# Feature Specification: Geek Calculator

**Feature Branch**: `001-build-a-single`  
**Created**: 2025-10-03  
**Status**: Draft  
**Input**: User description: "Build a single-file, offline-capable "Geek Calculator" that opens as index.html with no build step and no external CDN. User goals: - Perform basic arithmetic (+ − × ÷), parentheses, percentages, +/- toggle. - Power-user features: RPN mode toggle, keyboard-first operation, command palette (\"@\"), and history with re-run. - "Geek vibe" UI: dark terminal theme, monospace font, ASCII banner, blinking cursor in input, small Easter eggs. Constraints: - Pure HTML/CSS/JS, no frameworks, total payload < 50KB. - Works fully offline; no network requests. - A11y: full keyboard navigation, ARIA roles, high-contrast. Non-goals: - Scientific/trig functions, i18n for v1. Success metrics: - Lighthouse Perf/Best Practices/SEO/Accessibility ≥ 95 locally. - 100% keyboard coverage for primary flows. - Unit tests for core math + RPN stack."

## Execution Flow (main)
```
1. Parse user description from Input
   → If empty: ERROR "No feature description provided"
2. Extract key concepts from description
   → Identify: actors, actions, data, constraints
3. For each unclear aspect:
   → Mark with [NEEDS CLARIFICATION: specific question]
4. Fill User Scenarios & Testing section
   → If no clear user flow: ERROR "Cannot determine user scenarios"
5. Generate Functional Requirements
   → Each requirement must be testable
   → Mark ambiguous requirements
6. Identify Key Entities (if data involved)
7. Run Review Checklist
   → If any [NEEDS CLARIFICATION]: WARN "Spec has uncertainties"
   → If implementation details found: ERROR "Remove tech details"
8. Return: SUCCESS (spec ready for planning)
```

---

## ⚡ Quick Guidelines
- ✅ Focus on WHAT users need and WHY
- ❌ Avoid HOW to implement (no tech stack, APIs, code structure)
- 👥 Written for business stakeholders, not developers

### Section Requirements
- **Mandatory sections**: Must be completed for every feature
- **Optional sections**: Include only when relevant to the feature
- When a section doesn't apply, remove it entirely (don't leave as "N/A")

### For AI Generation
When creating this spec from a user prompt:
1. **Mark all ambiguities**: Use [NEEDS CLARIFICATION: specific question] for any assumption you'd need to make
2. **Don't guess**: If the prompt doesn't specify something (e.g., "login system" without auth method), mark it
3. **Think like a tester**: Every vague requirement should fail the "testable and unambiguous" checklist item
4. **Common underspecified areas**:
   - User types and permissions
   - Data retention/deletion policies  
   - Performance targets and scale
   - Error handling behaviors
   - Integration requirements
   - Security/compliance needs

---

## User Scenarios & Testing *(mandatory)*

### Primary User Story
As a user, I want to open the Geek Calculator and perform basic arithmetic operations (addition, subtraction, multiplication, division) with immediate results, so that I can quickly calculate mathematical expressions without needing an internet connection or installing additional software.

### Acceptance Scenarios
1. **Given** I am on the Geek Calculator page with a dark terminal-themed UI and monospace font, **When** I input a basic arithmetic expression like "5 + 3 * 2" using the keyboard, **Then** I see the correct result (11) displayed and formatted with the "geek vibe" aesthetic.
2. **Given** I want to perform calculations without a mouse, **When** I use keyboard shortcuts to enter numbers and operations, **Then** the calculator responds to my input and shows results instantly with keyboard-first navigation.
3. **Given** I have performed several calculations, **When** I access the history feature, **Then** I can see my previous calculations and re-run them to reproduce the same results.
4. **Given** I am a power user familiar with RPN calculators, **When** I toggle the RPN mode and use postfix notation for calculations, **Then** the calculator correctly processes expressions in RPN format rather than standard infix notation.

