From 7c3e2b9953564cabc78f8c270e9b2bad0f76588d Mon Sep 17 00:00:00 2001
From: Paul Romano <romano7@gmail.com>
Date: Fri, 11 Nov 2011 21:14:22 -0500
Subject: [PATCH] Fixed surface currents with multiple mesh bins.

---
 src/constants.f90 |  5 ++++-
 src/geometry.f90  | 15 +++++++++------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/src/constants.f90 b/src/constants.f90
index 106a38b0a4..7fec731d69 100644
--- a/src/constants.f90
+++ b/src/constants.f90
@@ -5,7 +5,7 @@ module constants
   ! Versioning numbers
   integer, parameter :: VERSION_MAJOR   = 0
   integer, parameter :: VERSION_MINOR   = 3
-  integer, parameter :: VERSION_RELEASE = 2
+  integer, parameter :: VERSION_RELEASE = 3
 
   ! Physical constants
   real(8), parameter ::            &
@@ -70,6 +70,9 @@ module constants
        & SENSE_POSITIVE = 1, &
        & SENSE_NEGATIVE = -1
 
+  ! Used for surface current tallies
+  real(8), parameter :: TINY_BIT = 1e-8
+
   ! Codes for read errors -- better hope these numbers are never used in an
   ! input file!
   integer, parameter :: ERROR_INT  = -huge(0)
diff --git a/src/geometry.f90 b/src/geometry.f90
index 9d5d4f7bd6..387fea4af6 100644
--- a/src/geometry.f90
+++ b/src/geometry.f90
@@ -215,7 +215,7 @@ contains
        ! TODO: Find a better solution to score surface currents than physically
        ! moving the particle forward slightly
 
-       p % xyz = p % xyz + 1e-6 * p % uvw
+       p % xyz = p % xyz + TINY_BIT * p % uvw
        call score_surface_current(p)
 
        ! Display message
@@ -229,6 +229,13 @@ contains
        ! =======================================================================
        ! PARTICLE REFLECTS FROM SURFACE
 
+       ! Score surface currents since reflection causes the direction of the
+       ! particle to change -- artificially move the particle slightly back in
+       ! case the surface crossing in coincident with a mesh boundary
+       p % xyz = p % xyz - TINY_BIT * p % uvw
+       call score_surface_current(p)
+       p % xyz = p % xyz + TINY_BIT * p % uvw
+
        ! Copy particle's direction cosines
        u = p % uvw(1)
        v = p % uvw(2)
@@ -321,12 +328,8 @@ contains
        p % cell = last_cell
        p % surface = -p % surface
 
-       ! Score surface currents since reflection causes the direction of the
-       ! particle to change
-       call score_surface_current(p)
-
        ! Set previous coordinate going slightly past surface crossing
-       p % last_xyz = p % xyz + 1e-6 * p % uvw
+       p % last_xyz = p % xyz + TINY_BIT * p % uvw
 
        ! Diagnostic message
        if (verbosity >= 10) then