Merge pull request #1134 from akiyamaneko/factorial_improved

优化阶乘相关的逻辑

hutool-core/src/main/java/cn/hutool/core/util/NumberUtil.java

@@ -43,6 +43,14 @@ public class NumberUtil {
 	 */
 	private static final int DEFAUT_DIV_SCALE = 10;
 
+	/**
+         * 0-20对应的阶乘，超过20的阶乘会超过Long.MAX_VALUE
+         */
+        private static final long[] FACTORIALS = new long[]{
+            1L, 1L, 2L, 6L, 24L, 120L, 720L, 5040L, 40320L, 362880L, 3628800L, 39916800L, 479001600L, 6227020800L,
+            87178291200L, 1307674368000L, 20922789888000L, 355687428096000L, 6402373705728000L, 121645100408832000L,
+            2432902008176640000L};
+	
 	/**
 	 * 提供精确的加法运算
 	 *
@@ -1423,13 +1431,31 @@ public class NumberUtil {
 	 * @since 4.1.0
 	 */
         public static long factorial(long start, long end) {
+                // 负数没有阶乘
+                if(start < 0 || end < 0) {
+                        throw new IllegalArgumentException(String.format("Factorial start and end both must be >= 0, " +
+                            "but got start=%d, end=%d", start, end));
+                }
                 if (0L == start || start == end) {
                         return 1L;
                 }
                 if (start < end) {
                         return 0L;
                 }
-		return start * factorial(start - 1, end);
+                return factorialMultiplyAndCheck(start, factorial(start - 1, end));
+        }
+	
+	/**
+         * 计算范围阶乘中校验中间的计算是否存在溢出，factorial提前做了负数和0的校验，因此这里没有校验数字的正负
+         * @param a 乘数
+         * @param b 被乘数
+         * @return 如果 a * b的结果没有溢出直接返回，否则抛出异常
+         */
+        private static long factorialMultiplyAndCheck(long a, long b) {
+                if (a <= Long.MAX_VALUE / b) {
+                        return a * b;
+                }
+                throw new IllegalArgumentException(String.format("Overflow in multiplication: {%d} * {%d}", a, b));
         }
 
         /**
@@ -1442,7 +1468,11 @@ public class NumberUtil {
          * @return 结果
          */
         public static long factorial(long n) {
-		return factorial(n, 1);
+                if (n < 0 || n > 20) {
+                        throw new IllegalArgumentException(String.format("Factorial must have n >= 0 and n <= 20 for n!, " +
+                            "but got n = %d", n));
+                }
+                return FACTORIALS[(int) n];
         }
 
 	/**

hutool-core/src/test/java/cn/hutool/core/util/NumberUtilTest.java

@@ -241,12 +241,17 @@ public class NumberUtilTest {
 		long factorial = NumberUtil.factorial(0);
 		Assert.assertEquals(1, factorial);
 
+		Assert.assertEquals(1L, NumberUtil.factorial(1));
+		Assert.assertEquals(1307674368000L, NumberUtil.factorial(15));
+		Assert.assertEquals(2432902008176640000L, NumberUtil.factorial(20));
+
 		factorial = NumberUtil.factorial(5, 0);
 		Assert.assertEquals(120, factorial);
 		factorial = NumberUtil.factorial(5, 1);
 		Assert.assertEquals(120, factorial);
     
 		Assert.assertEquals(5, NumberUtil.factorial(5, 4));
+		Assert.assertEquals(2432902008176640000L, NumberUtil.factorial(20, 0));
 	}
 
 	@Test