Match with slightly unequal timestep times

.travis.yml

@@ -7,10 +7,12 @@ sudo: false
 
 # specify different versions of python and numpy
 env:
-  - PYTHON=2.7  NUMPY_VERSION=1.13.1
-  - PYTHON=3.5  NUMPY_VERSION=1.13.1
-  - PYTHON=3.6  NUMPY_VERSION=1.13.1
-  - PYTHON=3.7  NUMPY_VERSION=1.15.1
+  - PYTHON=2.7  NUMPY_VERSION=1.10.4 CYTHON_VERSION=0.27
+  - PYTHON=2.7  NUMPY_VERSION=1.13.1 CYTHON_VERSION=0.27
+  - PYTHON=3.5  NUMPY_VERSION=1.13.1 CYTHON_VERSION=0.27
+  - PYTHON=3.6  NUMPY_VERSION=1.13.1 CYTHON_VERSION=0.27
+  - PYTHON=3.6  NUMPY_VERSION=1.15.1 CYTHON_VERSION=0.27
+  - PYTHON=3.6  NUMPY_VERSION=1.15.1 CYTHON_VERSION=0.29
 
 before_install:
   - wget http://repo.continuum.io/miniconda/Miniconda-latest-Linux-x86_64.sh -O miniconda.sh

tangos/config.py

@@ -13,9 +13,6 @@
 
 default_fileset_handler_class = "pynbody.PynbodyInputHandler"
 
-num_multihops_max_default = 100
-# the maximum number of links to follow when searching for related halos
-
 default_linking_threshold = 0.005
 # the percentage of particles in common between two objects before the database bothers to store the relationship
 
@@ -47,6 +44,10 @@
 mergertree_timeout = 15.0 # seconds before abandoning the construction of a merger tree in the web interface
 mergertree_max_hops = 500 # maximum number of timesteps to scan
 
+# relation finding paremeters for multi hop queries
+num_multihops_max_default = 100     # the maximum number of links to follow when searching for related halos
+max_relative_time_difference = 1e-4     # the maximum fractional difference in time between two contemporaneous timesteps when searching for related halos
+
 # On some network file systems, concurrency using sqlite is dodgy to say the least. After committing a transaction
 # on one node, and before attempting to open a new transaction on another node, it seems empirically helpful to
 # allow a significant time delay. This variable controls that delay.

tangos/relation_finding/multi_hop.py

@@ -17,13 +17,9 @@
 from .one_hop import HopStrategy
 
 from ..config import num_multihops_max_default as NHOPS_MAX_DEFAULT
+from ..config import max_relative_time_difference as SMALL_FRACTION
 from six.moves import range
 
-# Fractional increase or decrease in time to represent future or past when making hops
-# (prevents numerical accuracy issues making the db misunderstand a contemporaneous step
-# as being in the future or past)
-SMALL_FRACTION = 1e-4
-
 class MultiHopStrategy(HopStrategy):
     """An extension of the HopStrategy class that takes multiple hops across
     HaloLinks, up to a specified maximum, before finding the target halo."""
@@ -163,7 +159,8 @@ def _generate_link_filter(self, timestep_old, timestep_new, table):
                 existing_timestep_ids = self.session.query(core.Halo.timestep_id).\
                     select_from(self._link_orm_class).join(self._link_orm_class.halo_to).distinct()
                 recursion_filter &= ~timestep_new.id.in_(existing_timestep_ids)
-                recursion_filter &= sqlalchemy.func.abs(timestep_new.time_gyr - timestep_old.time_gyr) < SMALL_FRACTION
+                recursion_filter &= sqlalchemy.func.abs(timestep_new.time_gyr - timestep_old.time_gyr) \
+                                    < SMALL_FRACTION * timestep_old.time_gyr
             else:
                 raise ValueError("Unknown direction %r" % directed)