# Working Spec

## Overview

SDLC SIMPCALC3 — Simple Calculator 3 is a browser-based web application that provides the four fundamental arithmetic operations: addition, subtraction, multiplication, and division. The application is delivered as a single-page web app through the AI-SDLC pipeline. It is intended to be lightweight, accessible, and easy to use without any installation or account creation.

## Actors

| Actor | Description |
|---|---|
| **End User** | Any person who opens the web app in a browser to perform arithmetic calculations. No authentication or account is required. |

## Functional Requirements

1. **Addition** — The user can enter two numeric operands and compute their sum.
2. **Subtraction** — The user can enter two numeric operands and compute their difference.
3. **Multiplication** — The user can enter two numeric operands and compute their product.
4. **Division** — The user can enter two numeric operands and compute their quotient.
5. **Division by zero handling** — When the divisor is zero, the application displays a clear error message rather than crashing or displaying `Infinity`/`NaN`.
6. **Result display** — The computed result is displayed prominently on screen immediately after the user triggers the calculation.
7. **Clear / reset** — The user can clear the current inputs and result to start a new calculation.
8. **Keyboard input** — The user can enter operands and trigger calculations using a standard keyboard.
9. **Decimal number support** — The application accepts and correctly computes floating-point (decimal) operands.
10. **Operator selection** — The user can select the desired arithmetic operator via the UI (e.g., buttons or a dropdown).

## Non-Functional Requirements

| ID | Category | Requirement |
|---|---|---|
| NFR-1 | Performance | The result must appear within 100 ms of user action on any modern browser. |
| NFR-2 | Usability | The UI must be usable on desktop and mobile screen sizes (responsive design). |
| NFR-3 | Accessibility | The app must meet WCAG 2.1 Level AA contrast and keyboard-navigation requirements. |
| NFR-4 | Compatibility | The app must function correctly in the latest stable versions of Chrome, Firefox, Safari, and Edge. |
| NFR-5 | Maintainability | Code must be modular and covered by unit tests for all arithmetic logic. |
| NFR-6 | Deployability | The app must be deployable as a static site (no server-side runtime required). |

## Assumptions

1. The target user base requires no server-side persistence; all computation is performed client-side in the browser.
2. No user authentication, account management, or history/logging feature is in scope for this version.
3. The calculator operates on two operands per operation (binary operations only); chained multi-step expression input (e.g., a full expression parser) is out of scope.
4. Numeric precision follows standard IEEE 754 double-precision floating-point; no arbitrary-precision arithmetic is required.
5. The application will be delivered as a static web app (HTML/CSS/JavaScript); a specific front-end framework will be determined during technical architecture.
6. Internationalisation (i18n) and localisation (l10n) are out of scope for this version.
7. Offline (PWA/service-worker) support is not required for this version.