for (int i = 0; i < nGluons; i++) append(0,21,0.,-1);

  // If no other remnants found, add a light q-qbar pair or a photon
  // to carry momentum.
  if (size() == nInit) {
    if (!isHadron())
      append(0, 22, 0., -1);
    else {
      int idRemnant = int(3*rndmPtr->flat())+1;
      append(0, -idRemnant, 0., -1);
      append(0,  idRemnant, 0., -1);
      resolved[size()-2].companion(size() - 1);
      resolved[size()-1].companion(size() - 2);
    }
  }

  usedCol =  vector<bool>(size(),false);
  usedAcol = vector<bool>(size(),false);

  // If at least two valence quarks left in baryon and no junction formed.
  // First check if junction already was moved into beam.
  nDiffJuncs = nJuncs - nAjuncs - beamJunc;
  if (isBaryon() && nInitPlusVal - nInit >= 2 && (
      (nDiffJuncs > 0 && beamJunc < 0) ||
      (nDiffJuncs < 0 && beamJunc > 0)) ) {

    // If three, pick two at random to form junction, else trivial.
    int iQ1 = nInit;
    int iQ2 = nInit + 1;
    if (nInitPlusVal - nInit == 3) {
      double pickDq = 3. * rndmPtr->flat();
      if (pickDq > 1.) iQ2 = nInit + 2;
      if (pickDq > 2.) iQ1 = nInit + 1;
    }

    // Either form di-quark or (anti-)junction.
    if (beamJunction) {
      // Form antijunction.
      if (resolved[iQ1].id() < 0) {

        // Start by finding last colour in the out going particles.
        usedAcol[iQ1] = true;
        usedAcol[iQ2] = true;

        // Find matching anti-colour.
        int acol = findSingleCol(event, true, true);
        if ( acol == 0) return false;

        // Make the antijunction.
        int newCol1 = event.nextColTag();
        int newCol2 = event.nextColTag();
        resolved[iQ1].acol(newCol1);
        resolved[iQ2].acol(newCol2);
        event.appendJunction(2, resolved[iQ1].acol(), resolved[iQ2].acol(),
          acol);
        nDiffJuncs--;
      }

      // Form Junction.
      else {
        // Start by finding last colour in the out going particles.
        usedCol[iQ1] = true;
        usedCol[iQ2] = true;

        // Find matching colour.
        int col = findSingleCol(event, false, true);
        if (col == 0) return false;

        // Make the junction.
        int newCol1 = event.nextColTag();
        int newCol2 = event.nextColTag();
        resolved[iQ1].col(newCol1);
        resolved[iQ2].col(newCol2);
        event.appendJunction(1,resolved[iQ1].col(),resolved[iQ2].col(),col);
        nDiffJuncs++;
      }

    // Form diquark.
    } else {

    // Pick spin 0 or 1 according to SU(6) wave function factors.
      int idDq = flavSelPtr->makeDiquark( resolved[iQ1].id(),
        resolved[iQ2].id(), idBeam);

      // Overwrite with diquark flavour and remove one slot. No more junction.
      if (nInitPlusVal - nInit == 3)
        resolved[nInit + 2].id( resolved[3 * nInit + 3 - iQ2 - iQ1].id() );
      resolved[nInit].id(idDq);
      resolved.erase(resolved.begin() + nInit + 1);
      hasJunctionBeam = false;

      // Di-quark changes the baryon number.
      if (idDq > 0) nDiffJuncs++;
      else          nDiffJuncs--;
    }
  }

  // Form anti-junction out of any beam remnants if needed.
  while (nDiffJuncs > 0) {
    int acol1 = findSingleCol(event, true, false);
    int acol2 = findSingleCol(event, true, false);
    int acol3 = findSingleCol(event, true, true);
    event.appendJunction(2,acol1,acol2,acol3);
    nDiffJuncs--;
  }
  // Form junction out of any beam remnants if needed.
  while (nDiffJuncs < 0) {
    int col1 = findSingleCol(event, false, false);
    int col2 = findSingleCol(event, false, false);
    int col3 = findSingleCol(event, false, true);
    event.appendJunction(1,col1,col2,col3);
    nDiffJuncs++;
  }

  // Set remaining colours first in random order.
  for (int j = 0;j < NRANDOMTRIES; ++j) {
    int i = int(rndmPtr->flat() * (size() - nInit) + nInit );
    // Check if resolved has colour.
    if ( resolved[i].hasCol() && !usedCol[i]) {
      usedCol[i] = true;
      int acol = findSingleCol(event,true,true);
      if ( acol == 0) return false;
      resolved[i].col(acol);
    }
    // Check if resolved has anti colour.
    if ( resolved[i].hasAcol() && !usedAcol[i]) {
      usedAcol[i] = true;
      int col = findSingleCol(event, false, true);
      if (col == 0) return false;
      resolved[i].acol(col);
    }
  }

