Support non-covariant indices in evaluation expressions.

SeQuant/core/algorithm.hpp

@@ -138,34 +138,6 @@ bool next_permutation_parity(int& parity, BidirIt first, BidirIt last,
   }
 }
 
-///
-/// Given a size of a container @c n, returns a range of bitsets. The bitsets
-/// represent the sieve that can be used to construct the subsequences of the
-/// size-n container.
-///
-inline auto subset_indices(size_t n) {
-  using ranges::views::iota;
-  using ranges::views::transform;
-  return iota(size_t{1}, size_t(1 << n)) |
-         transform([n](auto i) { return boost::dynamic_bitset<>(n, i); });
-}
-
-///
-/// Given an iterable @c it and a bitset @c bs, select the elements in the
-/// iterable that correspond to an 'on' bit.
-///
-template <typename Iterable>
-auto subsequence(Iterable const& it, boost::dynamic_bitset<> const& bs) {
-  using ranges::views::filter;
-  using ranges::views::iota;
-  using ranges::views::transform;
-  using ranges::views::zip;
-
-  auto bits = iota(size_t{0}) | transform([&bs](auto i) { return bs.at(i); });
-  return zip(it, bits) | filter([](auto&& kv) { return std::get<1>(kv); }) |
-         transform([](auto&& kv) { return std::get<0>(kv); });
-}
-
 ///
 /// All elements in the vector belong to the integral range [-1,N)
 /// where N is the length of the [Expr] (ie. the iterable of expressions)

SeQuant/core/meta.hpp

@@ -9,6 +9,7 @@
 #include <memory>
 #include <range/v3/range/access.hpp>
 #include <range/v3/range/traits.hpp>
+#include <ranges>
 #include <type_traits>
 
 namespace sequant {
@@ -506,6 +507,23 @@ struct std_array_size<std::array<T, N>>
 template <typename T>
 constexpr inline std::size_t std_array_size_v = std_array_size<T>::value;
 
+///
+/// True if @tparam Rng is a range whose value type is convertible
+/// to @tparam V .
+///
+template <typename Rng, typename V>
+concept range_of = std::ranges::range<Rng> &&
+                   std::is_convertible_v<std::ranges::range_value_t<Rng>, V>;
+
+///
+/// True if @tparam Rng is a range whose value type is convertible
+/// to @tparam V and the size (std::ranges::distance) is known in constant time.
+///
+/// @see https://en.cppreference.com/w/cpp/ranges/sized_range.html
+///
+template <typename Rng, typename V>
+concept sized_range_of = std::ranges::sized_range<Rng> && range_of<Rng, V>;
+
 }  // namespace meta
 }  // namespace sequant
 

SeQuant/core/utility/indices.hpp

@@ -566,6 +566,46 @@ auto get_ket_idx(T&& container)
   }
 }
 
+auto subexpr_index_counts(meta::sized_range_of<Tensor> auto const& tnsrs) {
+  size_t const N = ranges::distance(tnsrs);
+  container::vector<container::map<Index, size_t, Index::FullLabelCompare>>
+      result((1 << N));
+  for (auto i = 1; i < result.size(); ++i) {
+    auto bs = boost::dynamic_bitset<>(N, i);
+    for (auto b = 0; b < N; ++b)
+      if (bs[b])
+        for (auto&& ix : ranges::at(tnsrs, b).indices())
+          if (auto [it, yn] = result[i].try_emplace(ix, 1); !yn)  //
+            ++(it->second);
+  }
+  return result;
+}
+
+template <meta::sized_range_of<Tensor> Rng, meta::range_of<Index> Ixs>
+auto subexpr_target_indices(Rng const& tnsrs, Ixs const& tixs) {
+  using IndexSet = container::set<Index, Index::FullLabelCompare>;
+  size_t const N = ranges::distance(tnsrs);
+  container::vector<IndexSet> result((1 << N));
+
+  if (result.empty()) return result;
+
+  for (auto i = 0; i < N; ++i) {
+    auto&& ixs = ranges::at(tnsrs, i).indices();
+    result[(1 << i)] = IndexSet(ranges::begin(ixs), ranges::end(ixs));
+  }
+
+  *result.rbegin() = IndexSet(ranges::begin(tixs), ranges::end(tixs));
+
+  auto counts = subexpr_index_counts(tnsrs);
+
+  for (auto i = 0; i < result.size(); ++i)
+    for (auto&& [k, v] : counts[i])
+      if (v == 1 || (v > 0 && counts.at(counts.size() - i - 1).contains(k)))
+        result[i].emplace(k);
+
+  return result;
+}
+
 }  // namespace sequant
 
 #endif  // SEQUANT_CORE_UTILITY_INDICES_HPP