From 462ed0ed905cc0e10c8e4b485a7c3302e2369a92 Mon Sep 17 00:00:00 2001
From: ferdymercury <ferdymercury@users.noreply.github.com>
Date: Thu, 9 Oct 2025 12:41:51 +0200
Subject: [PATCH] [io] Extend flexibility of array size I/O marker

Fixes https://its.cern.ch/jira/browse/ROOT-5144
---
 io/io/src/TStreamerInfo.cxx | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/io/io/src/TStreamerInfo.cxx b/io/io/src/TStreamerInfo.cxx
index 9cf26bfd20cfb..a8e120f3da6dc 100644
--- a/io/io/src/TStreamerInfo.cxx
+++ b/io/io/src/TStreamerInfo.cxx
@@ -569,6 +569,7 @@ void TStreamerInfo::Build(Bool_t isTransient)
       }
 
       TDataMember* dmCounter = 0;
+      int premultiplier = 1;
       if (dmIsPtr) {
          //
          // look for a pointer data member with a counter
@@ -576,11 +577,19 @@ void TStreamerInfo::Build(Bool_t isTransient)
          //
          //      int n;
          //      double* MyArray; //[n]
-         //
+         //      double* MyMatrix;//[3*n]
+
          const char* lbracket = TVirtualStreamerInfo::GetElementCounterStart(dmTitle);
-         const char* rbracket = ::strchr(dmTitle, ']');
+         const char* rbracket = std::strchr(dmTitle, ']');
          if (lbracket && rbracket) {
             const char* counterName = dm->GetArrayIndex();
+            auto multiple = std::strchr(counterName, '*');
+            if (multiple) {
+               std::string s;
+               s.assign(counterName, multiple - counterName);
+               premultiplier = std::stoi(s);
+               counterName = multiple + 1;
+            }
             TRealData* rdCounter = (TRealData*) fClass->GetListOfRealData()->FindObject(counterName);
             if (!rdCounter || rdCounter->TestBit(TRealData::kTransient)) {
                if (!isTransient)
@@ -622,7 +631,12 @@ void TStreamerInfo::Build(Bool_t isTransient)
          } else if (dmIsPtr && (dtype != kCharStar)) {
             if (dmCounter) {
                // data member is pointer to an array of basic types
-               element = new TStreamerBasicPointer(dmName, dmTitle, offset, dtype, dm->GetArrayIndex(), dmCounter->GetClass()->GetName(), dmCounter->GetClass()->GetClassVersion(), dmFull);
+               const char* counterName = dm->GetArrayIndex();
+               auto multiple = std::strchr(counterName, '*');
+               if (multiple) {
+                  counterName = multiple + 1;
+               }
+               element = new TStreamerBasicPointer(dmName, dmTitle, offset, dtype, counterName, dmCounter->GetClass()->GetName(), dmCounter->GetClass()->GetClassVersion(), dmFull);
             } else {
                if ((fName == "TString") || (fName == "TClass")) {
                   continue;
@@ -748,7 +762,7 @@ void TStreamerInfo::Build(Bool_t isTransient)
       if (!narr) {
          narr = 1;
       }
-      element->SetSize(dsize*narr);
+      element->SetSize(dsize*narr*premultiplier);
       element->SetStreamer(streamer);
       if (!streamer) {
          Int_t k = element->GetType();