make sure failed neighbor list searches trigger exhaustive searches

[gridley]

Fixes #1827

So, I slipped up slightly when removing recursion. I had assumed that if we try to iterate through neighbor lists, it will always touch the variable i_cell in find_cell_inner. Well, that's actually not true, since it's possible that the neighbor lists are empty, which in that case, i_cell will never be affected. As a result of this, we were never forming neighbor lists.

The fix to this is simple. If we're attempting a cell search based on neighbor lists, we should always return false if we failed to find the right cell in the neighbor list. This will cause an exhaustive search to happen, the result of which will be appended to the neighbor list.

Here is a simple 5x5 checkerboard input done directly using surfaces, so that the impact of neighbor lists is fairly high. On my computer, I saw the tracking rate go from around 190 kp/s to around 240 kp/s.

<?xml version='1.0' encoding='utf-8'?>
<geometry>
  <!-- 25 cells in total -->
  <cell material="1" id="1"  region="1 -5 3 -9" universe="0" />
  <cell material="2" id="2"  region="5 -6 3 -9" universe="0" />
  <cell material="1" id="3"  region="6 -7 3 -9" universe="0" />
  <cell material="2" id="4"  region="7 -8 3 -9" universe="0" />
  <cell material="1" id="5"  region="8 -2 3 -9" universe="0" />
  <cell material="2" id="6"  region="1 -5 9 -10" universe="0" />
  <cell material="1" id="7"  region="5 -6 9 -10" universe="0" />
  <cell material="2" id="8"  region="6 -7 9 -10" universe="0" />
  <cell material="1" id="9"  region="7 -8 9 -10" universe="0" />
  <cell material="2" id="10" region="8 -2 9 -10" universe="0" />
  <cell material="1" id="11" region="1 -5 10 -11" universe="0" />
  <cell material="2" id="12" region="5 -6 10 -11" universe="0" />
  <cell material="1" id="13" region="6 -7 10 -11" universe="0" />
  <cell material="2" id="14" region="7 -8 10 -11" universe="0" />
  <cell material="1" id="15" region="8 -2 10 -11" universe="0" />
  <cell material="2" id="16" region="1 -5 11 -12" universe="0" />
  <cell material="1" id="17" region="5 -6 11 -12" universe="0" />
  <cell material="2" id="18" region="6 -7 11 -12" universe="0" />
  <cell material="1" id="19" region="7 -8 11 -12" universe="0" />
  <cell material="2" id="20" region="8 -2 11 -12" universe="0" />
  <cell material="1" id="21" region="1 -5 12 -4" universe="0" />
  <cell material="2" id="22" region="5 -6 12 -4" universe="0" />
  <cell material="1" id="23" region="6 -7 12 -4" universe="0" />
  <cell material="2" id="24" region="7 -8 12 -4" universe="0" />
  <cell material="1" id="25" region="8 -2 12 -4" universe="0" />

  <surface boundary="reflective" coeffs="0" id="1" type="x-plane" />
  <surface boundary="reflective" coeffs="10" id="2" type="x-plane" />
  <surface boundary="reflective" coeffs="0" id="3" type="y-plane" />
  <surface boundary="reflective" coeffs="10" id="4" type="y-plane" />

  <surface coeffs="2" id="5"  type="x-plane" />
  <surface coeffs="4" id="6"  type="x-plane" />
  <surface coeffs="6" id="7"  type="x-plane" />
  <surface coeffs="8" id="8"  type="x-plane" />

  <surface coeffs="2" id="9"  type="y-plane" />
  <surface coeffs="4" id="10" type="y-plane" />
  <surface coeffs="6" id="11" type="y-plane" />
  <surface coeffs="8" id="12" type="y-plane" />
</geometry>
<?xml version='1.0' encoding='utf-8'?>
<materials>
  <material depletable="true" id="1" name="Fuel 3.1%" temperature="300">
    <density units="g/cc" value="10.30166" />
    <nuclide ao="1.9992419999999977" name="O16" />
    <nuclide ao="0.0007579999999999991" name="O17" />
    <nuclide ao="0.00028071399816198897" name="U234" />
    <nuclide ao="0.03140630756586438" name="U235" />
    <nuclide ao="0.9681691224623687" name="U238" />
    <nuclide ao="0.0001438559736051798" name="U236" />
  </material>
  <material id="2" name="Water" temperature="300">
    <density units="g/cc" value="0.7405820675158279" />
    <nuclide ao="0.0003217866044341838" name="B10" />
    <nuclide ao="0.001301758322075321" name="B11" />
    <nuclide ao="1.9964419358487546" name="H1" />
    <nuclide ao="0.00031097429822629077" name="H2" />
    <nuclide ao="0.9979980703970175" name="O16" />
    <nuclide ao="0.00037838467647285283" name="O17" />
  </material>
</materials>
<?xml version='1.0' encoding='utf-8'?>
<plots>
  <plot basis="xy" color_by="material" id="1" type="slice">
    <origin>5 5 0.5</origin>
    <width>10 10</width>
    <pixels>800 800</pixels>
  </plot>
</plots>
<?xml version='1.0' encoding='utf-8'?>
<settings>
  <run_mode>eigenvalue</run_mode>
  <particles>100000</particles>
  <batches>5</batches>
  <inactive>3</inactive>
  <source strength="1.0">
    <space type="box">
      <parameters>0 10 0 10 1 1</parameters>
    </space>
  </source>
</settings>

[paulromano]

Thanks for implementing a fix so quickly @gridley!

[gridley]

OK, the latest commit should address all the concerns.

[jtramm]

Looks good -- thanks for making the fix @gridley! This bug is further motivation for us to stand up an automated performance testing apparatus of some sort so we can have a way of catching performance regressions.

[gridley]

Looks good -- thanks for making the fix @gridley! This bug is further motivation for us to stand up an automated performance testing apparatus of some sort so we can have a way of catching performance regressions.

Agreed, started a discourse thread on it!