src/main/java/com/pt/utils/BdecodeUtils.java

package com.pt.utils;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.*;

public class BdecodeUtils {
    public static Object decode(byte[] data) throws IOException {
        try (ByteArrayInputStream in = new ByteArrayInputStream(data)) {
            return decodeNext(in);
        }
    }

    private static Object decodeNext(InputStream in) throws IOException {
        int prefix = in.read();
        if (prefix == -1) {
            throw new IOException("Unexpected end of stream");
        }

        if (prefix >= '0' && prefix <= '9') {
            // 字符串，回退一个字节给parseString处理
            in.reset();
            return parseString(in, prefix);
        } else if (prefix == 'i') {
            return parseInteger(in);
        } else if (prefix == 'l') {
            return parseList(in);
        } else if (prefix == 'd') {
            return parseDict(in);
        } else {
            throw new IOException("Invalid bencode prefix: " + (char) prefix);
        }
    }

    private static String parseString(InputStream in, int firstDigit) throws IOException {
        // 读长度前缀
        StringBuilder lenStr = new StringBuilder();
        lenStr.append((char) firstDigit);
        int b;
        while ((b = in.read()) != -1 && b != ':') {
            lenStr.append((char) b);
        }
        int length = Integer.parseInt(lenStr.toString());

        // 读内容
        byte[] buf = new byte[length];
        int read = in.read(buf);
        if (read < length) throw new IOException("Unexpected end of stream reading string");
        return new String(buf);
    }

    private static long parseInteger(InputStream in) throws IOException {
        StringBuilder intStr = new StringBuilder();
        int b;
        while ((b = in.read()) != -1 && b != 'e') {
            intStr.append((char) b);
        }
        return Long.parseLong(intStr.toString());
    }

    private static List<Object> parseList(InputStream in) throws IOException {
        List<Object> list = new ArrayList<>();
        int b;
        while ((b = in.read()) != 'e') {
            in.reset();
            list.add(decodeNext(in));
        }
        return list;
    }

    private static Map<String, Object> parseDict(InputStream in) throws IOException {
        Map<String, Object> map = new LinkedHashMap<>();
        int b;
        while ((b = in.read()) != 'e') {
            in.reset();
            String key = (String) decodeNext(in);
            Object value = decodeNext(in);
            map.put(key, value);
        }
        return map;
    }
}