CSCSegAlgoRU.cc

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#include "CSCSegAlgoRU.h"
#include "CSCSegFit.h"
#include "Geometry/CSCGeometry/interface/CSCLayer.h"
#include "DataFormats/GeometryVector/interface/GlobalPoint.h"
#include "DataFormats/Math/interface/deltaPhi.h"
#include "FWCore/ParameterSet/interface/ParameterSet.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include <algorithm>
#include <cmath>
#include <iostream>
#include <string>

CSCSegAlgoRU::CSCSegAlgoRU(const edm::ParameterSet& ps)
  : CSCSegmentAlgorithm(ps), myName("CSCSegAlgoRU"), sfit_(nullptr) {
  doCollisions = ps.getParameter<bool>("doCollisions");
  chi2_str_ = ps.getParameter<double>("chi2_str");
  chi2Norm_2D_ = ps.getParameter<double>("chi2Norm_2D_");
  dRMax = ps.getParameter<double>("dRMax");
  dPhiMax = ps.getParameter<double>("dPhiMax");
  dRIntMax = ps.getParameter<double>("dRIntMax");
  dPhiIntMax = ps.getParameter<double>("dPhiIntMax");
  chi2Max = ps.getParameter<double>("chi2Max");
  wideSeg = ps.getParameter<double>("wideSeg");
  minLayersApart = ps.getParameter<int>("minLayersApart");
 
  LogDebug("CSC") << myName << " has algorithm cuts set to: \n"
          << "--------------------------------------------------------------------\n"
          << "dRMax = " << dRMax << '\n'
          << "dPhiMax = " << dPhiMax << '\n'
          << "dRIntMax = " << dRIntMax << '\n'
          << "dPhiIntMax = " << dPhiIntMax << '\n'
          << "chi2Max = " << chi2Max << '\n'
          << "wideSeg = " << wideSeg << '\n'
          << "minLayersApart = " << minLayersApart << std::endl;

  //reset the thresholds for non-collision data
  if(!doCollisions){
    dRMax = 2.0;
    dPhiMax = 2*dPhiMax;
    dRIntMax = 2*dRIntMax;
    dPhiIntMax = 2*dPhiIntMax;
    chi2Norm_2D_ = 5*chi2Norm_2D_;
    chi2_str_ = 100;
    chi2Max = 2*chi2Max;
  }
}

std::vector<CSCSegment> CSCSegAlgoRU::run(const CSCChamber* aChamber, const ChamberHitContainer& rechits){
  theChamber = aChamber;
  return buildSegments(rechits);
}

std::vector<CSCSegment> CSCSegAlgoRU::buildSegments(const ChamberHitContainer& urechits) {
  ChamberHitContainer rechits = urechits;
  LayerIndex layerIndex(rechits.size());
  int recHits_per_layer[6] = {0,0,0,0,0,0};
  //skip events with high multiplicity of hits
  if (rechits.size()>150){
    return std::vector<CSCSegment>();
  }
  int iadd = 0;
  for(unsigned int i = 0; i < rechits.size(); i++) {
    recHits_per_layer[rechits[i]->cscDetId().layer()-1]++;//count rh per chamber
    layerIndex[i] = rechits[i]->cscDetId().layer();
  }
  double z1 = theChamber->layer(1)->position().z();
  double z6 = theChamber->layer(6)->position().z();
  if (std::abs(z1) > std::abs(z6)){
    reverse(layerIndex.begin(), layerIndex.end());
    reverse(rechits.begin(), rechits.end());
  }
  if (rechits.size() < 2) {
    return std::vector<CSCSegment>();
  }
  // We have at least 2 hits. We intend to try all possible pairs of hits to start
  // segment building. 'All possible' means each hit lies on different layers in the chamber.
  // after all same size segs are build we get rid of the overcrossed segments using the chi2 criteria
  // the hits from the segs that are left are marked as used and are not added to segs in future iterations
  // the hits from 3p segs are marked as used separately in order to try to assamble them in longer segments
  // in case there is a second pass
  // Choose first hit (as close to IP as possible) h1 and second hit
  // (as far from IP as possible) h2 To do this we iterate over hits
  // in the chamber by layer - pick two layers. Then we
  // iterate over hits within each of these layers and pick h1 and h2
  // these. If they are 'close enough' we build an empty
  // segment. Then try adding hits to this segment.
  // Initialize flags that a given hit has been allocated to a segment
  BoolContainer used(rechits.size(), false);
  BoolContainer used3p(rechits.size(), false);
  // This is going to point to fits to hits, and its content will be used to create a CSCSegment
  sfit_ = 0;
  // Define buffer for segments we build
  std::vector<CSCSegment> segments;
  ChamberHitContainerCIt ib = rechits.begin();
  ChamberHitContainerCIt ie = rechits.end();
  // Possibly allow 3 passes, second widening scale factor for cuts, third for segments from displaced vertices
  windowScale = 1.; // scale factor for cuts
  bool search_disp = false;
  strip_iadd = 1;
  chi2D_iadd = 1;
  int npass = (wideSeg > 1.)? 3 : 2;
  for (int ipass = 0; ipass < npass; ++ipass) {
    if(windowScale >1.){
      iadd = 1;
      strip_iadd = 2;
      chi2D_iadd = 2;
    }
    int used_rh = 0;
    for (ChamberHitContainerCIt i1 = ib; i1 != ie; ++i1) {
      if(used[i1-ib])used_rh++;
    }

