import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

/** 空值不参与，字典序，MD5 小写 */
public final class SignUtil {
    private SignUtil() {}

    public static String buildSign(Map<String, Object> params, String secretKey) throws Exception {
        TreeMap<String, Object> sorted = new TreeMap<>();
        for (Map.Entry<String, Object> e : params.entrySet()) {
            if ("Sign".equals(e.getKey())) {
                continue;
            }
            sorted.put(e.getKey(), e.getValue());
        }
        List<String> parts = new ArrayList<>();
        for (Map.Entry<String, Object> e : sorted.entrySet()) {
            Object v = e.getValue();
            if (v == null) {
                continue;
            }
            String s = String.valueOf(v);
            if (s.isEmpty()) {
                continue;
            }
            parts.add(e.getKey() + "=" + s);
        }
        String raw = String.join("&", parts) + "&SecretKey=" + secretKey;
        MessageDigest md = MessageDigest.getInstance("MD5");
        byte[] dig = md.digest(raw.getBytes(StandardCharsets.UTF_8));
        StringBuilder sb = new StringBuilder();
        for (byte b : dig) {
            sb.append(String.format("%02x", b));
        }
        return sb.toString();
    }
}