init geek calc

specs/001-build-a-single/data-model.md

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+# 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