# Construction Loan Interest Calculator

A web-based tool to model monthly interest accrual on a construction loan with:
- **Interest reserve** funded from the credit limit
- Reserve draws added to principal
- Optional capitalization of uncovered interest
- Multiple draws over the term
- **Day-count basis**: ACT/365 or ACT/360
- **Reserve analysis chart** to visualize trade-offs between total interest and out-of-pocket payments
- **Suggested reserve** calculation based on a user’s out-of-pocket tolerance
- Interactive **draggable handle** on the chart to set reserve

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## Features

### Core Loan Model
- Calculates interest for each month in the loan term.
- Handles multiple draws at arbitrary months.
- Reserve is funded from the credit limit and used to pay interest first.
- Any reserve draw increases principal.
- Option to capitalize uncovered interest (add unpaid interest to principal).

### Display
- **Blue pills** for monthly and total interest amounts.
- **Red pills** for uncovered interest (out-of-pocket) amounts greater than 0.
- Total uncovered interest highlighted prominently above the table.

### Reserve Curve Analysis
- Plots **total interest** (blue) and **out-of-pocket** uncovered interest (red) against different reserve amounts.
- **X-axis is clamped** so reserve never exceeds `credit limit − total planned draws`.
- Interactive vertical handle to adjust the reserve amount directly from the chart.
- Shows current reserve and suggested reserve markers.

### Suggested Reserve
- Input your **target uncovered ($)**.
- Tool runs a binary search to find the smallest reserve that keeps out-of-pocket uncovered ≤ target.
- Suggested reserve is displayed and marked on the chart.

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## Usage

1. **Open application**.
2. Enter:
   - **Credit limit**
   - **APR (%)**
   - **Term (months)**
   - **Start date**
   - **Interest reserve** (optional)
   - **Day-count basis**
   - **Capitalize uncovered interest?**
3. Add draws (date & amount).
4. Click **Compute schedule** to see the monthly breakdown and totals.
5. To analyze reserves, click **Analyze reserve curve**.
6. To get a suggested reserve for your out-of-pocket limit, enter **Target uncovered** and click **Suggest reserve**.
7. Drag the chart’s vertical handle or use arrow keys (← →) to change the reserve interactively.

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## Accessibility
- Chart is keyboard-accessible: use arrow keys to move the handle (Shift+Arrow = ±1000, Arrow = ±100).
- Color-coded interest and uncovered interest for quick scanning.

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## Tech Stack
- HTML, CSS, JavaScript (ES modules)
- [`date-fns`](https://date-fns.org/) for date calculations
- Canvas API for chart rendering

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## License
MIT