feat: add birthday paradox

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

@@ -0,0 +1,29 @@
+package me.hatter.math;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+
+public class BirthdayParadox {
+
+    public static void main(String[] args) {
+        System.out.println(p(1));
+        System.out.println(p(10));
+        System.out.println(p(20));
+        System.out.println(p(23));
+        System.out.println(p(30));
+        System.out.println(p(40));
+        System.out.println(p(50));
+        System.out.println(p(60));
+    }
+
+    public static BigDecimal p(int n) {
+        BigDecimal p = BigDecimal.ONE;
+        for (int i = 1; i < n; i++) {
+            p = p.multiply(BigDecimal.ONE.subtract(BigDecimal.valueOf(i).divide(BigDecimal.valueOf(365), 100, RoundingMode.FLOOR)));
+        }
+        if (!p.equals(BigDecimal.ONE)) {
+            p = p.divide(BigDecimal.ONE, 100, RoundingMode.FLOOR);
+        }
+        return BigDecimal.ONE.subtract(p);
+    }
+}