Partner_Project/src/model/PrimaryMaker.java

package model;

import java.util.Stack;

/**
 * 小学题目生成器，确保题目结果非负
 */
public class PrimaryMaker extends Student {
    private static final char[] OPERATORS = {'+', '-', '*', '/'};
    private static final int MIN_OPERANDS = 2;
    private static final int MAX_OPERANDS = 5;
    private static final int MAX_OPERAND_VALUE = 100;

    private int operandCount;
    private int[] operands;
    private int parenStart;
    private int parenEnd;

    public PrimaryMaker(String name, String password, String path) {
        super(name, password, path);
    }

    @Override
    protected String makeOneQuestion() {
        String question;
        while (true) {
            question = generateSingleQuestion();
            if (isQuestionValid(question)) {
                break;
            }
        }
        return question;
    }

    private String generateSingleQuestion() {
        generateRandomParameters();
        StringBuilder question = new StringBuilder();

        for (int j = 0; j < operandCount; j++) {
            appendOperandWithParentheses(question, j);

            if (j < operandCount - 1) {
                appendOperator(question, j);
            }
        }
        return question.toString();
    }

    private void generateRandomParameters() {
        operandCount = random.nextInt(MAX_OPERANDS - MIN_OPERANDS + 1) + MIN_OPERANDS;
        operands = new int[operandCount];

        for (int j = 0; j < operandCount; j++) {
            operands[j] = random.nextInt(MAX_OPERAND_VALUE) + 1;
        }

        int[] parenPos = bracketMaker(operandCount);
        parenStart = parenPos[0];
        parenEnd = parenPos[1];
    }

    private void appendOperandWithParentheses(StringBuilder question, int index) {
        if (index == parenStart) {
            question.append("( ");
        }
        question.append(operands[index]);
        if (index == parenEnd) {
            question.append(" )");
        }
    }

    private void appendOperator(StringBuilder question, int currentIndex) {
        char op = OPERATORS[random.nextInt(OPERATORS.length)];

        if (op == '-') {
            ensureSubtractionIsValid(currentIndex);
        }
        if (op == '/') {
            ensureDivisionIsValid(currentIndex);
        }

        question.append(" ").append(op).append(" ");
    }

    private void ensureSubtractionIsValid(int currentIndex) {
        if (operands[currentIndex] < operands[currentIndex + 1]) {
            int temp = operands[currentIndex];
            operands[currentIndex] = operands[currentIndex + 1];
            operands[currentIndex + 1] = temp;
        }
    }

    private void ensureDivisionIsValid(int currentIndex) {
        if (operands[currentIndex + 1] == 0) {
            operands[currentIndex + 1] = 1;
        }
    }

    private boolean isQuestionValid(String expression) {
        try {
            evaluateExpression(expression, true);
            return true;
        } catch (ArithmeticException e) {
            return false;
        }
    }

    private double evaluateExpression(String expression, boolean checkNegative)
            throws ArithmeticException {
        Stack<Double> numbers = new Stack<>();
        Stack<Character> ops = new Stack<>();

        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);
            if (c == ' ') {
                continue;
            }

            if (Character.isDigit(c)) {
                i = parseAndPushNumber(expression, i, numbers);
            } else if (c == '(') {
                ops.push(c);
            } else if (c == ')') {
                processClosingParenthesis(numbers, ops, checkNegative);
            } else if (isOperator(c)) {
                processOperator(c, numbers, ops, checkNegative);
            }
        }

        processRemainingOperators(numbers, ops, checkNegative);

        double finalResult = numbers.pop();
        validateResult(finalResult, checkNegative);
        return finalResult;
    }

    private int parseAndPushNumber(String expression, int startIndex,
                                   Stack<Double> numbers) {
        StringBuilder sb = new StringBuilder();
        int i = startIndex;

        while (i < expression.length()
                && (Character.isDigit(expression.charAt(i)) || expression.charAt(i) == '.')) {
            sb.append(expression.charAt(i++));
        }

        numbers.push(Double.parseDouble(sb.toString()));
        return i - 1;
    }

    private void processClosingParenthesis(Stack<Double> numbers, Stack<Character> ops,
                                           boolean checkNegative) {
        while (ops.peek() != '(') {
            double result = applyOp(ops.pop(), numbers.pop(), numbers.pop());
            validateResult(result, checkNegative);
            numbers.push(result);
        }
        ops.pop();
    }

    private void processOperator(char currentOp, Stack<Double> numbers,
                                 Stack<Character> ops, boolean checkNegative) {
        while (!ops.isEmpty() && hasPrecedence(currentOp, ops.peek())) {
            double result = applyOp(ops.pop(), numbers.pop(), numbers.pop());
            validateResult(result, checkNegative);
            numbers.push(result);
        }
        ops.push(currentOp);
    }

    private void processRemainingOperators(Stack<Double> numbers, Stack<Character> ops,
                                           boolean checkNegative) {
        while (!ops.isEmpty()) {
            double result = applyOp(ops.pop(), numbers.pop(), numbers.pop());
            validateResult(result, checkNegative);
            numbers.push(result);
        }
    }

    private void validateResult(double result, boolean checkNegative) {
        if (checkNegative && result < 0) {
            throw new ArithmeticException("Negative result");
        }
    }

    private boolean isOperator(char c) {
        return c == '+' || c == '-' || c == '*' || c == '/';
    }

    private boolean hasPrecedence(char op1, char op2) {
        if (op2 == '(' || op2 == ')') {
            return false;
        }
        return (op1 != '*' && op1 != '/') || (op2 != '+' && op2 != '-');
    }

    private double applyOp(char op, double b, double a) {
        switch (op) {
            case '+':
                return a + b;
            case '-':
                if (a < b) {
                    throw new ArithmeticException("Negative result in subtraction");
                }
                return a - b;
            case '*':
                return a * b;
            case '/':
                if (b == 0) {
                    throw new ArithmeticException("Cannot divide by zero");
                }
                return a / b;
            default:
                return 0;
        }
    }
}