projectone/ElementaryProblemGenerator.java

import java.text.DecimalFormat;
import java.util.Random;

public class ElementaryProblemGenerator extends AbstractProblemGenerator {
    public ElementaryProblemGenerator(Random random, DecimalFormat df) {
        super(random, df);
    }

    @Override
    public MathProblem generate() {
        // 小学题目要求：答案不得为负数。最多尝试若干次生成，若仍为负则回退到简单加法题。
        int maxAttempts = 50;
        for (int attempt = 0; attempt < maxAttempts; attempt++) {
            int operandCount = random.nextInt(4) + 2; // 2-5个操作数
            String[] operators = {"+", "-", "*", "/"};
            int[] operands = new int[operandCount];
            String[] selectedOperators = new String[operandCount - 1];
            for (int i = 0; i < operandCount; i++) operands[i] = random.nextInt(100) + 1;
            // 运算符选择（避免连乘/连除，调整概率）
            String previousOperator = null;
            double pPlus = 0.45, pMinus = 0.35, pTimes = 0.15, pDivide = 0.05;
            for (int i = 0; i < operandCount - 1; i++) {
                String operator;
                if ("*".equals(previousOperator) || "/".equals(previousOperator)) {
                    double randValue = random.nextDouble();
                    double total = pPlus + pMinus;
                    operator = randValue < (pPlus / total) ? "+" : "-";
                } else {
                    double randValue = random.nextDouble();
                    if (randValue < pPlus) operator = "+";
                    else if (randValue < pPlus + pMinus) operator = "-";
                    else if (randValue < pPlus + pMinus + pTimes) operator = "*";
                    else operator = "/";
                }
                selectedOperators[i] = operator;
                previousOperator = operator;
            }
            // 括号选择
            int startPos = findBracketStart(toDoubleArray(operands), selectedOperators, 0.3);
            String expression;
            double result;
            if (startPos != -1) {
                expression = buildExpressionWithBrackets(operands, selectedOperators, startPos);
                result = evaluateExpressionWithBrackets(operands, selectedOperators, startPos);
            } else {
                StringBuilder expBuilder = new StringBuilder();
                expBuilder.append(operands[0]);
                result = operands[0];
                for (int i = 0; i < operandCount - 1; i++) {
                    expBuilder.append(" ").append(selectedOperators[i]).append(" ").append(operands[i + 1]);
                    result = applyOperator(result, operands[i + 1], selectedOperators[i]);
                }
                expression = expBuilder.toString();
            }
            if (result >= 0) {
                return new MathProblem(expression, df.format(result), "小学");
            }
            // 如果为负数，继续下一次尝试
        }
        // 回退：生成一个保证非负的简单加法题
        int a = random.nextInt(100) + 1;
        int b = random.nextInt(100) + 1;
        String expression = a + " + " + b;
        double result = a + b;
        return new MathProblem(expression, df.format(result), "小学");
    }
}