# AutoFill Module data flow

## What It Does
AutoFill automatically fills cells using pattern recognition, series generation, and copying. Two-layer architecture: **AutoFill (UI)** handles user interactions, **WorkbookAutoFill (Core)** executes filling logic.

## Entry Points

**AutoFill (UI Layer)**
- `performAutoFill()` - Triggered by drag handle interaction
- `getAutoFillRange()` - Calculates fill range from drag position

**WorkbookAutoFill (Core Layer)**
- `autoFill()` - Main coordinator routing to fillSeries() or copyCells()
- `getFillInfo()` - Analyzes data and returns fillType + disabled menu items
- Event listener on `setAutoFill` notification

## Core Logic Flow

```
User Drag → performAutoFill() → getAutoFillRange() → setAutoFill Notification
    ↓
WorkbookAutoFill.autoFill()
    ↓
getFillInfo()
    ↓
(fillType = numeric? → fillSeries() : copyCells())
```

## Operations Handled

1. **fillSeries()** - Detects pattern (number/date/formula/string/time) and generates next values using:
   - `getDataPattern()` - Identifies value types and format
   - `getPattern()` - Generates predictions via linear regression
   - `getPredictionValue()` - Calculates a, b coefficients for trend line

2. **copyCells()** - Simple duplication:
   - Copies all cell properties cyclically across target range
   - Optional: preserve original values (FillFormattingOnly)
   - Handles read-only/validation/notes removal

3. **Pattern Types**:
   - Numbers: Linear/geometric progression (1,2,3 or 2,4,6)
   - Dates: Calendar-aware increments (1/1, 1/2, 1/3)
   - Formulas: Updates references (A1→A2, $A$1→$A$1, mixed)
   - Strings: Month/day sequences
   - Time: Duration patterns (1:00, 1:15, 1:30)

## Fill Types

| Type | Behavior |
|------|----------|
| CopyCells | Repeat values + formatting |
| FillSeries | Calculate series + formatting |
| FillFormattingOnly | Copy format, keep values |
| FillWithoutFormatting | Calculate series, no format |

## Validation & Safety

- **Read-only cells**: Alert + stop
- **Protected sheets**: Check locked cells, prevent fill
- **Merged cells**: Handle with adjusted ranges
- **Hidden rows/cols**: Skip silently
- **Conditional formats**: Refresh after fill
- **Hyperlinks**: Preserve/remove per fillType

## Desired Outputs

**User-Facing:**
- Autofill handle at selection corner
- Dropdown menu with 4 fill options
- Smart series/duplication based on source
- Expandable selection on drag

**System-Level:**
- Updated cells with calculated/copied values
- Formatting applied per fillType
- Validation rules preserved
- Row heights adjusted
- Undo/redo recorded
- Events: beginAction, completeAction