# Getting Around the Hierarchy: Linked Lists in Enzo¶

There are two primary linked lists in Enzo; HierarchyEntry and LevelHierarchyEntry. They’re both used to traverse the hierarchy, but in very different ways. HierarchyEntry is used to traverse down the hierarchy, from a parent to its children. LevelHierarchyEntry is used to traverse across the hierarchy, on a single level.

One of the primary things to note about the two lists is that NextGridThisLevel (which exists in both) serve different purposes.

In LevelHierarchyEntry, NextGridThisLevel links all the grids on a given level together.

In HierarchyEntry, NextGridThisLevel only counts things on a given level that share a parent.

Below we will present a description of the structures and their creation and usage in Enzo.

## HierarchyEntry¶

The HierarchyEntry linked list is used for traversing down the hierarchy, from parents to children.

This is the contents of the definition of the structure, which you can find in src/enzo/Hierarchy.h.

struct HierarchyEntry
{
HierarchyEntry *NextGridThisLevel; /* pointer to the next grid on level */
HierarchyEntry *NextGridNextLevel; /* pointer to first child of this grid */
HierarchyEntry *ParentGrid;        /* pointer to this grid's parent */
grid           *GridData;          /* pointer to this grid's data */
};


NextGridThisLevel connects all children of a parent. NextGridNextLevel points to the first child of the given grid. ParentGrid connects to the parent, and GridData points to the actual grid structure.

### Usage of HierarchyEntry lists¶

The HierarchyEntry list is used (among other things) whenever communication between child and parent grids needs to be done. The typical pattern for looping over all the children of a parent grid is as following:

 1 2 3 4 5 6 7 8  HierarchyEntry * NextGrid = ParentGrid->NextGridNextLevel; while (NextGrid != NULL ){ if (NextGrid->GridData->SomeFunctionOnChildren(args) == FAIL ) fprintf(stderr, "Error in your function\n"); return FAIL; } NextGrid = NextGrid->NextGridThisLevel; } 

Line 1 sets the pointer NextGrid to the “first” child of the parent grid.

Line 2 starts the while loop.

Lines 3-6 is the standard function call pattern in Enzo.

Line 7 advances the pointer to the next child on the child level.

This loop stops once all the children of ParentGrid have been accessed, because the last child grid of a given parent has NULL as NextGridThisLevel.

### Generation of HierarchyEntry lists¶

The HierarchyEntry linked list is generated in several different points in the code. The details are slightly different for each place it’s used, depending on the details of what that linked list is used for and the assumed structure of the hierarchy at that point. The list most used in the code is the one generated in src/enzo/FindSubgrids.C, called in src/enzo/RebuildHierarchy.C. This code is called on a single ‘Parent Grid’ at a time. Paraphrased and annotated:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 HierarchyEntry *, *ThisGrid; PreviousGrid = &ParentGrid; for (i = 0; i < NumberOfSubgrids; i++) { ThisGrid = new HierarchyEntry; if (PreviousGrid == &ParentGrid) ParentGrid.NextGridNextLevel = ThisGrid; else PreviousGrid->NextGridThisLevel = ThisGrid; ThisGrid->NextGridNextLevel = NULL; ThisGrid->NextGridThisLevel = NULL; ThisGrid->ParentGrid = &ParentGrid; ThisGrid->GridData = new grid; ThisGrid->GridData = Setup Functions Skipped for clarity; PreviousGrid = ThisGrid; } 

Line 1 starts the HierarchyEntry list with ParentGrid. (Called simply Grid in the source, changed here for clarity.)

Line 5 creates the next HierarchyEntry to be added to the list.

Line 7-8 attaches the new subgrid, and the ensuing subgrid chain, to the parent grid (note that this is only done for the first new subgrid)

line 10 attaches all subsequent new subgrids to the NextGridThisLevel chain.

Lines 11 and 12 ensure that both lists terminate with this new grid. NextGridThisLevel will be replaced if there is in fact a next grid. Since this routine is called only on a single Parent at a time, one can now see that for HierarchyEntry, the NextGridThisLevel list only links children that belong to the same Parent Grid.

