# LogiSheets Controller

`Controller` is the core crate for LogiSheets.
`Controller` handles the user actions and returns the display information.

Here we will introduce some implementations.

## Important Components

- `Cell Id Manager` assigns Cell Id for every cell.
Cell Id identifies a cell even this cell is removed.
Cell Id is different from the cell position which would be changed after
inserting or removing some rows or columns.
You may find that we also use **Sheetid** to identify a worksheet,
and use **NameId** to identify a name. That's because in a spreadsheet
application, almost everything can be renamed or moved. Id Manager is
the foundation.

- `Container` stores the **CellId** and **Cell** map. The content
  of a cell should be visited by its **CellId**.

- `Navigator` is responsible for making convertion between cell id and
cell position. For now, LogiSheets do only support inserting or removing rows and columns,
we use a simple algorithm to realize this.
`Navigator` stores a vector of **RowId** and a vector of **ColId**, and we use a tuple
(**RowId**, **ColId**) as a **CellId**. After a `navigator` of a sheet is initialized,
every row and column has an Id and therefore, every cell can be identified by (**RowId**, **ColId**).
For example, when indexing a cell by position, like **B4** or r4c2, `Navigator` takes
the 4th **RowId** and 2nd **ColId**, and you can then get the **CellId**.
Users' inserting or removing rows/cols will do some effect on the **RowId** vector or **ColId** vector.

- `VertexManager` is responsible for recording the dependencies for calculating, updating
  the fomula ast and providing the dirty vertices and ast to `CalcEngine`.

- `Connectors` implements the traits that helps communicate between components.
  We define some traits to make abstraction barrier between components, which helps to decouple
  them and makes it easier to write unittests.

## Undo / Redo

We use [persistent data structure](https://en.wikipedia.org/wiki/Persistent_data_structure)
to store the state of a workbook, which helps us easyily implement an undo/redo system.

We use the [`im`](https://github.com/bodil/im-rs) crate to do this. It would be nice if
you have similar background to maintain this crate.