[treeplayer] Fix precision loss in TTree::Scan for 64-bit integers

The `"lld"` column format passed without an embedded size (e.g. via
`"colsize=N col=lld"`) was not recognized as a `"long long"` modifier in
**TTreeFormula::PrintValue**, so 64-bit integer branches (e.g.
`ULong64_t`) were evaluated as double and rounded above 2^53.

This commit fixes a off-by-one problem in the length-modifier detection
to address the issue, adding also a regression test.

Closes #7844.

🤖 Done with the help of [Claude Code](https://claude.com/claude-code) (Claude Opus 4.8)

Author: guitargeek

tree/treeplayer/src/TTreeFormula.cxx

@@ -5052,12 +5052,12 @@ char *TTreeFormula::PrintValue(Int_t mode, Int_t instance, const char *decform)
             Ssiz_t len = strlen(decform);
             Char_t outputSizeLevel = 1;
             char *expo = nullptr;
-            if (len>2) {
+            if (len > 1) {
                switch (decform[len-2]) {
                   case 'l':
                   case 'L': {
                      outputSizeLevel = 2;
-                     if (len>3 && tolower(decform[len-3])=='l') {
+                     if (len > 2 && tolower(decform[len - 3]) == 'l') {
                         outputSizeLevel = 3;
                      }
                      break;

tree/treeplayer/test/regressions.cxx

@@ -585,3 +585,68 @@ TEST(TTreeScan, TTreeGetBranchOfFriendTChain)
          throw std::runtime_error("Could not retrieve TTreePlayer from main tree!");
    }
 }
+
+// https://github.com/root-project/root/issues/7844
+// TTree::Scan() used to lose precision when printing 64-bit integer branches
+// (e.g. ULong64_t): the "lld" column format, when given without an embedded
+// column size (i.e. via "colsize=N col=lld"), was not recognized as a
+// "long long" modifier because of an off-by-one in the length-modifier
+// detection in TTreeFormula::PrintValue. As a consequence the value was
+// evaluated and printed as a double, rounding anything above 2^53.
+TEST(TTreeScan, ULong64Precision)
+{
+   // 1617047019150033926 needs 61 bits, so it cannot be represented exactly
+   // by a double (53-bit mantissa).
+   constexpr ULong64_t value{1617047019150033926ULL};
+
+   struct DatasetRAII {
+      const char *fTreeName{"tree_7844"};
+      const char *fFileName{"tree_7844.root"};
+      DatasetRAII(ULong64_t v)
+      {
+         auto file = std::make_unique<TFile>(fFileName, "recreate");
+         auto tree = std::make_unique<TTree>(fTreeName, fTreeName);
+
+         ULong64_t x = v;
+         tree->Branch("x", &x, "x/l");
+         tree->Fill();
+         file->Write();
+      }
+
+      ~DatasetRAII() { std::remove(fFileName); }
+   } dataset{value};
+
+   auto file = std::make_unique<TFile>(dataset.fFileName);
+   std::unique_ptr<TTree> tree{file->Get<TTree>(dataset.fTreeName)};
+
+   std::ostringstream strCout;
+   {
+      if (auto *treePlayer = static_cast<TTreePlayer *>(tree->GetPlayer())) {
+         struct FileRAII {
+            const char *fPath;
+            FileRAII(const char *name) : fPath(name) {}
+            ~FileRAII() { std::remove(fPath); }
+         } redirectFile{"tree_7844_regression_redirect.txt"};
+         treePlayer->SetScanRedirect(true);
+         treePlayer->SetScanFileName(redirectFile.fPath);
+         // The "lld" format without an embedded size (the size is given
+         // separately via "colsize=") must print the exact 64-bit value, as
+         // well as the exact result of integer arithmetic with large constants.
+         tree->Scan("x:x-1617047019150033925:x-1617047019150033000", "", "colsize=21 col=lld:lld:lld");
+
+         std::ifstream redirectStream(redirectFile.fPath);
+         std::stringstream redirectOutput;
+         redirectOutput << redirectStream.rdbuf();
+
+         const static std::string expectedScanOut{
+            R"Scan(************************************************************************************
+*    Row   *                     x * x-1617047019150033925 * x-1617047019150033000 *
+************************************************************************************
+*        0 *   1617047019150033926 *                     1 *                   926 *
+************************************************************************************
+)Scan"};
+         EXPECT_EQ(redirectOutput.str(), expectedScanOut);
+      } else
+         throw std::runtime_error("Could not retrieve TTreePlayer from main tree!");
+   }
+}