# Data Model: Geek Calculator

## Entities

### Calculation Expression
- **Representation**: String containing numbers, operators (+, -, ×, ÷), parentheses, percentage, +/- signs
- **Validation**: Must follow valid mathematical expression syntax rules
- **State transitions**: In-progress expression → evaluated expression → stored in history

### Calculation Result 
- **Representation**: Number (JavaScript number type, may be Infinity or finite value)
- **Validation**: Must be a numeric result from evaluation
- **State transitions**: Calculated from expression → displayed → stored in history

### RPN Stack
- **Representation**: Array of numbers representing operands in Reverse Polish Notation
- **Operations**: Push, pop, peek, clear, size
- **Validation**: Elements must be valid numbers
- **State transitions**: Empty → populated with operands → modified through RPN operations

### Calculation History Entry
- **Fields**: 
  - expression (string): The original input expression
  - result (number): The calculated result
  - timestamp (Date): When the calculation was completed
  - id (string): Unique identifier for recall
- **Validation**: Expression and result must be valid
- **State transitions**: New entry → stored → accessed → potentially deleted when limit reached

### Application Settings
- **Fields**:
  - theme (string): 'dark' or other theme options
  - mode (string): 'standard' or 'rpn' for calculation mode
  - historyLimit (number): Maximum number of history entries to store
- **Validation**: Values must be from predefined sets
- **State transitions**: Default settings → user modified → saved to localStorage

### User Input State
- **Fields**:
  - currentValue (string): The current value being entered
  - operator (string): The current operator in use
  - previousValue (number): The previous operand
  - calculationPending (boolean): Whether a calculation is ready to execute
- **Validation**: Values must be consistent with calculator state
- **State transitions**: Initial state → value entry → operator selection → result calculation