[hist] Remove asserts in RBinIndex operators

Arithmetic operations on special values and if wrapping around,
go to InvalidIndex.

Author: hahnjo

hist/histv7/inc/ROOT/RBinIndex.hxx

@@ -50,10 +50,17 @@ public:
 
    RBinIndex &operator+=(std::size_t a)
    {
-      assert(IsNormal());
-      fIndex += a;
-      // TODO: is it possible to catch overflows?
-      assert(IsNormal());
+      if (!IsNormal()) {
+         // Arithmetic operations on special values go to InvalidIndex.
+         fIndex = InvalidIndex;
+      } else {
+         std::size_t old = fIndex;
+         fIndex += a;
+         if (fIndex < old || !IsNormal()) {
+            // The addition wrapped around, go to InvalidIndex.
+            fIndex = InvalidIndex;
+         }
+      }
       return *this;
    }
 
@@ -79,10 +86,15 @@ public:
 
    RBinIndex &operator-=(std::size_t a)
    {
-      assert(IsNormal());
-      assert(fIndex >= a);
-      fIndex -= a;
-      assert(IsNormal());
+      if (!IsNormal()) {
+         // Arithmetic operations on special values go to InvalidIndex.
+         fIndex = InvalidIndex;
+      } else if (fIndex >= a) {
+         fIndex -= a;
+      } else {
+         // The operation would wrap around, go to InvalidIndex.
+         fIndex = InvalidIndex;
+      }
       return *this;
    }
 

hist/histv7/test/hist_index.cxx

@@ -45,6 +45,17 @@ TEST(RBinIndex, Plus)
       const auto index3 = index1 + 2;
       EXPECT_EQ(index3.GetIndex(), 3);
    }
+
+   // Arithmetic operations on special values go to InvalidIndex.
+   for (auto index : {RBinIndex::Underflow(), RBinIndex::Overflow(), RBinIndex()}) {
+      index++;
+      EXPECT_TRUE(index.IsInvalid());
+   }
+
+   // Matches RBinIndex::UnderflowIndex
+   static constexpr std::size_t UnderflowIndex = -3;
+   EXPECT_TRUE((RBinIndex(0) + UnderflowIndex).IsInvalid());
+   EXPECT_TRUE((RBinIndex(3) + UnderflowIndex).IsInvalid());
 }
 
 TEST(RBinIndex, Minus)
@@ -74,6 +85,15 @@ TEST(RBinIndex, Minus)
       const auto index1 = index3 - 2;
       EXPECT_EQ(index1.GetIndex(), 1);
    }
+
+   // Arithmetic operations on special values go to InvalidIndex.
+   for (auto index : {RBinIndex::Underflow(), RBinIndex::Overflow(), RBinIndex()}) {
+      index--;
+      EXPECT_TRUE(index.IsInvalid());
+   }
+
+   EXPECT_TRUE((RBinIndex(0) - 1).IsInvalid());
+   EXPECT_TRUE((RBinIndex(0) - 4).IsInvalid());
 }
 
 TEST(RBinIndex, Equality)