### Edge Cases
- What happens when I input an invalid expression like "5 // 0" or "5 + * 3"? [NEEDS CLARIFICATION: What error handling behavior is expected for invalid expressions?]
- How does the system handle very large numbers or calculations that result in overflow? [NEEDS CLARIFICATION: What is the expected behavior for calculations that exceed numerical limits?]
- What happens when the calculation history becomes very long - is there a limit or does it scroll? [NEEDS CLARIFICATION: Are there limits to calculation history storage?]
- How does the calculator handle the percentage operation in different contexts (e.g., "50% of 100" vs "100 + 50%")? [NEEDS CLARIFICATION: What are the specific percentage operation behaviors expected?]
- What happens if someone tries to use scientific functions (non-goal) - does it show an error or ignore? [NEEDS CLARIFICATION: How should the system respond to inputs that exceed the v1 scope?]

## Requirements *(mandatory)*

### Functional Requirements
- **FR-001**: System MUST perform basic arithmetic operations (+, -, ×, ÷) with correct order of operations precedence
- **FR-002**: System MUST support parentheses for expression grouping and control of evaluation order
- **FR-003**: Users MUST be able to use percentage operations in calculations
- **FR-004**: Users MUST be able to toggle the sign of numbers using a +/- feature
- **FR-005**: System MUST provide an RPN mode toggle that switches between standard and Reverse Polish Notation calculation methods
- **FR-006**: System MUST support keyboard-first operation with all functions accessible via keyboard shortcuts
- **FR-007**: System MUST provide a command palette feature accessible via the "@" key
- **FR-008**: System MUST maintain a calculation history that users can review and re-run
- **FR-009**: System MUST provide a "geek vibe" UI with dark terminal theme, monospace font, and ASCII banner
- **FR-010**: System MUST display a blinking cursor in the input field for terminal-like experience
- **FR-011**: System MUST include small Easter eggs for enhanced user experience
- **FR-012**: System MUST work fully offline with no network requests required
- **FR-013**: System MUST store calculation history locally for offline access
- **FR-014**: System MUST provide full keyboard navigation for accessibility compliance
- **FR-015**: System MUST implement ARIA roles for accessibility compliance
- **FR-016**: System MUST support high-contrast mode for accessibility
- **FR-017**: System MUST maintain total payload under 50KB
- **FR-018**: System MUST provide unit tests for core math operations
- **FR-019**: System MUST provide unit tests for RPN stack operations

### Key Entities
- **Calculation Expression**: Represents the mathematical expression being entered or evaluated, including operands, operators, and parentheses grouping
- **Calculation Result**: The numerical result of a completed calculation, with formatting that matches the "geek vibe" aesthetic
- **RPN Stack**: A data structure used in Reverse Polish Notation mode to manage operands during calculations
- **Calculation History Entry**: A record of previous calculations, including the expression, result, and timestamp, that can be recalled and re-executed
- **Application Settings**: Configuration options for the calculator including theme preferences, RPN vs standard mode, keyboard shortcuts, and accessibility settings
- **User Input**: The current expression being entered by the user, which may be in progress or ready for evaluation

---

## Review & Acceptance Checklist
*GATE: Automated checks run during main() execution*

### Content Quality
- [ ] No implementation details (languages, frameworks, APIs)
- [x] Focused on user value and business needs
- [x] Written for non-technical stakeholders
- [x] All mandatory sections completed

### Requirement Completeness
- [ ] No [NEEDS CLARIFICATION] markers remain
- [x] Requirements are testable and unambiguous  
- [x] Success criteria are measurable
- [x] Scope is clearly bounded
- [x] Dependencies and assumptions identified

---

## Execution Status
*Updated by main() during processing*

- [x] User description parsed
- [x] Key concepts extracted
- [x] Ambiguities marked
- [x] User scenarios defined
- [x] Requirements generated
- [x] Entities identified
- [ ] Review checklist passed

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