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## Thursday, July 19, 2012

Q: A spreadsheet consists of a two-dimensional array of cells, labelled A1, A2, etc. Rows are identified using letters, columns by numbers. Each cell contains either a numeric value or an expression. Expressions contain numbers, cell references, and any combination of '+', '-', '*', '/' (4 basic operators). Task: Compute all RPN expressions, or point out one of the Cyclic dependency paths.

--------- Here is "our" design ----------
I feel it's unwise to start by detecting circles. Better concretize as many cells as possible in Phase 1.

* First pass -- construct all the RPN objects and corresponding cell objects. An RPN holds all the concrete or symbolic tokens. A cell has an rpn and also a cell name. If a cell is completely concrete, then calculate the result, and add the cell to a FIFO queue.

Also construct a p2d or precedent2dependent<Name, Set<Name> > map. It's a look-up map of <Name, set<Name> > This will help us fire update events. If you wonder why use Name. In this context, name is a unique identifier for a cell. I use a simple hash map<Name,Cell>.

* 2nd pass -- process the queue of concrete cells. For every cell removed from the queue, get its name and concrete value into a pair (call it ppair since it's a Precedent). Look up p2d to get all dependents. Fire update events by feeding the ppair to each dependent cell, which will use the ppair to concretize (part of) its expression. If any dependent cell gets fully concretized, add it to the queue.

Remember to remove the ppair cell from p2d.

Only 2 data structures needed -- queue and p2d.

* Phase 1 over when queue depletes. If p2d is still non-empty, we have cyclic dependency (Phase 2). All concrete cells have been "applied" on the spreadsheet yet some cells still reference other cells.

All remaining cells are guaranteed to be involved in some circle(s). To print out one circle, just start from any cell and follow first link and you are bound to hit the starting point.