+/*
+ * This file is part of the GROMACS molecular simulation package.
+ *
+ * Copyright (c) 2021, by the GROMACS development team, led by
+ * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
+ * and including many others, as listed in the AUTHORS file in the
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+/*! \libinternal \file
+ *
+ * \brief This file makes declarations used for storing bitfields
+ * describing each atom so that other modules can efficiently process
+ * them.
+ *
+ * \inlibraryapi
+ */
+
+#ifndef GMX_MDTYPES_ATOMINFO_H
+#define GMX_MDTYPES_ATOMINFO_H
+
+#include <vector>
+
+namespace gmx
+{
+
+/*! \brief Constants whose bit describes properties of atoms in
+ * AtomInfoWithinMoleculeBlock.atomInfo.
+ *
+ * No bit should exceed 1 << 63, so that it fits into a 64-bit
+ * integer.
+ *
+ * Since the tpx format support max 256 energy groups, we do the same
+ * here, reserving bits 0-7 for the energy-group ID.
+ */
+//! \{
+static constexpr int sc_atomInfo_FreeEnergyPerturbation = 1 << 15;
+static constexpr int sc_atomInfo_Exclusion = 1 << 17;
+static constexpr int sc_atomInfo_Constraint = 1 << 20;
+static constexpr int sc_atomInfo_Settle = 1 << 21;
+static constexpr int sc_atomInfo_BondCommunication = 1 << 22;
+static constexpr int sc_atomInfo_HasVdw = 1 << 23;
+static constexpr int sc_atomInfo_HasCharge = 1 << 24;
+//! \}
+//! The first 8 bits are reserved for energy-group ID
+static constexpr int sc_atomInfo_EnergyGroupIdMask = 0b11111111;
+
+/*! \brief Contains information about each atom in a molecule block of
+ * the global topology. */
+struct AtomInfoWithinMoleculeBlock
+{
+ //! Index within the system of the first atom in the molecule block
+ int indexOfFirstAtomInMoleculeBlock = 0;
+ //! Index within the system of the last atom in the molecule block
+ int indexOfLastAtomInMoleculeBlock = 0;
+ /*! \brief Atom info for each atom in the block.
+ *
+ * The typical case is that all atoms are identical for each
+ * molecule of the block, and if so this vector has size equal to
+ * the number of atoms in the molecule.
+ *
+ * An example of an atypical case is QM/MM, where multiple
+ * molecules might be present and different molecules have
+ * different atoms within any one QM group or region. Now there are
+ * multiple kinds of molecules with the same connectivity, so we simply
+ * write out the atom info for the entire molecule block. Then the
+ * size equals the product of the number of atoms in the
+ * molecule and the number of molecules.
+ *
+ * The vector needs to be indexed accordingly.
+ */
+ std::vector<int> atomInfo;
+};
+
+} // namespace gmx
+
+#endif