feat: add wallis product

README.md

@@ -1,9 +1,10 @@
 # math
 
 | Class                   | Comments           | MuBoard Link                                                                                                                |
-|--------------------------|--------------------|-----------------------------------------------------------------------------------------------------------------------------|
+|-------------------------|--------------------|-----------------------------------------------------------------------------------------------------------------------------|
 | ChineseRemainderTheorem | 中国剩余定理             | [chinese-remainder-theorem](https://hatter.ink/static/resource/muboard/?id=chinese-remainder-theorem&version=latest&full=1) |
 | EulerTheorem            | 欧拉定理               | [euler-theorem](https://hatter.ink/static/resource/muboard/?id=euler-theorem&version=latest&full=1)                         |
 | ModularInv              | 求乘法逆元【扩展欧几里得法】     | -                                                                                                                           |
 | Sqrt10                  | 求 $\sqrt{10}$【连分数】 | [resolve-sqrt-10](https://hatter.ink/static/resource/muboard/?id=resolve-sqrt-10&version=latest&full=1)                     |
+| WallisProduct           | 沃里斯乘积              | [wallis-product](https://hatter.ink/static/resource/muboard/?id=wallis-product&version=latest&full=1)                       |
 

src/main/java/me/hatter/math/WallisProduct.java

@@ -0,0 +1,26 @@
+package me.hatter.math;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+
+public class WallisProduct {
+
+    public static void main(String[] args) {
+        // 3.1415925750499739534746260502048989270653181072075621931977986489848855062212872415726332946132240849
+        System.out.println(pi());
+    }
+
+    public static BigDecimal pi() {
+        BigDecimal p = BigDecimal.valueOf(2);
+        for (long i = 1; i < 10000000; i++) {
+            BigDecimal doubleI = BigDecimal.valueOf(i * 2);
+            p = p.multiply(
+                    doubleI.divide(doubleI.subtract(BigDecimal.ONE), 100, RoundingMode.FLOOR)
+            ).multiply(
+                    doubleI.divide(doubleI.add(BigDecimal.ONE), 100, RoundingMode.FLOOR)
+            );
+            p = p.divide(BigDecimal.ONE, 100, RoundingMode.FLOOR);
+        }
+        return p;
+    }
+}