  // Add all missed colours from the random assignment.
  for (int i = nInit;i < size();i++) {
    // Check if resolved has colour.
    if ( resolved[i].hasCol() && !usedCol[i]) {
      usedCol[i] = true;
      int acol = findSingleCol(event,true,true);
      if ( acol == 0) return false;
      resolved[i].col(acol);
    }
    // Check if resolved has anti colour.
    if ( resolved[i].hasAcol() && !usedAcol[i]) {
      usedAcol[i] = true;
      int col = findSingleCol(event, false, true);
      if (col == 0) return false;
      resolved[i].acol(col);
    }
  }

  // Need to end in a colour singlet.
  if (cols.size() != 0 || acols.size() != 0) {
    infoPtr->errorMsg("Error in BeamParticle::RemnantFlavours: "
      "Colour not conserved in beamRemnants");
    return false;
  }

  // Set initiator and remnant masses.
  for (int i = 0; i < size(); ++i) {
    if (i < nInit) resolved[i].m(0.);
    else resolved[i].m( particleDataPtr->m0( resolved[i].id() ) );
  }

  // For debug purposes: reject beams with resolved junction topology.
  if (hasJunctionBeam && !allowJunction) return false;

  // Done.
  return true;

}

//--------------------------------------------------------------------------

// Set initial colours.

void BeamParticle::setInitialCol(Event& event) {

  // Set beam colours equal to those in the event record.
  for (int i = 0;i < size(); ++i) {
    if (event[resolved[i].iPos()].col() != 0)
      resolved[i].col(event[resolved[i].iPos()].col());
    if (event[resolved[i].iPos()].acol() != 0)
      resolved[i].acol(event[resolved[i].iPos()].acol());
  }
}

//--------------------------------------------------------------------------

// Find a single (anti-) colour in the beam, if it is a
// beam remnant set the new colour.

int BeamParticle::findSingleCol(Event& event, bool isAcol,
  bool useHardScatters) {

  // Look in the already scattered parton list.
  if (useHardScatters) {
    if (isAcol) {
      if (acols.size() > 0) {
        int iAcol = int(acols.size() * rndmPtr->flat());
        int acol = acols[iAcol];
        acols.erase(acols.begin() + iAcol);
        return acol;
      }
    } else {
      if (int(cols.size()) > 0) {
        int iCol = int(cols.size() * rndmPtr->flat());
        int col = cols[iCol];
        cols.erase(cols.begin() + iCol);
        return col;
      }
    }
  }

  // Look inside the beam remnants.
  if (isAcol) {
    for (int i = 0;i < NMAX; ++i) {
      int iBeam = int((size() - nInit) * rndmPtr->flat()) + nInit;
      if (resolved[iBeam].hasAcol() && !usedAcol[iBeam]) {
        int acol = event.nextColTag();
        resolved[iBeam].acol(acol);
        usedAcol[iBeam] = true;
        return acol;
      }
    }
  } else {
    for (int i = 0; i < NMAX; ++i) {
      int iBeam = int((size() - nInit) * rndmPtr->flat()) + nInit;
      if (resolved[iBeam].hasCol() && !usedCol[iBeam]) {
        int col = event.nextColTag();
        resolved[iBeam].col(col);
        usedCol[iBeam] = true;
        return col;
      }
    }
  }

  // Return 0 if no particle was found.
  infoPtr->errorMsg("Error in BeamParticle::findSingleCol: "
                    "could not find matching anti colour");
  return 0;
}

//--------------------------------------------------------------------------

// Update list of all colours in beam.

void BeamParticle::updateCol(vector<pair<int,int> > colourChanges) {

  for (int iCol = 0;iCol < int(colourChanges.size()); ++iCol) {
    int oldCol = colourChanges[iCol].first;
    int newCol = colourChanges[iCol].second;

    // Update acols and cols.
    for (int i = 0;i < int(cols.size()); ++i)
      if (cols[i] == oldCol) cols[i] = newCol;
    for (int i = 0;i < int(acols.size()); ++i)
      if (acols[i] == oldCol) acols[i] = newCol;

    // Update resolved partons colours.
    for (int i = 0;i < int(resolved.size()); ++i) {
      if (resolved[i].acol() == oldCol) resolved[i].acol(newCol);
      if (resolved[i].col() == oldCol) resolved[i].col(newCol);
    }
  }
  return;
}

//--------------------------------------------------------------------------

void BeamParticle::updateSingleCol(int oldCol, int newCol) {

  // Update acols and cols.
  for (int i = 0; i < int(cols.size()); ++i)
    if (cols[i] == oldCol) cols[i] = newCol;

  for ( int i = 0; i < int(acols.size()); ++i)
    if (acols[i] == oldCol) acols[i] = newCol;

  // Update resolved partons colours.
  for (int i = 0; i < int(resolved.size()); ++i) {
    if (resolved[i].acol() == oldCol) resolved[i].acol(newCol);
    if (resolved[i].col() == oldCol) resolved[i].col(newCol);
  }

  colUpdates.push_back(make_pair(oldCol,newCol));
}

//==========================================================================

} // end namespace Pythia8
