// RPN (Reverse Polish Notation) calculator module
const RPNCalculator = (function() {
    class RPNCalculator {
        constructor() {
            this.clear();
        }

        // Push a number onto the stack
        push(value) {
            if (typeof value === 'number' || !isNaN(parseFloat(value))) {
                this.stack.push(parseFloat(value));
            }
        }

        // Pop a number from the stack
        pop() {
            if (this.stack.length > 0) {
                return this.stack.pop();
            }
            return undefined;
        }

        // Perform an operation on stack values
        operate(operator) {
            if (this.stack.length < 2) {
                return 'Error'; // Need at least 2 values for binary operations
            }

            const b = this.pop();
            const a = this.pop();
            let result;

            switch (operator) {
                case '+':
                    result = a + b;
                    break;
                case '-':
                    result = a - b;
                    break;
                case '*':
                    result = a * b;
                    break;
                case '/':
                    if (b === 0) {
                        result = Infinity;
                    } else {
                        result = a / b;
                    }
                    break;
                case '^':
                case '**':
                    result = Math.pow(a, b);
                    break;
                default:
                    return 'Error'; // Unknown operator
            }

            // Check for invalid results
            if (isNaN(result) || !isFinite(result)) {
                this.push('Error');
                return 'Error';
            }

            this.push(result);
            return result;
        }

        // Clear the entire stack
        clear() {
            this.stack = [];
        }

        // Get current stack state
        getStack() {
            return [...this.stack]; // Return a copy to prevent external modification
        }

        // Execute RPN expression (e.g., "3 4 +")
        evaluate(rpnExpression) {
            try {
                // Split the expression into tokens
                const tokens = rpnExpression.trim().split(/\s+/);
                
                this.clear(); // Clear the stack before evaluation
                
                for (const token of tokens) {
                    if (['+', '-', '*', '/', '^', '**'].includes(token)) {
                        // It's an operator
                        const result = this.operate(token);
                        if (result === 'Error') {
                            return 'Error';
                        }
                    } else {
                        // It's a number
                        const num = parseFloat(token);
                        if (isNaN(num)) {
                            return 'Error'; // Invalid token
                        }
                        this.push(num);
                    }
                }

                // Return the top of the stack if there's a result
                if (this.stack.length === 1) {
                    return this.stack[0];
                } else if (this.stack.length === 0) {
                    return 0; // Empty stack, return 0
                } else {
                    return this.stack[this.stack.length - 1]; // Return top of stack
                }
            } catch (error) {
                return 'Error';
            }
        }

        // Process a single RPN input token
        processInput(token) {
            if (['+', '-', '*', '/', '^', '**'].includes(token)) {
                // Operator
                return this.operate(token);
            } else if (token === 'ENTER' || token === 'E') {
                // ENTER doesn't do anything in implementation, just pushes numbers that were already processed
                return this.getStack();
            } else if (token === 'C' || token === 'CE') {
                this.clear();
                return 0;
            } else {
                // Number
                const num = parseFloat(token);
                if (!isNaN(num)) {
                    this.push(num);
                    return num;
                } else {
                    return 'Error';
                }
            }
        }
    }

    return { RPNCalculator };
})();