    //change the tresholds if it's time to look for displaced mu segments
    if(doCollisions && search_disp && int(rechits.size()-used_rh)>2){//check if there are enough recHits left to build a segment from displaced vertices
      doCollisions = false;
      windowScale = 1.; // scale factor for cuts
      dRMax = 2.0;
      dPhiMax = 2*dPhiMax;
      dRIntMax = 2*dRIntMax;
      dPhiIntMax = 2*dPhiIntMax;
      chi2Norm_2D_ = 5*chi2Norm_2D_;
      chi2_str_ = 100;
      chi2Max = 2*chi2Max;
    }else{
      search_disp = false;//make sure the flag is off
    }

    for(unsigned int n_seg_min = 6u; n_seg_min > 2u + iadd; --n_seg_min){
      BoolContainer common_used(rechits.size(),false);
      std::array<BoolContainer, 120> common_used_it = {};
      for (unsigned int i = 0; i < common_used_it.size(); i++) {
    common_used_it[i] = common_used;
      }
      ChamberHitContainer best_proto_segment[120];
      float min_chi[120] = {9999};
      int common_it = 0;
      bool first_proto_segment = true;
      for (ChamberHitContainerCIt i1 = ib; i1 != ie; ++i1) {
    bool segok = false;
    //skip if rh is used and the layer tat has big rh multiplicity(>25RHs)
    if(used[i1-ib] || recHits_per_layer[int(layerIndex[i1-ib])-1]>25 || (n_seg_min == 3 && used3p[i1-ib])) continue;
    int layer1 = layerIndex[i1-ib];
    const CSCRecHit2D* h1 = *i1;
    for (ChamberHitContainerCIt i2 = ie-1; i2 != i1; --i2) {
      if(used[i2-ib] || recHits_per_layer[int(layerIndex[i2-ib])-1]>25 || (n_seg_min == 3 && used3p[i2-ib])) continue;
      int layer2 = layerIndex[i2-ib];
      if((abs(layer2 - layer1) + 1) < int(n_seg_min)) break;//decrease n_seg_min
      const CSCRecHit2D* h2 = *i2;
      if (areHitsCloseInR(h1, h2) && areHitsCloseInGlobalPhi(h1, h2)) {
        proto_segment.clear();
        if (!addHit(h1, layer1))continue;
        if (!addHit(h2, layer2))continue;
        // Can only add hits if already have a segment
        if ( sfit_ )tryAddingHitsToSegment(rechits, used, layerIndex, i1, i2);
        segok = isSegmentGood(rechits);
        if (segok) {
          if(proto_segment.size() > n_seg_min){
        baseline(n_seg_min);
        updateParameters();
          }
          if(sfit_->chi2() > chi2Norm_2D_*chi2D_iadd || proto_segment.size() < n_seg_min) proto_segment.clear();
          if (!proto_segment.empty()) {
        updateParameters();
        //add same-size overcrossed protosegments to the collection
        if(first_proto_segment){
          flagHitsAsUsed(rechits, common_used_it[0]);
          min_chi[0] = sfit_->chi2();
          best_proto_segment[0] = proto_segment;
          first_proto_segment = false;
        }else{ //for the rest of found proto_segments
          common_it++;
          flagHitsAsUsed(rechits, common_used_it[common_it]);
          min_chi[common_it] = sfit_->chi2();
          best_proto_segment[common_it] = proto_segment;
          ChamberHitContainerCIt hi, iu, ik;
          int iter = common_it;
          for(iu = ib; iu != ie; ++iu) {
            for(hi = proto_segment.begin(); hi != proto_segment.end(); ++hi) {
              if(*hi == *iu) {
            int merge_nr = -1;
            for(int k = 0; k < iter+1; k++){
              if(common_used_it[k][iu-ib] == true){
                if(merge_nr != -1){
                  //merge the k and merge_nr collections of flaged hits into the merge_nr collection and unmark the k collection hits
                  for(ik = ib; ik != ie; ++ik) {
                if(common_used_it[k][ik-ib] == true){
                  common_used_it[merge_nr][ik-ib] = true;
                  common_used_it[k][ik-ib] = false;
                }
                  }
                  //change best_protoseg if min_chi_2 is smaller
                  if(min_chi[k] < min_chi[merge_nr]){
                min_chi[merge_nr] = min_chi[k];
                best_proto_segment[merge_nr] = best_proto_segment[k];
                best_proto_segment[k].clear();
                min_chi[k] = 9999;
                  }
                  common_it--;
                }
                else{
                  merge_nr = k;
                }
              }//if(common_used[k][iu-ib] == true)
            }//for k
              }//if
            }//for proto_seg
          }//for rec_hits
        }//else
          }//proto seg not empty
        }
      } // h1 & h2 close
      if (segok)
        break;
    } // i2
      } // i1


