/**
 * 超级计算器——两个大数相乘
 * 
 * @author GaoHuanjie
 */
public class SuperCalculator {
	public static void main(String[] args) {
		String multiplier1 = "12";
		String multiplier2 = "70";
		
//		String multiplier1 = "13286754398172596";
//		String multiplier2 = "2397567453241147";

		System.out.println(multiplier1+"x" + multiplier2+"="+product(multiplier1, multiplier2));
	} 

	/**
	 * 字符串顺序取反
	 */
	private static String reverse(String str) {
		return new StringBuffer(str).reverse().toString();
	}
	/**
	 * 将char类型的数据转int类型
	 */
	private static int covertInt(char str){
		return Integer.parseInt(String.valueOf(str));
	}

	public static String product(String multiplier1, String multiplier2) {//以12x70为例
		char[] multiplierArray1 = reverse(multiplier1).toCharArray();// 高低位对调 {2,1}
		char[] multiplierArray2 = reverse(multiplier2).toCharArray();// 高低位对调 {0,7}
		int multiplierLength1 = multiplierArray1.length;// 2
		int multiplierLength2 = multiplierArray2.length;// 2
		int productSize = multiplierLength1 + multiplierLength2;//两个数的乘积的最大长度  4
		int[] productArray = new int[productSize];//乘积数组 {0,0,0,0}
		for (int j = 0; j < multiplierLength2; j++) {// 对齐逐位相乘  {0,7}
			for (int i = 0; i < multiplierLength1; i++) {// {2,1}
				productArray[i + j] = productArray[i + j] + (covertInt(multiplierArray1[i])* covertInt(multiplierArray2[j]));
			}
		}
		//到此productArray元素为{0,14,7,0}
		
		for (int i = 0; i < productSize; i++) {// 进位处理
													//i=0 i=1  i=2 i=3 
			int quotient = productArray[i] / 10;//商   0   1    0   0
			productArray[i] = productArray[i] % 10;//  0   4    8   0
			if (quotient > 0) {
				productArray[i + 1] = productArray[i + 1] + quotient;
			}
		}
		//到此productArray元素为 {0,4,8,0}
		
		int m = 0;
		for (m = productSize - 1; m >= 0;) {// 找到最高位
			if (productArray[m] > 0) {
				break;
			}
			m--;
		}
		//至此m的值为 2

		StringBuffer stringBuffer = new StringBuffer();
		for (int n = 0; n <= m; n++) {// 由最高位开始打印乘积
			stringBuffer.append(productArray[m - n]);
		}
		// 至此乘积为840
		return stringBuffer.toString();
	}
}