ENH: add TPR position read support

* Add support for reading positions from
GMX `.tpr` files at `tpx` version `134`.

package/MDAnalysis/coordinates/TPR.py

@@ -0,0 +1,73 @@
+# -*- Mode: python; tab-width: 4; indent-tabs-mode:nil; coding:utf-8 -*-
+# vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4
+
+from . import base
+from ..lib import util
+from .timestep import Timestep
+import MDAnalysis.topology.tpr.utils as tpr_utils
+import MDAnalysis.topology.tpr.setting as S
+
+
+import numpy as np
+
+
+class TPRReader(base.SingleFrameReaderBase):
+    # TODO: reduce duplication with `TPRparser`;
+    # we could share some state for the position
+    # in the binary file to avoid re-reading topology
+    # or perhaps combine the topology and coordinate reading
+    # with some inheritance shenanigans?
+    format = "TPR"
+    units = {"length": "nm"}
+    _Timestep = Timestep
+
+    def _read_first_frame(self):
+        # Read header/move over topology
+        # TODO: reduce duplication with TPRparser perhaps...
+        with util.openany(self.filename, mode='rb') as infile:
+            tprf = infile.read()
+        data = tpr_utils.TPXUnpacker(tprf)
+        try:
+            th = tpr_utils.read_tpxheader(data)                    # tpxheader
+        except (EOFError, ValueError):
+            msg = f"{self.filename}: Invalid tpr file or cannot be recognized"
+            logger.critical(msg)
+            raise IOError(msg)
+
+        self.ts = ts = self._Timestep(th.natoms, **self._ts_kwargs)
+        self.n_atoms = th.natoms
+
+        # Starting with gromacs 2020 (tpx version 119), the body of the file
+        # is encoded differently. We change the unpacker accordingly.
+        if th.fver >= S.tpxv_AddSizeField and th.fgen >= 27:
+            actual_body_size = len(data.get_buffer()) - data.get_position()
+            if actual_body_size == 4 * th.sizeOfTprBody:
+                # See issue #2428.
+                msg = (
+                    "TPR files produced with beta versions of gromacs 2020 "
+                    "are not supported."
+                )
+                logger.critical(msg)
+                raise IOError(msg)
+            data = tpr_utils.TPXUnpacker2020.from_unpacker(data)
+
+        state_ngtc = th.ngtc         # done init_state() in src/gmxlib/tpxio.c
+        if th.bBox:
+            tpr_utils.extract_box_info(data, th.fver)
+
+        if state_ngtc > 0:
+            if th.fver < 69:                      # redundancy due to  different versions
+                tpr_utils.ndo_real(data, state_ngtc)
+            tpr_utils.ndo_real(data, state_ngtc)        # relevant to Berendsen tcoupl_lambda
+
+        if th.bTop:
+            tpr_top = tpr_utils.do_mtop(data, th.fver,
+                                        tpr_resid_from_one=True)
+        else:
+            msg = f"{self.filename}: No topology found in tpr file"
+            logger.critical(msg)
+            raise IOError(msg)
+
+        if th.bX:
+            self.ts._pos = np.asarray(tpr_utils.ndo_rvec(data, th.natoms),
+                                      dtype=np.float32)

package/MDAnalysis/coordinates/__init__.py

@@ -776,6 +776,7 @@ class can choose an appropriate reader automatically.
 from . import PDB
 from . import PDBQT
 from . import PQR
+from . import TPR
 from . import TRC
 from . import TRJ
 from . import TRR

package/MDAnalysis/topology/tpr/utils.py

@@ -423,6 +423,48 @@ def do_mtop(data, fver, tpr_resid_from_one=False):
         elements = Elements(np.array(elements, dtype=object))
         top.add_TopologyAttr(elements)
 
+    # NOTE: the tpr striding code below serves the
+    # purpose of placing us in a suitable "seek" position
+    # in the binary file such that we are well placed
+    # for reading coords and velocities if they are present
+    # the order of operations is based on an analysis of
+    # the C++ code in do_mtop() function in the GROMACS
+    # source at:
+    # src/gromacs/fileio/tpxio.cpp
+    # TODO: expand tpx version support for striding to
+    # the coordinates
+    if fver == 134:
+        # TODO: the following value is important, and not sure
+        # how to access programmatically yet...
+        # from GMX source code:
+        # api/legacy/include/gromacs/topology/topology_enums.h
+        # worst case scenario we hard code it based on
+        # tpx/GMX version?
+        SimulationAtomGroupType_size = 10
+        n_atoms = data.unpack_int()
+        interm = data.unpack_uchar()
+        ngrid = data.unpack_int()
+        grid_spacing = data.unpack_int()
+        n_elements = grid_spacing ** 2
+        for i in range(ngrid):
+            for j in range(n_elements):
+                ndo_real(data, 4)
+        for i in range(SimulationAtomGroupType_size):
+            group_size = data.unpack_int()
+            ndo_int(data, group_size)
+        n_group_names = data.unpack_int()
+        for i in range(n_group_names):
+            data.unpack_int()
+        for i in range(SimulationAtomGroupType_size):
+            n_grp_numbers = data.unpack_int()
+            if n_grp_numbers != 0:
+                for i in range(n_grp_numbers):
+                    data.unpack_uchar()
+        im_excl_grp_size = data.unpack_int()
+        ndo_int(data, im_excl_grp_size)
+        # TODO: why is this needed?
+        data.unpack_int()
+
     return top
 
 

testsuite/MDAnalysisTests/coordinates/test_tpr.py

@@ -0,0 +1,34 @@
+from MDAnalysisTests.datafiles import (TPR2024_4_bonded,
+                                       TPR_EXTRA_2024_4,
+                                       TPR2024_4)
+import MDAnalysis as mda
+
+
+import pytest
+from numpy.testing import assert_allclose, assert_equal
+
+
+@pytest.mark.parametrize("tpr_file, exp_first_atom, exp_last_atom, exp_shape", [
+    (TPR2024_4_bonded,
+     [4.446, 4.659, 2.384],
+     [4.446, 4.659, 2.384],
+     (14, 3),
+    ),
+    # same coordinates, different shape
+    (TPR_EXTRA_2024_4,
+     [4.446, 4.659, 2.384],
+     [4.446, 4.659, 2.384],
+     (18, 3),
+    ),
+    # different coordinates and different shape
+    (TPR2024_4,
+     [3.25000e-01,  1.00400e+00,  1.03800e+00],
+     [-2.56000e-01,  1.37300e+00,  3.59800e+00],
+     (2263, 3),
+    ),
+])
+def test_basic_read_tpr(tpr_file, exp_first_atom, exp_last_atom, exp_shape):
+    u = mda.Universe(tpr_file)
+    assert_allclose(u.atoms.positions[0, ...], exp_first_atom)
+    assert_allclose(u.atoms.positions[-1, ...], exp_last_atom)
+    assert_equal(u.atoms.positions.shape, exp_shape)