      //add the reconstructed segments
      for(int j = 0;j < common_it+1; j++){
    proto_segment = best_proto_segment[j];
    best_proto_segment[j].clear();
    //SKIP empty proto-segments
    if(proto_segment.size() == 0) continue;
    updateParameters();
    // Create an actual CSCSegment - retrieve all info from the fit
    CSCSegment temp(sfit_->hits(), sfit_->intercept(),
            sfit_->localdir(), sfit_->covarianceMatrix(), sfit_->chi2());
    sfit_ = 0;
    segments.push_back(temp);
    //if the segment has 3 hits flag them as used in a particular way
    if(proto_segment.size() == 3){
      flagHitsAsUsed(rechits, used3p);
    }
    else{
      flagHitsAsUsed(rechits, used);
    }
    proto_segment.clear();
      }
    }//for n_seg_min

    if(search_disp){
      //reset params and flags for the next chamber
      search_disp = false;
      doCollisions = true;
      dRMax = 2.0;
      dPhiMax = dPhiMax/2;
      dRIntMax = dRIntMax/2;
      dPhiIntMax = dPhiIntMax/2;
      chi2Norm_2D_ = chi2Norm_2D_/5;
      chi2_str_ = 100;
      chi2Max = chi2Max/2;
    }

    std::vector<CSCSegment>::iterator it =segments.begin();
    bool good_segs = false;
    while(it != segments.end()) {
      if ((*it).nRecHits() > 3){
    good_segs = true;
    break;
      }
      ++it;
    }
    if (good_segs && doCollisions) { // only change window if not enough good segments were found (bool can be changed to int if a >0 number of good segs is required)
      search_disp = true;
      continue;//proceed to search the segs from displaced vertices
    }

    // Increase cut windows by factor of wideSeg only for collisions
    if(!doCollisions && !search_disp) break;
    windowScale = wideSeg;
  } // ipass

  //get rid of enchansed 3p segments
  std::vector<CSCSegment>::iterator it =segments.begin();
  while(it != segments.end()) {
    if((*it).nRecHits() == 3){
      bool found_common = false;
      const std::vector<CSCRecHit2D>& theseRH = (*it).specificRecHits();
      for (ChamberHitContainerCIt i1 = ib; i1 != ie; ++i1) {
    if(used[i1-ib] && used3p[i1-ib]){
      const CSCRecHit2D* sh1 = *i1;
      CSCDetId id = sh1->cscDetId();
      int sh1layer = id.layer();
      int RH_centerid = sh1->nStrips()/2;
      int RH_centerStrip = sh1->channels(RH_centerid);
      int RH_wg = sh1->hitWire();
      std::vector<CSCRecHit2D>::const_iterator sh;
      for(sh = theseRH.begin(); sh != theseRH.end(); ++sh){
        CSCDetId idRH = sh->cscDetId();
        //find segment hit coord
        int shlayer = idRH.layer();
        int SegRH_centerid = sh->nStrips()/2;
        int SegRH_centerStrip = sh->channels(SegRH_centerid);
        int SegRH_wg = sh->hitWire();
        if(sh1layer == shlayer && SegRH_centerStrip == RH_centerStrip && SegRH_wg == RH_wg){
          //remove the enchansed 3p segment
          segments.erase(it,(it+1));
          found_common = true;
          break;
        }
      }//theserh
    }
    if(found_common) break;//current seg has already been erased
      }//camber hits
      if(!found_common)++it;
    }//its a 3p seg
    else{
      ++it;//go to the next seg
    }
  }//while
  // Give the segments to the CSCChamber
  return segments;
}//build segments

