[NOT READY] Add a fused multiply add for AD tensors

.gitignore

@@ -4,3 +4,8 @@ build
 installed
 .clang-format
 TAGS
+src/.libs/*
+src/apps/.deps/*
+src/utilities/src/.libs/*
+src/utilities/src/.deps/*
+test/.deps/*

src/numerics/include/metaphysicl/dualnumber.h

@@ -288,8 +288,20 @@ derivative_multiply_helper(DualNumber<T, D, asd> & out, const DualNumber<T2, D2,
   if (asd && !(out.take_derivatives() || in.take_derivatives()))
     return;
 
-  out.derivatives() *= in.value();
-  out.derivatives() += out.value() * in.derivatives();
+  // Fused multiply-add: out.deriv = in.value * out.deriv + out.value * in.deriv
+  //
+  // Replaces:
+  //   out.derivatives() *= in.value();
+  //   out.derivatives() += out.value() * in.derivatives();
+  //
+  // The naive version above costs:
+  //   1) scalar *=: cheap, no sparsity_union, no alloc
+  //   2) out.value() * in.derivatives(): allocates a temporary sparse number (no sparsity_union)
+  //   3) operator+=: sparsity_union(temp.indices()) + merge loop + temp dealloc
+  //
+  // The fused version below calls sparsity_union once and eliminates the temporary entirely,
+  // reducing the cost from 1 alloc + 1 dealloc + 1 sparsity_union to 0 allocs + 1 sparsity_union.
+  out.derivatives().multiply_union_add(in.value(), out.value(), in.derivatives());
 }
 
 template <typename T,

src/numerics/include/metaphysicl/dynamicsparsenumberbase.h

@@ -61,7 +61,7 @@ template <typename Data2, typename Indices2, class... SubTypeArgs2>
 METAPHYSICL_INLINE
 DynamicSparseNumberBase<Data, Indices, SubType, SubTypeArgs...>::
 DynamicSparseNumberBase(const DynamicSparseNumberBase<Data2, Indices2, SubType, SubTypeArgs2...> & src)
-{ 
+{
   this->resize(src.size());
   const auto & src_indices = src.nude_indices();
   const auto & src_data = src.nude_data();
@@ -612,6 +612,46 @@ DynamicSparseNumberBase<Data, Indices, SubType, SubTypeArgs...>::operator+= (con
 }
 
 
+template <typename Data, typename Indices, template <class...> class SubType, class... SubTypeArgs>
+template <class... SubTypeArgs2>
+METAPHYSICL_INLINE
+SubType<SubTypeArgs...>&
+DynamicSparseNumberBase<Data, Indices, SubType, SubTypeArgs...>::multiply_union_add(
+    const typename Data::value_type scalar_a,
+    const typename Data::value_type scalar_b,
+    const SubType<SubTypeArgs2...>& other)
+{
+  // Single sparsity union — may resize _data and _indices
+  this->sparsity_union(other.nude_indices());
+
+  // Scale all existing entries by scalar_a (indices unchanged by scalar multiply)
+  for (auto & d : _data)
+    d *= scalar_a;
+
+  // Walk the sorted index lists and add scalar_b * other[i] into matching entries.
+  // After sparsity_union above, every index in other is guaranteed to be in _indices,
+  // so the inner while loop will always find a match.
+  auto data_it   = _data.begin();
+  auto index_it  = _indices.begin();
+  auto data2_it  = other.nude_data().begin();
+  auto index2_it = other.nude_indices().begin();
+
+  for (; data2_it != other.nude_data().end(); ++data2_it, ++index2_it)
+    {
+      while (*index_it < *index2_it) {
+        ++index_it;
+        ++data_it;
+        metaphysicl_assert(index_it != _indices.end());
+      }
+      metaphysicl_assert_equal_to(*index_it, *index2_it);
+
+      *data_it += scalar_b * (*data2_it);
+    }
+
+  return static_cast<SubType<SubTypeArgs...>&>(*this);
+}
+
+
 template <typename Data, typename Indices, template <class...> class SubType, class... SubTypeArgs>
 template <class... SubTypeArgs2>
 METAPHYSICL_INLINE