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#ifndef TEMPLAT_UTIL_RANGEITERATION_SUM_IN_RANGE_H
#define TEMPLAT_UTIL_RANGEITERATION_SUM_IN_RANGE_H

/* This file is part of TempLat, available at https://cosmolattice.github.io/templat .
   Copyright 2021-2026 The TempLat authors, see AUTHORS.md.
   Released under the MIT license, see LICENSE.md. */

// File info: Main contributor(s): Adrien Florio, Franz R. Sattler, Year: 2026

#include "TempLat/lattice/algebra/constants/zerotype.h"
#include "TempLat/lattice/algebra/operators/add.h"

namespace TempLat
{
  template <int begin, typename F, int... INT>
  constexpr auto sum_in_range_impl(F &&func, std::integer_sequence<int, INT...> iseq)
  {
    return (func(Tag<begin + INT>()) + ... + ZeroType());
  }

  template <int begin, int end, typename F> constexpr auto sum_in_range(F &&func)
  {
    return sum_in_range_impl<begin>(std::forward<F>(func), std::make_integer_sequence<int, end - begin>());
  }
} // namespace TempLat

// NOTE: the empty-range guard is keyed on the (end < beg) range test, NOT on `requires { expr; }`.
// A `requires { expr; }` guard would place a nested Total's inner `[&](auto j){...}` lambda inside a
// requires-expression, which clang rejects ("non-local lambda expression cannot have a capture-default"),
// breaking nested `Total(i, ..., Total(j, ...))` (B2SU2, theta_ijk, ...). Gating on the range keeps the
// empty-range safety (for end < beg the `expr` branch is never instantiated) without that hazard.
#define Total(i, beg, end, expr)                                                                                       \
  TempLat::sum_in_range<beg, end + 1>([&](auto i) {                                                                    \
    if constexpr ((end) < (beg)) {                                                                                     \
      return TempLat::ZeroType();                                                                                      \
    } else {                                                                                                           \
      return expr;                                                                                                     \
    }                                                                                                                  \
  })

#endif