void CSCSegAlgoRU::tryAddingHitsToSegment(const ChamberHitContainer& rechits,
                      const BoolContainer& used, const LayerIndex& layerIndex,
                      const ChamberHitContainerCIt i1, const ChamberHitContainerCIt i2) {
  // Iterate over the layers with hits in the chamber
  // Skip the layers containing the segment endpoints
  // Test each hit on the other layers to see if it is near the segment
  // If it is, see whether there is already a hit on the segment from the same layer
  // - if so, and there are more than 2 hits on the segment, copy the segment,
  // replace the old hit with the new hit. If the new segment chi2 is better
  // then replace the original segment with the new one (by swap)
  // - if not, copy the segment, add the hit. If the new chi2/dof is still satisfactory
  // then replace the original segment with the new one (by swap)
  ChamberHitContainerCIt ib = rechits.begin();
  ChamberHitContainerCIt ie = rechits.end();
  for (ChamberHitContainerCIt i = ib; i != ie; ++i) {
    int layer = layerIndex[i-ib];
    if (hasHitOnLayer(layer) && proto_segment.size() <= 2)continue;  
    if (layerIndex[i-ib] == layerIndex[i1-ib] || layerIndex[i-ib] == layerIndex[i2-ib] || used[i-ib])continue;
    
    const CSCRecHit2D* h = *i;
    if (isHitNearSegment(h)) {
      // Don't consider alternate hits on layers holding the two starting points
      if (hasHitOnLayer(layer)) {
    if (proto_segment.size() <= 2)continue;
    compareProtoSegment(h, layer);
      }
      else{
    increaseProtoSegment(h, layer, chi2D_iadd);
      }
    } // h & seg close
  } // i
}

bool CSCSegAlgoRU::areHitsCloseInR(const CSCRecHit2D* h1, const CSCRecHit2D* h2) const {
  float maxWG_width[10] = {0, 0, 4.1, 5.69, 2.49, 5.06, 2.49, 5.06, 1.87, 5.06};
  CSCDetId id = h1->cscDetId();
  int iStn = id.iChamberType()-1;
  //find maxWG_width for ME11 (tilt = 29deg)
  int wg_num = h2->hitWire();
  if(iStn == 0 || iStn == 1){
    if (wg_num == 1){
      maxWG_width[0] = 9.25;
      maxWG_width[1] = 9.25;
    }
    if (wg_num > 1 && wg_num < 48){
      maxWG_width[0] = 3.14;
      maxWG_width[1] = 3.14;
    }
    if (wg_num == 48){
      maxWG_width[0] = 10.75;
      maxWG_width[1] = 10.75;
    }
  }
  const CSCLayer* l1 = theChamber->layer(h1->cscDetId().layer());
  GlobalPoint gp1 = l1->toGlobal(h1->localPosition());
  const CSCLayer* l2 = theChamber->layer(h2->cscDetId().layer());
  GlobalPoint gp2 = l2->toGlobal(h2->localPosition());
  //find z to understand the direction
  float h1z = gp1.z();
  float h2z = gp2.z();
  //switch off the IP check for non collisions case
  if (!doCollisions){
    h1z = 1;
    h2z = 1;
  }
  if (gp2.perp() > ((gp1.perp() - dRMax*maxWG_width[iStn])*h2z)/h1z && gp2.perp() < ((gp1.perp() + dRMax*maxWG_width[iStn])*h2z)/h1z){
    return true;
  }
  else{
    return false;
  }
}

bool CSCSegAlgoRU::areHitsCloseInGlobalPhi(const CSCRecHit2D* h1, const CSCRecHit2D* h2) const {
  float strip_width[10] = {0.003878509, 0.002958185, 0.002327105, 0.00152552, 0.00465421, 0.002327105, 0.00465421, 0.002327105, 0.00465421, 0.002327105};//in rad
  const CSCLayer* l1 = theChamber->layer(h1->cscDetId().layer());
  GlobalPoint gp1 = l1->toGlobal(h1->localPosition());
  const CSCLayer* l2 = theChamber->layer(h2->cscDetId().layer());
  GlobalPoint gp2 = l2->toGlobal(h2->localPosition());
  float err_stpos_h1 = h1->errorWithinStrip();
  float err_stpos_h2 = h2->errorWithinStrip();
  CSCDetId id = h1->cscDetId();
  int iStn = id.iChamberType()-1;
  float dphi_incr = 0;
  if(err_stpos_h1>0.25*strip_width[iStn] || err_stpos_h2>0.25*strip_width[iStn])dphi_incr = 0.5*strip_width[iStn];
  float dphi12 = deltaPhi(gp1.barePhi(),gp2.barePhi());
  return (fabs(dphi12) < (dPhiMax*strip_iadd+dphi_incr))? true:false; // +v
}