Lines 13-17 finish setting up this grid.

If you’re writing a new problem generator, and have been brought here by the AMR problem generation page, we advise that you examine one of the other code patterns that are used in Enzo. They look fairly similar to the above code, though have some details different. Some suggestions are:

For adding a single subgrid, visit src/enzo/SphericalInfallInitialize.C.

For adding a single stack of nested subgrids, see /src/enzo/ProtostellarCollapseInitialize.C.

For a completely general, though more complex setup, see src/enzo/CosmologySimulationInitialize.C.

Another notable routine that generates HierarchyEntry lists is src/enzo/CommunicationPartitionGrid.C, which breaks the TopGrid pointer across multiple processors.

## LevelHierarchyEntry and LevelArray¶

The LevelHierarchyEntry Linked List is used for traversing all the grids on a given level. It’s a simpler structure than HierarchyEntry. The source can be found in src/enzo/LevelHierarchy.h.

struct LevelHierarchyEntry
{
LevelHierarchyEntry *NextGridThisLevel;  /* next entry on this level */
grid                *GridData;           /* pointer to this entry's grid */
HierarchyEntry      *GridHierarchyEntry; /* pointer into hierarchy */
};


NextGridThisLevel connects all grids on a given level. GridData points to the actual grid object, and GridHierarchyEntry points to the (unique) HierarchyEntry node discussed above.

The LevelHierarchyEntry lists, one for each populated level, are all bundled together in the LevelArray object. Both data structures will be discussed presently.

### Usage of LevelHierarchyEntry and LevelArray¶

The main usage of the LevelHierarchyEntry list is quite similar to the main loop for HierarchyEntry lists.

LevelHierarchyEntry *Temp = LevelArray[level];
while (Temp != NULL) {
if (Temp->GridData->MyCode(MyArgs) == FAIL) {
fprintf(stderr, "Error in grid->SetExternalBoundaryValues.\n");
return FAIL;
}
Temp = Temp->NextGridThisLevel;
}


This calls MyCode for each grid on level.

### Generation of LevelHierarchyEntry and LevelArray¶

This is done in two places in the code: in src/enzo/main.C main.C and src/enzo/RebuildHierarchy.C. It’s done by the code src/enzo/LevelHierarchy_AddLevel.C, which is described below.

The setup, prep in main.C:

for (int level = 0; level < MAX_DEPTH_OF_HIERARCHY; level++)
LevelArray[level] = NULL;


The call in main():

AddLevel(LevelArray, &TopGrid, 0);


The fill:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 void AddLevel(LevelHierarchyEntry *LevelArray[], HierarchyEntry *Grid, int level) { LevelHierarchyEntry *ThisLevel; /* create a new LevelHierarchyEntry for the HierarchyEntry Grid and insert it into the head of the linked list (LevelArray[level]). */ ThisLevel = new LevelHierarchyEntry; ThisLevel->GridData = Grid->GridData; ThisLevel->NextGridThisLevel = LevelArray[level]; ThisLevel->GridHierarchyEntry = Grid; LevelArray[level] = ThisLevel; /* recursively call this for the next grid on this level. */ if (Grid->NextGridThisLevel != NULL) AddLevel(LevelArray, Grid->NextGridThisLevel, level); /* ... and then descend the tree. */ if (Grid->NextGridNextLevel != NULL) AddLevel(LevelArray, Grid->NextGridNextLevel, level+1); } 

This is a recursive function that takes LevelArray that’s to be filled, the HierarchyEntry list that fills it, and a counter for the level. It’s recursive in both HierarchyEntry’s lists, both NextGridNextLevel and NextGridThisLevel. The most notable lines are 11, 13, and 17. In lines 11 and 13, one can see that the current HierarchyEntry is attached to the HEAD of the list, but line 17 shows that the HierarchyEntry list is traversed from its head to its tail: so the LevelArray list is backwards from the HierarchyEntry. This is only really needed information on the top grid.

## Traversing the Entire Hierarchy¶

Sometimes the user needs to traverse the entire hierarchy. This is done with a recursive function call on the HierarchyEntry. This should be done in a manner akin to the AddLevel code above.