Issue 10 | Phase A: Parallel Development of Computing Engine

app.js (created by logic-dev) — Shunting-yard Calculation Engine

// Operator precedence
const PRECEDENCE = {
  '+': 1, '-': 1,
  '×': 2, '*': 2, '÷': 2, '/': 2,
  '%': 3, 'unary-': 4
};

// Lexical analysis
function tokenize(expression) {
  // "1+2*3" → ['1', '+', '2', '*', '3']
}

// Shunting-yard algorithm → RPN
function parseExpression(tokens) {
  // ['1', '+', '2', '*', '3'] → ['1', '2', '3', '*', '+']
}

// RPN evaluation
function evaluate(rpn) {
  // Stack-based evaluation, returns "Error" on division by zero
}

// Real-time preview (incomplete expressions are not calculated)
function previewResult(expression) {
  // Mismatched parentheses → returns ''
  // Ends with an operator → returns ''
}

// Input helpers (pure functions, no DOM dependency)
function appendNumber(current, digit) { /* ... */ }
function appendOperator(current, op) { /* ... */ }
function appendDecimal(current) { /* ... */ }
function toggleSign(current) { /* ... */ }

// DOM binding layer (separated from pure functions)
function initCalculator() {
  initElements();
  loadModePreference();
  bindButtonEvents();
  bindKeyboardEvents();
  bindModeToggle();
  updateDisplay();
}

// Export (Node.js test compatibility)
if (typeof module !== 'undefined' && module.exports) {
  module.exports = { tokenize, parseExpression, evaluate, /* ... */ };
}

Architectural Key: Pure Functions and DOM Separation

┌─────────────────────────────────┐
│  Pure Function Layer (DOM-independent) │
│  tokenize → parseExpression → evaluate  │
│  appendNumber, appendOperator, ...     │
│  ✅ Independently testable (Node.js) │
└──────────────┬──────────────────┘
               │
┌──────────────▼──────────────────┐
│  DOM Interaction Layer           │
│  initCalculator, handleButtonClick │
│  bindKeyboardEvents, toggleMode    │
│  ⚠️ Requires browser environment for testing │
└─────────────────────────────────┘