Use fastAtan2 with protection against radial tracks (#15642)

Author: shahor02

Detectors/Base/include/DetectorsBase/MatLayerCyl.h

@@ -107,7 +107,7 @@ class MatLayerCyl : public o2::gpu::FlatObject
   GPUd() int getZBinID(float z) const
   {
     int idz = int((z - getZMin()) * getDZInv()); // cannot be negative since before isZOutside is applied
-    return idz < getNZBins() ? idz : getNZBins() - 1;
+    return idz < getNZBins() ? (idz > 0 ? idz : 0) : getNZBins() - 1;
   }
 
   // lower boundary of Z slice

Detectors/Base/include/DetectorsBase/Ray.h

@@ -79,7 +79,7 @@ class Ray
 
   GPUd() float getPhi(float t) const
   {
-    float p = o2::gpu::CAMath::ATan2(mP[1] + t * mD[1], mP[0] + t * mD[0]);
+    float p = o2::math_utils::fastATan2(mP[1] + t * mD[1], mP[0] + t * mD[0]); // instead of float p = o2::gpu::CAMath::ATan2(mP[1] + t * mD[1], mP[0] + t * mD[0]);
     o2::math_utils::bringTo02Pi(p);
     return p;
   }

Detectors/Base/src/MatLayerCylSet.cxx

@@ -348,6 +348,12 @@ GPUd() MatBudget MatLayerCylSet::getMatBudget(float x0, float y0, float z0, floa
           if (tEndPhi == Ray::InvalidT) {
             break; // ray parallel to radial line, abandon check for phi bin change
           }
+          const auto tMarginPhi = 1.e-6f + 1.e-5f * (cross1 - cross2);
+          // if (!(tEndPhi >= cross2 - tMarginPhi) | !(tEndPhi <= cross1 + tMarginPhi)) { // use non-short-circuit | to reject eventual NANs
+          if (tEndPhi < cross2 - tMarginPhi || tEndPhi > cross1 + tMarginPhi) {
+            tEndPhi = cross2;
+            checkMorePhi = false;
+          }
         }
         auto zID = lr.getZBinID(ray.getZ(tStartPhi));
         auto zIDLast = lr.getZBinID(ray.getZ(tEndPhi));