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# Source code for ase.io.dlp4

    """ Read/Write DL_POLY_4 CONFIG files """
    import itertools
    import re
    from typing import IO, Generator, Iterable, List, Optional, Tuple, Union

    import numpy as np

    from ase.atoms import Atoms
    from ase.calculators.singlepoint import SinglePointCalculator
    from ase.data import chemical_symbols
    from ase.units import _amu, _auf, _auv
    from ase.utils import reader, writer

    __all__ = ["read_dlp4", "write_dlp4"]

    # dlp4 labels will be registered in atoms.arrays[DLP4_LABELS_KEY]
    DLP4_LABELS_KEY = "dlp4_labels"
    DLP4_DISP_KEY = "dlp4_displacements"
    DLP_F_SI = 1.0e-10 * _amu / (1.0e-12 * 1.0e-12)  # Å*Da/ps^2
    DLP_F_ASE = DLP_F_SI / _auf
    DLP_V_SI = 1.0e-10 / 1.0e-12  # Å/ps
    DLP_V_ASE = DLP_V_SI / _auv

    def _get_frame_positions(fd: IO) -> Tuple[int, int, int, List[int]]:
        """Get positions of frames in HISTORY file."""
        init_pos = fd.tell()

        fd.seek(0)

        record_len = len(fd.readline())  # system name, and record size

        items = fd.readline().strip().split()

        if len(items) not in (3, 5):
            raise RuntimeError("Cannot determine version of HISTORY file format.")

        levcfg, imcon, natoms = map(int, items[0:3])

        if len(items) == 3:   # DLPoly classic
            # we have to iterate over the entire file
            pos = [fd.tell() for line in fd if "timestep" in line]
        else:
            nframes = int(items[3])
            pos = [((natoms * (levcfg + 2) + 4) * i + 3) * record_len
                   for i in range(nframes)]

        fd.seek(init_pos)
        return levcfg, imcon, natoms, pos

    @reader
    def read_dlp_history(fd: IO,
                         index: Optional[Union[int, slice]] = None,
                         symbols: Optional[List[str]] = None) -> List[Atoms]:
        """Read a HISTORY file.

        Compatible with DLP4 and DLP_CLASSIC.

        *Index* can be integer or a slice.

        Provide a list of element strings to symbols to ignore naming
        from the HISTORY file.
        """
        return list(iread_dlp_history(fd, index, symbols))

    @reader
    def iread_dlp_history(fd: IO,
                          index: Optional[Union[int, slice]] = None,
                          symbols: Optional[List[str]] = None
                          ) -> Generator[Atoms, None, None]:
        """Generator version of read_dlp_history

        Compatible with DLP4 and DLP_CLASSIC.

        *Index* can be integer or a slice.

        Provide a list of element strings to symbols to ignore naming
        from the HISTORY file.
        """
        levcfg, imcon, natoms, pos = _get_frame_positions(fd)

        positions: Iterable[int] = ()
        if index is None:
            positions = pos
        elif isinstance(index, int):
            positions = (pos[index],)
        elif isinstance(index, slice):
            positions = itertools.islice(pos, index.start, index.stop, index.step)

        for pos_i in positions:
            fd.seek(pos_i + 1)
            yield read_single_image(fd, levcfg, imcon, natoms, is_trajectory=True,
                                    symbols=symbols)

    @reader
    def read_dlp4(fd: IO,
                  symbols: Optional[List[str]] = None) -> Atoms:
        """Read a DL_POLY_4 config/revcon file.

        Typically used indirectly through read("filename", atoms, format="dlp4").

        Can be unforgiving with custom chemical element names.
        Please complain to alin@elena.space for bugs."""

        fd.readline()
        levcfg, imcon = map(int, fd.readline().split()[0:2])

        position = fd.tell()
        is_trajectory = fd.readline().split()[0] == "timestep"

        if not is_trajectory:
            # Difficult to distinguish between trajectory and non-trajectory
            # formats without reading one line too many.
            fd.seek(position)

        natoms = (int(fd.readline().split()[2]) if is_trajectory else None)
        return read_single_image(fd, levcfg, imcon, natoms, is_trajectory,
                                 symbols)

    def read_single_image(fd: IO, levcfg: int, imcon: int,
                          natoms: Optional[int], is_trajectory: bool,
                          symbols: Optional[List[str]] = None) -> Atoms:
        """ Read a DLP frame """
        sym = symbols if symbols else []
        positions = []
        velocities = []
        forces = []
        charges = []
        masses = []
        disps = []
        labels = []

        is_pbc = imcon > 0

        cell = np.zeros((3, 3), dtype=np.float64)
        if is_pbc or is_trajectory:
            for j, line in enumerate(itertools.islice(fd, 3)):
                cell[j, :] = np.array(line.split(), dtype=np.float64)

        i = 0
        for i, line in enumerate(itertools.islice(fd, natoms), 1):
            match = re.match(r"\s*([A-Za-z][a-z]?)(\S*)", line)
            if not match:
                raise OSError(f"Line {line} does not match valid format.")

