add comment to patch_system::compute_synchronize_Jacobian()

describing how to compute the Jacobian taking into account the 3-phase
algorithm


git-svn-id: http://svn.einsteintoolkit.org/cactus/EinsteinAnalysis/AHFinderDirect/trunk@686 f88db872-0e4f-0410-b76b-b9085cfa78c5

1 files changed, 20 insertions, 0 deletions

diff --git a/src/patch/patch_system.cc b/src/patch/patch_system.cc
index 4f409f0..142ad97 100644
--- a/src/patch/patch_system.cc
+++ b/src/patch/patch_system.cc
@@ -1410,6 +1410,26 @@ void patch_system::synchronize(int ghosted_min_gfn_to_sync,
 //
 // This function computes the Jacobian of  synchronize()  into internal
 // buffers, taking into account synchronize()'s full 3-phase algorithm.
+// There are several cases:
+// - ghost zone is symmetry && x point is in non-corner
+//   --> x value was computed by a phase 1 symmetry operation,
+//       from nominal-grid data
+//   ==> use the ghost zone's Jacobian unchanged
+// - ghost zone is symmetry && x point is in corner
+//   --> x value was computed by a phase 3 symmetry operation,
+//       - from a symmetry ghost zone,
+//         which must have been computed in phase 1 from nominal-grid data
+//         ==> use the two symmetry ghost zones' combined Jacobian
+//       - from an interpatch ghost zone
+//         ==> use that interpatch ghost zone's Jacobian
+// - ghost zone is interpatch
+//   --> x value was computed by a phase 2 interpatch interpolation
+//       - from (only) nominal-grid data
+//         ==> use that interpatch ghost zone's Jacobian unchanged
+//       - from a mixture of nominal-grid data
+//         and data computed by a phase 1 symmetry operation
+//         ==> "fold" the interpatch ghost zone's Jacobian
+//             to reflect the phase 1 symmetry operation
 //
 // It also properly sets  global_{min,max}_m_ .
 //