Fixed error writing a DCD file for a very long trajectory

wrappers/python/simtk/openmm/app/dcdfile.py

@@ -77,6 +77,7 @@ class DCDFile(object):
         if is_quantity(dt):
             dt = dt.value_in_unit(picoseconds)
         dt /= 0.04888821
+        self._dt = dt
         boxFlag = 0
         if topology.getUnitCellDimensions() is not None:
             boxFlag = 1
@@ -116,9 +117,19 @@ class DCDFile(object):
             raise ValueError('Particle position is infinite')
         file = self._file
 
+        self._modelCount += 1
+        if self._interval > 1 and self._firstStep+self._modelCount*self._interval > 1<<31:
+            # This will exceed the range of a 32 bit integer.  To avoid crashing or producing a corrupt file,
+            # update the header to say the trajectory consisted of a smaller number of larger steps (so the
+            # total trajectory length remains correct).
+            self._firstStep //= self._interval
+            self._dt *= self._interval
+            self._interval = 1
+            file.seek(0, os.SEEK_SET)
+            file.write(struct.pack('<i4c9if', 84, b'C', b'O', b'R', b'D', 0, self._firstStep, self._interval, 0, 0, 0, 0, 0, 0, self._dt))
+
         # Update the header.
 
-        self._modelCount += 1
         file.seek(8, os.SEEK_SET)
         file.write(struct.pack('<i', self._modelCount))
         file.seek(20, os.SEEK_SET)

wrappers/python/tests/TestDcdFile.py

@@ -17,6 +17,17 @@ class TestDCDFile(unittest.TestCase):
             for i in range(5):
                 dcd.writeModel([mm.Vec3(random(), random(), random()) for j in range(natom)]*unit.angstroms)
         os.remove(fname)
+    
+    def testLongTrajectory(self):
+        """Test writing a trajectory that has more than 2^31 steps."""
+        fname = tempfile.mktemp(suffix='.dcd')
+        pdbfile = app.PDBFile('systems/alanine-dipeptide-implicit.pdb')
+        natom = len(list(pdbfile.topology.atoms()))
+        with open(fname, 'wb') as f:
+            dcd = app.DCDFile(f, pdbfile.topology, 0.001, interval=1000000000)
+            for i in range(5):
+                dcd.writeModel([mm.Vec3(random(), random(), random()) for j in range(natom)]*unit.angstroms)
+        os.remove(fname)
 
 if __name__ == '__main__':
     unittest.main()