After hacking for about two hours the cell dependencies yesterday using dicts, I found myself saying "how can I check if the dependency graph is cyclical?"

And of course it hit me. That's the dependency **graph**. Use a graph library!

I looked for about 2 minutes before finding one that's probably not optimal, but is damn simple and in the public domain: graph_lib.py by Nathan Denny.

First interesting data point, the first two lines on the file (yes, I found out there is a later version):

#--Version 1.0.0
#--Nathan Denny, May 27, 1999

Yup. In two days this piece of code is turning 8 years old and untouched. But it works just fine and dandy!

Here's the piece of code that makes the engine run which has grown from a humble 10 LOC to almost a whooping 40 LOC:

```
class SpreadSheet(QtCore.QObject):
_cells = {}
tools = {}
graph=Graph()
def __init__(self,parent):
QtCore.QObject.__init__(self,parent)
for name in dir(functions):
self.tools[name]=eval('functions.'+name)
def __setitem__(self, key, formula):
key=key.lower()
c=traxcompile('%s=%s;'%(key,formula))
self._cells[key] = [c[key][0],False,compile(c[key][0],"Formula for %s"%key,'eval')]
# Dependency graph
if not self.graph.has_node(key):
self.graph.add_node(key)
for edge in self.graph.in_arcs(key):
self.graph.delete_edge(edge)
for cell in c[key][1]:
self.graph.add_edge(cell,key)
try:
print 'GRAPH(TOPO): ',self.graph.topological_sort()
self._cells[key][1]=False
print 'GRAPH(BFS) : ',self.graph.bfs(key)
for cell in self.graph.bfs(key)[1:]:
self.emit(QtCore.SIGNAL('changed'),(cell))
except Graph_topological_error:
# We made the graph cyclic
# So, mark this cell as evil
self._cells[key][1]=True
# And remove all incoming edges to go back to
# status quo
for edge in self.graph.in_arcs(key):
self.graph.delete_edge(edge)
def getformula(self, key):
key=key.lower()
return self._cells[key][0]
def __getitem__(self, key ):
print self._cells[key]
if self._cells[key][1]:
return "ERROR: cyclic dependency"
else:
print 'evaluating [%s]: '%key,type(self._cells[key][0]),self._cells[key][0]
return eval(self._cells[key][0], self.tools, self)
```

This engine supports:

User defined functions (just add them in formula.py)

Cell dependencies with detection of cicles, of any length

Unlimited size spreadsheets

Notification of cell changes to other modules

Automatic recalculation

A custom formula language (ok, that's not in this piece of code ;-)

The whole project is now 1167 LOC, of which 591 are generated or graph_lib, which means everything including the traxter "compiler" and the UI is less than 600 lines of code.

Not too shabby.

My goal is a functional spreadsheet with a working GUI and supporting the most common functions and pieces of the formula language in .... 2000 LOC, and I am willing to work on it for about two weeks.

Let's see how it turns out. Of course anyone with 5 free minutes can contribute his favourite spreadsheet function (I already have sum ;-)