大數字符串加法

最近在整理總結舊知識，自己寫的一個大數字符串加法算法，供大家參考：

char *StrAddStr(const char *sub1, const char *sub2)
{
	if (sub1 == NULL &&
		sub2 == NULL)
		return "0";

	if (sub2 == NULL)
		return (char*)sub1;

	if (sub1 == NULL)
		return (char*)sub2;

	int sub1_len = strlen(sub1);
	int sub2_len = strlen(sub2);
	if (sub1_len > 256 || sub2_len > 256)
		return NULL;	// 不支持該長度

	int sub1_order[256] = {0}, sub2_order[256] = {0};
	int result[256] = {0};
	static char strres[256];
	memset(strres, 0, 256);

	int j = 0;
	// 變成數字並反序
	for (int i=sub1_len-1; i>=0; i--)
		sub1_order[j++] = *(sub1 + i) - '0';

	j = 0;
	for (int i=sub2_len-1; i>=0; i--)
		sub2_order[j++] = *(sub2 + i) - '0';

	int len = 0;	// 記錄計算結果長度
	for (int i=0; i<(sub1_len>sub2_len?sub1_len:sub2_len); i++)
	{
		int res = result[i] + sub1_order[i] + sub2_order[i];
		if (res >= 10)
		{
			result[i] = res - 10;
			result[i+1] += 1;	// 進位
			len = i+1;
		}
		else
		{
			result[i] = res;
			len = i;
		}
	}

	// 正序，變成字符串
	j=0;
	for (int i= len; i>=0; i--)
	{
		strres[j++] = result[i] + '0';
	}

	return strres;
}

簡單的測試代碼：

int main ()
{
	char *strnum1 = "12345";
	char *strnum2 = "234";
	char *res = StrAddStr(strnum1, strnum2);
	printf("12345+234=%s\r\n", res);

	strnum1 = "9876";
	strnum2 = "132089";
	res = StrAddStr(strnum1, strnum2);
	printf("9876+132089=%s\r\n", res);

	strnum1 = "876123089982";
	strnum2 = "673526156533";
	res = StrAddStr(strnum1, strnum2);
	printf("876123089982+673526156533=%s\r\n", res);
	return 0;
}

結果如下：

我沒有做更加嚴謹的測試，如果發現有問題的話可以給我留言，一起討論。