From e0045d993703f30c5bb905a02fa598a2230f6942 Mon Sep 17 00:00:00 2001
From: jdkio <jeffrey.kleykamp@gmail.com>
Date: Wed, 19 Aug 2026 15:03:38 -0400
Subject: [PATCH] Prevent track matcher index underflow loops

---
 CHANGELOG.md     |  1 +
 src/TMS_Reco.cpp | 35 ++++++++++++++++++++++++++++-------
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 524b1d62..b23c5195 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ Geometry releases will be tagged as `Descriptive_tag_v_X.Y.Z`.
 ## [MAIN] - Unreleased
 - Changed MergeTracks default to True.
 - Hits trailing the end of a Hough Track can now be added to the track they 'should' correspond to. ON by default, switchable in config.
+- Prevent 3D track matching from looping indefinitely when the last hit in a required view lies outside the detector bounds.
 
 ## [Version 0.2] - 2023-12-13
 - Areal density calculation fixed, affected Muon KE reconstruction
diff --git a/src/TMS_Reco.cpp b/src/TMS_Reco.cpp
index d691167a..5769ef9f 100644
--- a/src/TMS_Reco.cpp
+++ b/src/TMS_Reco.cpp
@@ -1320,21 +1320,32 @@ std::vector<TMS_Track> TMS_TrackFinder::TrackMatching3D() {
 //        if (Xrun) std::cout <<"itX:"<<itX<<" ,Y: " << XTracks[itX].GetNotZ() <<  ", Z: " << XTracks[itX].GetZ() <<", PlaneNumber: " << XTracks[itX].GetPlaneNumber()<< std::endl;
         // Sanity check for hits being in the detector volume (x and z)
         bool hit_outside = false;
+        bool view_exhausted = false;
         if (std::abs(UTracks[itU].GetNotZ()) > 4000.0 || UTracks[itU].GetZ() < 11000 || UTracks[itU].GetZ() > 20000) {
-          --itU;
+          if (itU == 0) view_exhausted = true;
+          else --itU;
           hit_outside = true;
         }
         if (std::abs(VTracks[itV].GetNotZ()) > 4000.0 || VTracks[itV].GetZ() < 11000 || VTracks[itV].GetZ() > 20000) {
-          --itV;
+          if (itV == 0) view_exhausted = true;
+          else --itV;
           hit_outside = true;
         }
         if (Xrun && Xback_match && Xfront_match) {
           if (XTracks[itX].GetNotZ() > 400. || XTracks[itX].GetNotZ() < -4000. || XTracks[itX].GetZ() < 11000 || XTracks[itX].GetZ() > 20000) {
-            --itX;
+            if (itX == 0) view_exhausted = true;
+            else --itX;
             hit_outside = true;
           }
         }
-        if (hit_outside) continue;
+        // There is no valid hit left in at least one required view. Continuing
+        // would decrement its index below zero and read outside the vector.
+        if (view_exhausted) break;
+        if (hit_outside) {
+          sane = (itU > 0 || itV > 0);
+          if (Xrun && Xback_match && Xfront_match) sane = sane || itX > 0;
+          continue;
+        }
 
         // Stereo check
         bool stereo_view = true;
@@ -2137,17 +2148,27 @@ std::vector<TMS_Track> TMS_TrackFinder::TrackMatching3D_XY() {
 //                std::cout <<"itX:"<<itX<<" ,Y: " << XTracks[itX].GetNotZ() <<  ", Z: " << XTracks[itX].GetZ() <<", PlaneNumber: " << XTracks[itX].GetPlaneNumber()<< std::endl;
                 // Sanity check for hits being in the detector volume (x and z)
                 bool hit_outside = false;
+                bool view_exhausted = false;
                 if (XTracks[itX].GetNotZ() > 500.0 || XTracks[itX].GetNotZ() == 0. || XTracks[itX].GetNotZ() < -4000.0
                         || XTracks[itX].GetZ() < 11000 || XTracks[itX].GetZ() > 20000) {
-                    --itX;
+                    if (itX == 0) view_exhausted = true;
+                    else --itX;
                     hit_outside = true;
                 }
                 if (std::abs(YTracks[itY].GetNotZ()) > 4000.0 or YTracks[itY].GetNotZ() == 0.
                         || YTracks[itY].GetZ() < 11000 || YTracks[itY].GetZ() > 20000) {
-                    --itY;
+                    if (itY == 0) view_exhausted = true;
+                    else --itY;
                     hit_outside = true;
                 }
-                if (hit_outside) continue;
+                // There is no valid hit left in at least one required view.
+                // Continuing would decrement its index below zero, skip the
+                // loop-condition update, and repeatedly read outside the vector.
+                if (view_exhausted) break;
+                if (hit_outside) {
+                    sane = (itX > 0 || itY > 0);
+                    continue;
+                }
 
                 // Stereo check
                 bool stereo_view = true;