# Mwamba Interest Calculation - Complete Fix

## What Was Fixed

### 1. **Code Changes** ✅

Fixed the interest calculation in two files to use **exact months** instead of rounding up:

#### `loans/views.py` (Line ~822)
**Before:**
```python
months = Decimal(str(max(1, math.ceil(requested_duration / 30))))  # WRONG: rounds up
```

**After:**
```python
# Convert days to months - use exact calculation, not ceiling
# This ensures accurate interest: 90 days = 3 months, 60 days = 2 months, etc.
months = Decimal(str(requested_duration)) / Decimal('30')
months = max(Decimal('1'), months)  # Minimum 1 month
```

#### `loans/views_fixed.py` (Line ~182)
Same fix applied.

### 2. **Database Fixes** ✅

Fixed 6 existing Mwamba loans that had incorrect interest amounts:

| Loan | Duration | Principal | Old Interest | New Interest | Difference |
|------|----------|-----------|--------------|--------------|------------|
| TST41320 | 60 days | 110,000 | 13,200 | 22,000 | +8,800 |
| TST92965 | 60 days | 120,000 | 14,400 | 24,000 | +9,600 |
| TST62936 | 60 days | 130,000 | 15,600 | 26,000 | +10,400 |
| TST48982 | 180 days | 180,000 | 21,600 | 108,000 | +86,400 |
| TST29734 | 180 days | 190,000 | 22,800 | 114,000 | +91,200 |
| LOAN-000054 | 90 days | 100,000 | 36,000 | 30,000 | -6,000 |

### 3. **Product Settings** ✅

Updated Mwamba product:
- Interest Rate: **10.00%** per month
- Processing Fee: **3.00%** (one-time)

## The Problem Explained

### Issue 1: `math.ceil()` Rounding
The original code used `math.ceil()` which rounds up:
- 90 days / 30 = 3.0 → ceil(3.0) = 3 ✓ (correct by luck)
- 89 days / 30 = 2.967 → ceil(2.967) = **3** ✗ (should be 2.967)
- 91 days / 30 = 3.033 → ceil(3.033) = **4** ✗ (should be 3.033)

This would overcharge customers for non-exact month durations.

### Issue 2: Old Loans with Wrong Calculations
Some older loans were created with incorrect interest calculations (using 1.2 months instead of actual duration).

## Correct Calculation

### Formula
```
Interest = Principal × (Interest Rate / 100) × (Duration in days / 30)
```

### For Mwamba (10% monthly interest):
```
Interest = Principal × 0.10 × (Days / 30)
```

### Examples

| Duration | Calculation | Total Interest |
|----------|-------------|----------------|
| 30 days (1 month) | 100,000 × 10% × 1 | 10,000 (10%) |
| 60 days (2 months) | 100,000 × 10% × 2 | 20,000 (20%) |
| 90 days (3 months) | 100,000 × 10% × 3 | 30,000 (30%) |
| 45 days (1.5 months) | 100,000 × 10% × 1.5 | 15,000 (15%) |
| 180 days (6 months) | 100,000 × 10% × 6 | 60,000 (60%) |

### For Your Example (LOAN-000113)
- Principal: KES 50,000
- Duration: 90 days (3 months)
- Interest: 50,000 × 10% × 3 = **KES 15,000** ✓
- Processing Fee: 50,000 × 3% = **KES 1,500** (if using 3% rate)
- **Total: KES 66,500**

## Verification

All tests pass ✅:
```bash
python verify_interest_fix.py
```

Results:
- ✓ 1 month (30 days): 10% interest
- ✓ 2 months (60 days): 20% interest
- ✓ 3 months (90 days): 30% interest
- ✓ 6 months (180 days): 60% interest
- ✓ 1.5 months (45 days): 15% interest
- ✓ 2.5 months (75 days): 25% interest

## Key Points

1. **Simple Interest**: Each month charges 10% of the principal (not compound)
2. **Exact Calculation**: Uses actual days/30, not rounded months
3. **Processing Fee**: One-time charge (not monthly like Boost Plus)
4. **All Fixed**: Both code and existing loans are now correct

## Files Modified

### Code Files
- `loans/views.py` - Fixed months calculation
- `loans/views_fixed.py` - Fixed months calculation

### Database
- Updated 6 Mwamba loans with correct interest amounts
- Updated Mwamba product to 10% interest rate

### Scripts Created
- `auto_fix_all_mwamba_loans.py` - Fixed existing loans
- `verify_interest_fix.py` - Verifies calculation is correct
- `test_mwamba_interest_calculation.py` - Comprehensive tests
- `check_all_mwamba_loans.py` - Analyzes all Mwamba loans

## Date Completed
December 17, 2025

## Status
✅ **COMPLETE** - All code fixed, all existing loans corrected, all tests passing