bool CSCSegAlgoRU::isHitNearSegment(const CSCRecHit2D* h) const {
  // Is hit near segment?
  // Requires deltaphi and deltaR within ranges specified in parameter set.
  // Note that to make intuitive cuts on delta(phi) one must work in
  // phi range (-pi, +pi] not [0, 2pi)
  float strip_width[10] = {0.003878509, 0.002958185, 0.002327105, 0.00152552, 0.00465421, 0.002327105, 0.00465421, 0.002327105, 0.00465421, 0.002327105};//in rad
  const CSCLayer* l1 = theChamber->layer((*(proto_segment.begin()))->cscDetId().layer());
  GlobalPoint gp1 = l1->toGlobal((*(proto_segment.begin()))->localPosition());
  const CSCLayer* l2 = theChamber->layer((*(proto_segment.begin()+1))->cscDetId().layer());
  GlobalPoint gp2 = l2->toGlobal((*(proto_segment.begin()+1))->localPosition());
  float err_stpos_h1 = (*(proto_segment.begin()))->errorWithinStrip();
  float err_stpos_h2 = (*(proto_segment.begin()+1))->errorWithinStrip();
  const CSCLayer* l = theChamber->layer(h->cscDetId().layer());
  GlobalPoint hp = l->toGlobal(h->localPosition());
  float err_stpos_h = h->errorWithinStrip();
  float hphi = hp.phi(); // in (-pi, +pi]
  if (hphi < 0.)
    hphi += 2.*M_PI; // into range [0, 2pi)
  float sphi = phiAtZ(hp.z()); // in [0, 2*pi)
  float phidif = sphi-hphi;
  if (phidif < 0.)
    phidif += 2.*M_PI; // into range [0, 2pi)
  if (phidif > M_PI)
    phidif -= 2.*M_PI; // into range (-pi, pi]
  SVector6 r_glob;
  CSCDetId id = h->cscDetId();
  int iStn = id.iChamberType()-1;
  float dphi_incr = 0;
  float pos_str = 1;
  //increase dPhi cut if the hit is on the edge of the strip
  float stpos = (*h).positionWithinStrip();
  bool centr_str = false;
  if(iStn != 0 && iStn != 1){
    if (stpos > -0.25 && stpos < 0.25) centr_str = true;
  }
  if(err_stpos_h1<0.25*strip_width[iStn] || err_stpos_h2<0.25*strip_width[iStn] || err_stpos_h < 0.25*strip_width[iStn]){
    dphi_incr = 0.5*strip_width[iStn];
  }else{
    if(centr_str) pos_str = 1.3;
  }
  r_glob((*(proto_segment.begin()))->cscDetId().layer()-1) = gp1.perp();
  r_glob((*(proto_segment.begin()+1))->cscDetId().layer()-1) = gp2.perp();
  float R = hp.perp();
  int layer = h->cscDetId().layer();
  float r_interpolated = fit_r_phi(r_glob,layer);
  float dr = fabs(r_interpolated - R);
  float maxWG_width[10] = {0, 0, 4.1, 5.69, 2.49, 5.06, 2.49, 5.06, 1.87, 5.06};
  //find maxWG_width for ME11 (tilt = 29deg)
  int wg_num = h->hitWire();
  if(iStn == 0 || iStn == 1){
    if (wg_num == 1){
      maxWG_width[0] = 9.25;
      maxWG_width[1] = 9.25;
    }
    if (wg_num > 1 && wg_num < 48){
      maxWG_width[0] = 3.14;
      maxWG_width[1] = 3.14;
    }
    if (wg_num == 48){
      maxWG_width[0] = 10.75;
      maxWG_width[1] = 10.75;
    }
  }
  return (fabs(phidif) < dPhiIntMax*strip_iadd*pos_str+dphi_incr && fabs(dr) < dRIntMax*maxWG_width[iStn])? true:false;
}

float CSCSegAlgoRU::phiAtZ(float z) const {
  if ( !sfit_ ) return 0.;
  // Returns a phi in [ 0, 2*pi )
  const CSCLayer* l1 = theChamber->layer((*(proto_segment.begin()))->cscDetId().layer());
  GlobalPoint gp = l1->toGlobal(sfit_->intercept());
  GlobalVector gv = l1->toGlobal(sfit_->localdir());
  float x = gp.x() + (gv.x()/gv.z())*(z - gp.z());
  float y = gp.y() + (gv.y()/gv.z())*(z - gp.z());
  float phi = atan2(y, x);
  if (phi < 0.f ) phi += 2. * M_PI;
  return phi ;
}

