# Iterative calculation

Iterative calculation allows circular references to converge over repeated evaluations, similar to Excel. This is useful for finance models where the result depends on its own output.

## Enable it

```ts
const state = useSpreadsheetState({
  iterativeCalculation: {
    enabled: true,
    maxIterations: 100,
    maxChange: 0.001,
  },
});
```

Notes:

* Defaults match Excel (100 iterations, 0.001 max change).
* When disabled, circular references return `#REF!`.
* When enabled, non-converging formulas return `#NUM!`.

## Supported modes

* **Single-threaded:** `useCalculation` runs iterative loops on the UI thread.
* **Worker mode:** `useCalculationWorker` delegates iterative groups to the worker.

## Example formulas to test

Converging loop:

```
// A1
0
// A1 (replace)
=(A1+10)/2
```

Expected: A1 converges to \~10.

Non-converging loop:

```
// A2
0
// A2 (replace)
=A2+1
```

Expected: A2 returns `#NUM!`.

## Testing

Unit tests for single-threaded iterative calculation:

```sh
yarn workspace @rowsncolumns/spreadsheet-state test -- use-calculation.spec.ts
```

Worker iterative tests:

```sh
yarn workspace @rowsncolumns/calculation-worker test
```