Fix etl::histogram copy/move not preserving start_index (#1485)

The run-time-offset specialization of etl::histogram stores a start_index
member used to map keys to bins (accumulator[key - start_index]). Its copy
constructor, move constructor, copy assignment, and move assignment copied
only the accumulator and left start_index uninitialized, so a copied or
moved histogram indexed the wrong bin in operator[]/add(), causing
out-of-bounds access (undefined behavior).

All four special member functions now also copy start_index. The
compile-time-offset specialization is unaffected, as its start index is a
template constant rather than a data member.

Add regression tests covering copy/move construction and assignment.

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>

include/etl/histogram.h

@@ -296,6 +296,7 @@ namespace etl
     histogram(const histogram& other)
     {
       this->accumulator = other.accumulator;
+      start_index       = other.start_index;
     }
 
 #if ETL_USING_CPP11
@@ -305,6 +306,7 @@ namespace etl
     histogram(histogram&& other)
     {
       this->accumulator = etl::move(other.accumulator);
+      start_index       = other.start_index;
     }
 #endif
 
@@ -314,6 +316,7 @@ namespace etl
     histogram& operator=(const histogram& rhs)
     {
       this->accumulator = rhs.accumulator;
+      start_index       = rhs.start_index;
 
       return *this;
     }
@@ -325,6 +328,7 @@ namespace etl
     histogram& operator=(histogram&& rhs)
     {
       this->accumulator = etl::move(rhs.accumulator);
+      start_index       = rhs.start_index;
 
       return *this;
     }

test/test_histogram.cpp

@@ -248,6 +248,68 @@ namespace
       CHECK_EQUAL(0U, histogram2.count());
     }
 
+    //*************************************************************************
+    // operator[] of the run time offset histogram uses the stored start index,
+    // so copying/moving must preserve it. Regression test: the copy/move
+    // constructors and assignment operators used to forget to copy start_index.
+    //*************************************************************************
+    TEST(test_int_runtime_offset_copy_constructor_preserves_start_index)
+    {
+      IntRuntimeOffsetHistogram histogram1(Start, input2.begin(), input2.end());
+
+      IntRuntimeOffsetHistogram histogram2(histogram1);
+
+      for (int key = Start; key < Start + static_cast<int>(Size); ++key)
+      {
+        CHECK_EQUAL(static_cast<int>(histogram1[key]), static_cast<int>(histogram2[key]));
+      }
+    }
+
+    //*************************************************************************
+    TEST(test_int_runtime_offset_copy_assignment_preserves_start_index)
+    {
+      IntRuntimeOffsetHistogram histogram1(Start, input2.begin(), input2.end());
+
+      IntRuntimeOffsetHistogram histogram2(Start);
+      histogram2 = histogram1;
+
+      for (int key = Start; key < Start + static_cast<int>(Size); ++key)
+      {
+        CHECK_EQUAL(static_cast<int>(histogram1[key]), static_cast<int>(histogram2[key]));
+      }
+    }
+
+#if ETL_USING_CPP11
+    //*************************************************************************
+    TEST(test_int_runtime_offset_move_constructor_preserves_start_index)
+    {
+      IntRuntimeOffsetHistogram histogram1(Start, input2.begin(), input2.end());
+      IntRuntimeOffsetHistogram histogram_source(Start, input2.begin(), input2.end());
+
+      IntRuntimeOffsetHistogram histogram2(etl::move(histogram_source));
+
+      for (int key = Start; key < Start + static_cast<int>(Size); ++key)
+      {
+        CHECK_EQUAL(static_cast<int>(histogram1[key]), static_cast<int>(histogram2[key]));
+      }
+    }
+
+    //*************************************************************************
+    TEST(test_int_runtime_offset_move_assignment_preserves_start_index)
+    {
+      IntRuntimeOffsetHistogram histogram1(Start, input2.begin(), input2.end());
+      IntRuntimeOffsetHistogram histogram_source(Start, input2.begin(), input2.end());
+
+      IntRuntimeOffsetHistogram histogram2(Start);
+      histogram2 = etl::move(histogram_source);
+
+      for (int key = Start; key < Start + static_cast<int>(Size); ++key)
+      {
+        CHECK_EQUAL(static_cast<int>(histogram1[key]), static_cast<int>(histogram2[key]));
+      }
+    }
+#endif
+
     //*************************************************************************
     TEST(test_string_histogram_constructor)
     {