bool CSCSegAlgoRU::isSegmentGood(const ChamberHitContainer& rechitsInChamber) const {
  // If the chamber has 20 hits or fewer, require at least 3 hits on segment
  // If the chamber has >20 hits require at least 4 hits
  //@@ THESE VALUES SHOULD BECOME PARAMETERS?
  bool ok = false;
  unsigned int iadd = ( rechitsInChamber.size() > 20)? 1 : 0;
  if (windowScale > 1.)
    iadd = 1;
  if (proto_segment.size() >= 3+iadd)
    ok = true;
  return ok;
}

void CSCSegAlgoRU::flagHitsAsUsed(const ChamberHitContainer& rechitsInChamber,
                  BoolContainer& used ) const {
  // Flag hits on segment as used
  ChamberHitContainerCIt ib = rechitsInChamber.begin();
  ChamberHitContainerCIt hi, iu;
  for(hi = proto_segment.begin(); hi != proto_segment.end(); ++hi) {
    for(iu = ib; iu != rechitsInChamber.end(); ++iu) {
      if(*hi == *iu)
    used[iu-ib] = true;
    }
  }
}

bool CSCSegAlgoRU::addHit(const CSCRecHit2D* aHit, int layer) {
  // Return true if hit was added successfully
  // (and then parameters are updated).
  // Return false if there is already a hit on the same layer, or insert failed.
  ChamberHitContainer::const_iterator it;
  for(it = proto_segment.begin(); it != proto_segment.end(); it++)
    if (((*it)->cscDetId().layer() == layer) && (aHit != (*it)))
      return false;
  proto_segment.push_back(aHit);
  // make a fit
  updateParameters();
  return true;
}

void CSCSegAlgoRU::updateParameters() {
  // Delete input CSCSegFit, create a new one and make the fit
  // delete sfit_;
  sfit_.reset(new CSCSegFit( theChamber, proto_segment ));
  sfit_->fit();
}

float CSCSegAlgoRU::fit_r_phi(SVector6 points, int layer) const{
  //find R or Phi on the given layer using the given points for the interpolation
  float Sx = 0;
  float Sy = 0;
  float Sxx = 0;
  float Sxy = 0;
  for (int i=1;i<7;i++){
    if (points(i-1)== 0.) continue;
    Sy = Sy + (points(i-1));
    Sx = Sx + i;
    Sxx = Sxx + (i*i);
    Sxy = Sxy + ((i)*points(i-1));
  }
  float delta = 2*Sxx - Sx*Sx;
  float intercept = (Sxx*Sy - Sx*Sxy)/delta;
  float slope = (2*Sxy - Sx*Sy)/delta;
  return (intercept + slope*layer);
}

void CSCSegAlgoRU::baseline(int n_seg_min){
  int nhits = proto_segment.size();
  ChamberHitContainer::const_iterator iRH_worst;
  //initialise vectors for strip position and error within strip
  SVector6 sp;
  SVector6 se;
  unsigned int init_size = proto_segment.size();
  ChamberHitContainer buffer;
  buffer.clear();
  buffer.reserve(init_size);
  while (buffer.size()< init_size){
    ChamberHitContainer::iterator min;
    int min_layer = 10;
    for(ChamberHitContainer::iterator k = proto_segment.begin(); k != proto_segment.end(); k++){
      const CSCRecHit2D* iRHk = *k;
      CSCDetId idRHk = iRHk->cscDetId();
      int kLayer = idRHk.layer();
      if(kLayer < min_layer){
    min_layer = kLayer;
    min = k;
      }
    }
    buffer.push_back(*min);
    proto_segment.erase(min);
  }//while

  proto_segment.clear();
  for (ChamberHitContainer::const_iterator cand = buffer.begin(); cand != buffer.end(); cand++) {
    proto_segment.push_back(*cand);
  }

  for(ChamberHitContainer::const_iterator iRH = proto_segment.begin(); iRH != proto_segment.end(); iRH++){
    const CSCRecHit2D* iRHp = *iRH;
    CSCDetId idRH = iRHp->cscDetId();
    int kRing = idRH.ring();
    int kStation = idRH.station();
    int kLayer = idRH.layer();
    // Find the strip containing this hit
    int centerid = iRHp->nStrips()/2;
    int centerStrip = iRHp->channels(centerid);
    float stpos = (*iRHp).positionWithinStrip();
    se(kLayer-1) = (*iRHp).errorWithinStrip();
    // Take into account half-strip staggering of layers (ME1/1 has no staggering)
    if (kStation == 1 && (kRing == 1 || kRing == 4)) sp(kLayer-1) = stpos + centerStrip;
    else{
      if (kLayer == 1 || kLayer == 3 || kLayer == 5) sp(kLayer-1) = stpos + centerStrip;
      if (kLayer == 2 || kLayer == 4 || kLayer == 6) sp(kLayer-1) = stpos - 0.5 + centerStrip;
    }
  }
  float chi2_str;
  fitX(sp, se, -1, -1, chi2_str);

