feat: add wallis product

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;
+    }
+}