GPU TPC: Do not interpolate with too few NDF

Author: davidrohr

GPU/Common/GPUCommonMath.h

@@ -74,6 +74,11 @@ class GPUCommonMath
   GPUhdni() constexpr static float Sqrt(float x);
   GPUd() static float InvSqrt(float x);
   template <class T>
+  GPUdi() constexpr static T Square(T x)
+  {
+    return x * x;
+  }
+  template <class T>
   GPUhd() constexpr static T Abs(T x);
   GPUd() constexpr static float ASin(float x);
   GPUd() constexpr static float ACos(float x);

GPU/GPUTracking/Merger/GPUTPCGMPropagator.cxx

@@ -653,11 +653,18 @@ GPUd() int32_t GPUTPCGMPropagator::InterpolateReject(const GPUParam& GPUrestrict
   if (rejectChi2 == rejectInterFill) {
     inter->posY = mP[0];
     inter->posZ = mP[1];
-    inter->errorY = mC[0];
-    inter->errorZ = mC[2];
+    if (mT->NDF() <= 0) {
+      inter->errorY = inter->errorZ = 100.f;
+    } else {
+      inter->errorY = mC[0];
+      inter->errorZ = mC[2];
+    }
   } else if (rejectChi2 == rejectInterReject) {
     float chi2Y, chi2Z;
-    if (mFitInProjections || mT->NDF() <= 0) {
+    if (mT->NDF() <= 0) {
+      chi2Y = CAMath::Square((float)inter->posY - posY) / ((float)inter->errorY + err2Y);
+      chi2Z = CAMath::Square((float)inter->posZ - posZ) / ((float)inter->errorZ + err2Z);
+    } else if (mFitInProjections) {
       const float Iz0 = inter->posY - mP[0];
       const float Iz1 = inter->posZ - mP[1];
       const float Iw0 = 1.f / (mC[0] + (float)inter->errorY);