            symbol, label = match.group(1, 2)
            symbol = symbol.capitalize()

            if is_trajectory:
                mass, charge, *disp = map(float, line.split()[2:])
                charges.append(charge)
                masses.append(mass)
                disps.extend(disp if disp else [None])  # type: ignore[list-item]

            if not symbols:
                if symbol not in chemical_symbols:
                    raise ValueError(
                        f"Symbol '{symbol}' not found in chemical symbols table"
                    )
                sym.append(symbol)

            # make sure label is not empty
            labels.append(label if label else "")

            positions.append([float(x) for x in next(fd).split()[:3]])
            if levcfg > 0:
                velocities.append([float(x) * DLP_V_ASE
                                   for x in next(fd).split()[:3]])
            if levcfg > 1:
                forces.append([float(x) * DLP_F_ASE
                               for x in next(fd).split()[:3]])

        if symbols and (i != len(symbols)):
            raise ValueError(
                f"Counter is at {i} but {len(symbols)} symbols provided."
            )

        if imcon == 0:
            pbc = (False, False, False)
        elif imcon in (1, 2, 3):
            pbc = (True, True, True)
        elif imcon == 6:
            pbc = (True, True, False)
        elif imcon in (4, 5):
            raise ValueError(f"Unsupported imcon: {imcon}")
        else:
            raise ValueError(f"Invalid imcon: {imcon}")

        atoms = Atoms(positions=positions,
                      symbols=sym,
                      cell=cell,
                      pbc=pbc,
                      # Cell is centered around (0, 0, 0) in dlp4:
                      celldisp=-cell.sum(axis=0) / 2)

        if is_trajectory:
            atoms.set_masses(masses)
            atoms.set_array(DLP4_DISP_KEY, disps, float)
            atoms.set_initial_charges(charges)

        atoms.set_array(DLP4_LABELS_KEY, labels, str)
        if levcfg > 0:
            atoms.set_velocities(velocities)
        if levcfg > 1:
            atoms.calc = SinglePointCalculator(atoms, forces=forces)

        return atoms

    @writer
    def write_dlp4(fd: IO, atoms: Atoms,
                   levcfg: int = 0,
                   title: str = "CONFIG generated by ASE"):
        """Write a DL_POLY_4 config file.

        Typically used indirectly through write("filename", atoms, format="dlp4").

        Can be unforgiven with custom chemical element names.
        Please complain to alin@elena.space in case of bugs"""

        def float_format(flt):
            return format(flt, "20.10f")

        natoms = len(atoms)

        if tuple(atoms.pbc) == (False, False, False):
            imcon = 0
        elif tuple(atoms.pbc) == (True, True, True):
            imcon = 3
        elif tuple(atoms.pbc) == (True, True, False):
            imcon = 6
        else:
            raise ValueError(f"Unsupported pbc: {atoms.pbc}. "
                             "Supported pbc are 000, 110, and 111.")

        print(f"{title:72s}", file=fd)
        print(f"{levcfg:10d}{imcon:10d} {natoms}", file=fd)

        if imcon > 0:
            for row in atoms.get_cell():
                print("".join(map(float_format, row)), file=fd)

        vels = atoms.get_velocities() / DLP_V_ASE if levcfg > 0 else []
        forces = atoms.get_forces() / DLP_F_ASE if levcfg > 1 else []

        labels = atoms.arrays.get(DLP4_LABELS_KEY)

        for i, atom in enumerate(atoms):

            sym = atom.symbol
            if labels is not None:
                sym += labels[i]

            print(f"{sym:8s}{atom.index + 1:10d}", file=fd)

            to_write = (atom.x, atom.y, atom.z)
            print("".join(map(float_format, to_write)), file=fd)

            if levcfg > 0:
                to_write = (vels[atom.index, :]
                            if vels is not None else (0.0, 0.0, 0.0))
                print("".join(map(float_format, to_write)), file=fd)

            if levcfg > 1:
                to_write = (forces[atom.index, :]
                            if forces is not None else (0.0, 0.0, 0.0))
                print("".join(map(float_format, to_write)), file=fd)