  //-----------------------------------------------------
  // Optimal point rejection method
  //-----------------------------------------------------
  float minSum = 1000;
  int i1b = 0;
  int i2b = 0;
  int iworst = -1;
  int bad_layer = -1;
  ChamberHitContainer::const_iterator rh_to_be_deleted_1;
  ChamberHitContainer::const_iterator rh_to_be_deleted_2;
  if ( (chi2_str) > chi2_str_*chi2D_iadd){
    for (ChamberHitContainer::const_iterator i1 = proto_segment.begin(); i1 != proto_segment.end();++i1) {
      ++i1b;
      const CSCRecHit2D* i1_1 = *i1;
      CSCDetId idRH1 = i1_1->cscDetId();
      int z1 = idRH1.layer();
      i2b = i1b;
      for (ChamberHitContainer::const_iterator i2 = i1+1; i2 != proto_segment.end(); ++i2) {
    ++i2b;
    const CSCRecHit2D* i2_1 = *i2;
    CSCDetId idRH2 = i2_1->cscDetId();
    int z2 = idRH2.layer();
    int irej = 0;
    for ( ChamberHitContainer::const_iterator ir = proto_segment.begin(); ir != proto_segment.end(); ++ir) {
      ++irej;
      if (ir == i1 || ir == i2) continue;
      float dsum = 0;
      int hit_nr = 0;
      const CSCRecHit2D* ir_1 = *ir;
      CSCDetId idRH = ir_1->cscDetId();
      int worst_layer = idRH.layer();
      for (ChamberHitContainer::const_iterator i = proto_segment.begin(); i != proto_segment.end(); ++i) {
        ++hit_nr;
        const CSCRecHit2D* i_1 = *i;
        if (i == i1 || i == i2 || i == ir) continue;
        float slope = (sp(z2-1)-sp(z1-1))/(z2-z1);
        float intersept = sp(z1-1) - slope*z1;
        CSCDetId idRH = i_1->cscDetId();
        int z = idRH.layer();
        float di = fabs(sp(z-1) - intersept - slope*z);
        dsum = dsum + di;
      }//i
      if (dsum < minSum){
        minSum = dsum;
        bad_layer = worst_layer;
        iworst = irej;
        rh_to_be_deleted_1 = ir;
      }
    }//ir
      }//i2
    }//i1
    fitX(sp, se, bad_layer, -1, chi2_str);
  }//if chi2prob<1.0e-4

  //find worst from n-1 hits
  int iworst2 = -1;
  int bad_layer2 = -1;
  if (iworst > -1 && (nhits-1) > n_seg_min && (chi2_str) > chi2_str_*chi2D_iadd){
    iworst = -1;
    float minSum = 1000;
    int i1b = 0;
    int i2b = 0;
    for (ChamberHitContainer::const_iterator i1 = proto_segment.begin(); i1 != proto_segment.end();++i1) {
      ++i1b;
      const CSCRecHit2D* i1_1 = *i1;
      CSCDetId idRH1 = i1_1->cscDetId();
      int z1 = idRH1.layer();
      i2b = i1b;
      for ( ChamberHitContainer::const_iterator i2 = i1+1; i2 != proto_segment.end(); ++i2) {
    ++i2b;
    const CSCRecHit2D* i2_1 = *i2;
    CSCDetId idRH2 = i2_1->cscDetId();
    int z2 = idRH2.layer();
    int irej = 0;
    for ( ChamberHitContainer::const_iterator ir = proto_segment.begin(); ir != proto_segment.end(); ++ir) {
      ++irej;
      int irej2 = 0;
      if (ir == i1 || ir == i2 ) continue;
      const CSCRecHit2D* ir_1 = *ir;
      CSCDetId idRH = ir_1->cscDetId();
      int worst_layer = idRH.layer();
      for ( ChamberHitContainer::const_iterator ir2 = proto_segment.begin(); ir2 != proto_segment.end(); ++ir2) {
        ++irej2;
        if (ir2 == i1 || ir2 == i2 || ir2 ==ir ) continue;
        float dsum = 0;
        int hit_nr = 0;
        const CSCRecHit2D* ir2_1 = *ir2;
        CSCDetId idRH = ir2_1->cscDetId();
        int worst_layer2 = idRH.layer();
        for ( ChamberHitContainer::const_iterator i = proto_segment.begin(); i != proto_segment.end(); ++i) {
          ++hit_nr;
          const CSCRecHit2D* i_1 = *i;
          if (i == i1 || i == i2 || i == ir|| i == ir2 ) continue;
          float slope = (sp(z2-1)-sp(z1-1))/(z2-z1);
          float intersept = sp(z1-1) - slope*z1;
          CSCDetId idRH = i_1->cscDetId();
          int z = idRH.layer();
          float di = fabs(sp(z-1) - intersept - slope*z);
          dsum = dsum + di;
        }//i
        if (dsum < minSum){
          minSum = dsum;
          iworst2 = irej2;
          iworst = irej;
          bad_layer = worst_layer;
          bad_layer2 = worst_layer2;
          rh_to_be_deleted_1 = ir;
          rh_to_be_deleted_2 = ir2;
        }
      }//ir2
    }//ir
      }//i2
    }//i1
    fitX(sp, se, bad_layer ,bad_layer2, chi2_str);
  }//if prob(n-1)<e-4

