# Dynamic Sheet Handling Fix

## Problem
The application was failing with "Sheet 'US URGENT' not found in data" error when working with Excel files that don't contain a sheet named 'US URGENT'. The code needed to be made dynamic to work with any sheet names.

## Root Cause
The issue was in the web interface JavaScript where:
1. On page load, `analyzeData()` was called immediately
2. The sheet filter dropdown was empty at that time
3. An empty string was being sent as `sheet_filter` to the backend
4. The backend wasn't properly handling empty strings

## Fixes Applied

### 1. Frontend JavaScript Improvements (web_gui.py)

#### Fixed `analyzeData()` function:
```javascript
// Before: const sheetFilter = document.getElementById('sheetFilter').value;
// After: 
const sheetFilterElement = document.getElementById('sheetFilter');
const sheetFilter = sheetFilterElement.value || null; // Use null if empty
```

#### Improved `updateSheetFilter()` function:
- Added handling for empty sheet lists
- Added loading state for sheet dropdown
- Better initialization logic

#### Enhanced page load handling:
```javascript
window.onload = function() {
    // Initialize sheet filter with loading state
    updateSheetFilter([], null);
    analyzeData();
};
```

### 2. Backend Improvements (web_gui.py)

#### Enhanced request handling:
```python
# Handle empty string as None
if sheet_filter == '' or sheet_filter == 'undefined':
    sheet_filter = None
```

#### Added debugging information:
```python
# Debug: Print available sheets
available_sheets = list(comparator_instance.data.keys())
print(f"Available sheets: {available_sheets}")
print(f"Requested sheet_filter: {repr(sheet_filter)}")
```

### 3. Core Logic Improvements (data_comparator.py)

#### Enhanced error handling:
```python
# Validate that the requested sheet exists
if sheet_filter not in sheet_names:
    raise ValueError(f"Sheet '{sheet_filter}' not found in data. Available sheets: {sheet_names}")
```

## How It Works Now

1. **Page Load**: 
   - Sheet dropdown shows "Loading sheets..."
   - `analyzeData()` is called with `sheet_filter = null`

2. **Backend Processing**:
   - Loads Excel file and gets available sheet names
   - If `sheet_filter` is `null` or empty, defaults to first sheet
   - Validates sheet exists before processing

3. **Response**:
   - Returns results with `sheet_names` array and `current_sheet_filter`
   - Frontend populates dropdown with actual sheet names
   - User can then select different sheets

4. **Sheet Selection**:
   - User selects different sheet from dropdown
   - `filterBySheet()` calls `analyzeData()` with selected sheet
   - Backend processes the specific sheet

## Benefits

✅ **Dynamic**: Works with any Excel file regardless of sheet names
✅ **Robust**: Handles empty/invalid sheet names gracefully  
✅ **User-friendly**: Shows available sheets in dropdown
✅ **Error-resistant**: Provides clear error messages
✅ **Backward compatible**: Still works with existing functionality

## Test Coverage

Created `test_dynamic_sheets.py` to verify:
- Default sheet selection (no filter)
- Each available sheet works
- Invalid sheet names are handled
- Empty string sheet names default properly

## Usage Examples

```python
# Works with any Excel file
comparator = KSTCoordiComparator("any-file.xlsx")
comparator.load_data()

# Auto-selects first sheet
summary = comparator.get_comparison_summary()

# Or specify any existing sheet
summary = comparator.get_comparison_summary("Sheet1")
summary = comparator.get_comparison_summary("Data")
summary = comparator.get_comparison_summary("US URGENT")  # Only if it exists
```

The application now dynamically adapts to any Excel file structure!