  //----------------------------------
  //erase bad_hits
  //----------------------------------
  if( iworst2-1 >= 0 && iworst2 <= int(proto_segment.size()) ) {
    proto_segment.erase( rh_to_be_deleted_2);
  }
  if( iworst-1 >= 0 && iworst <= int(proto_segment.size()) ){
    proto_segment.erase(rh_to_be_deleted_1);
  }
}

float CSCSegAlgoRU::fitX(SVector6 points, SVector6 errors, int ir, int ir2, float &chi2_str){
  float S = 0;
  float Sx = 0;
  float Sy = 0;
  float Sxx = 0;
  float Sxy = 0;
  float sigma2 = 0;
  for (int i=1;i<7;i++){
    if (i == ir || i == ir2 || points(i-1) == 0.) continue;
    sigma2 = errors(i-1)*errors(i-1);
    float i1 = i - 3.5;
    S = S + (1/sigma2);
    Sy = Sy + (points(i-1)/sigma2);
    Sx = Sx + ((i1)/sigma2);
    Sxx = Sxx + (i1*i1)/sigma2;
    Sxy = Sxy + (((i1)*points(i-1))/sigma2);
  }
  float delta = S*Sxx - Sx*Sx;
  float intercept = (Sxx*Sy - Sx*Sxy)/delta;
  float slope = (S*Sxy - Sx*Sy)/delta;
  float chi_str = 0;
  chi2_str = 0;
  // calculate chi2_str
  for (int i=1;i<7;i++){
    if (i == ir || i == ir2 || points(i-1) == 0.) continue;
    chi_str = (points(i-1) - intercept - slope*(i-3.5))/(errors(i-1));
    chi2_str = chi2_str + chi_str*chi_str;
  }
  return (intercept + slope*0);
}

bool CSCSegAlgoRU::hasHitOnLayer(int layer) const {
  // Is there is already a hit on this layer?
  ChamberHitContainerCIt it;
  for(it = proto_segment.begin(); it != proto_segment.end(); it++)
    if ((*it)->cscDetId().layer() == layer)
      return true;
  return false;
}

bool CSCSegAlgoRU::replaceHit(const CSCRecHit2D* h, int layer) {
  // replace a hit from a layer
  ChamberHitContainer::const_iterator it;
  for (it = proto_segment.begin(); it != proto_segment.end();) {
    if ((*it)->cscDetId().layer() == layer)
      it = proto_segment.erase(it);
    else
      ++it;
  }
  return addHit(h, layer);
}

void CSCSegAlgoRU::compareProtoSegment(const CSCRecHit2D* h, int layer) {
  // Copy the input CSCSegFit
  std::unique_ptr<CSCSegFit> oldfit;
  oldfit.reset(new CSCSegFit( theChamber, proto_segment ));
  oldfit->fit();
  ChamberHitContainer oldproto = proto_segment;
  
  // May create a new fit
  bool ok = replaceHit(h, layer);
  if ( (sfit_->chi2() >= oldfit->chi2() ) || !ok ) {
    // keep original fit
    proto_segment = oldproto;
    sfit_ = std::move(oldfit); // reset to the original input fit
  }
}

void CSCSegAlgoRU::increaseProtoSegment(const CSCRecHit2D* h, int layer, int chi2_factor) {
  // Creates a new fit
  std::unique_ptr<CSCSegFit> oldfit;
  ChamberHitContainer oldproto = proto_segment;
  oldfit.reset(new CSCSegFit( theChamber, proto_segment ));
  oldfit->fit();

  bool ok = addHit(h, layer);
  //@@ TEST ON ndof<=0 IS JUST TO ACCEPT nhits=2 CASE??
  if ( !ok || ( (sfit_->ndof() > 0) && (sfit_->chi2()/sfit_->ndof() >= chi2Max)) ) {
    // reset to original fit
    proto_segment = oldproto;
    sfit_ = std::move(oldfit);
  }
}