From ce1f7944478bedc6e9a7074166c2ec404d04e0c1 Mon Sep 17 00:00:00 2001
From: Hatter Jiang <jht5945@gmail.com>
Date: Sat, 19 Jul 2025 20:19:12 +0800
Subject: [PATCH] feat: add math util

---
 .../java/me/hatter/math/util/MathUtil.java    | 45 +++++++++++++++++
 .../me/hatter/math/util/MathUtilTest.java     | 48 +++++++++++++++++++
 2 files changed, 93 insertions(+)
 create mode 100644 src/main/java/me/hatter/math/util/MathUtil.java
 create mode 100644 src/test/java/me/hatter/math/util/MathUtilTest.java

diff --git a/src/main/java/me/hatter/math/util/MathUtil.java b/src/main/java/me/hatter/math/util/MathUtil.java
new file mode 100644
index 0000000..042bd8b
--- /dev/null
+++ b/src/main/java/me/hatter/math/util/MathUtil.java
@@ -0,0 +1,45 @@
+package me.hatter.math.util;
+
+import java.math.BigDecimal;
+import java.math.BigInteger;
+import java.math.RoundingMode;
+
+public class MathUtil {
+
+    public static final BigDecimal E = new BigDecimal(
+            "2.7182818284590452353602874713526624977572470936999595749669676277240766303535475945713821785251664238"
+    );
+
+    public static final BigDecimal PI = new BigDecimal(
+            "3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679"
+                    .replace(" ", ""));
+
+    public static final BigDecimal LITTLE_NUMBER = new BigDecimal(
+            "0.000000000000000000000000000000000000000001");
+
+    public static BigInteger factorial(long n) {
+        if (n < 0) {
+            throw new RuntimeException("Not supported n < 0");
+        }
+        if (n == 0) {
+            return BigInteger.ONE;
+        }
+        BigInteger p = BigInteger.ONE;
+        for (long i = 1; i <= n; i++) {
+            p = p.multiply(BigInteger.valueOf(i));
+        }
+        return p;
+    }
+
+    public static boolean bigDecimalEquals(BigDecimal a, BigDecimal b) {
+        return a.subtract(b).abs().compareTo(LITTLE_NUMBER) < 0;
+    }
+
+    public static BigDecimal simpleContinuedFraction(long[] nums) {
+        BigDecimal v = BigDecimal.ZERO;
+        for (int i = nums.length - 1; i > 0; i--) {
+            v = BigDecimal.ONE.divide(v.add(BigDecimal.valueOf(nums[i])), 100, RoundingMode.FLOOR);
+        }
+        return v.add(BigDecimal.valueOf(nums[0]));
+    }
+}
diff --git a/src/test/java/me/hatter/math/util/MathUtilTest.java b/src/test/java/me/hatter/math/util/MathUtilTest.java
new file mode 100644
index 0000000..5bc499e
--- /dev/null
+++ b/src/test/java/me/hatter/math/util/MathUtilTest.java
@@ -0,0 +1,48 @@
+package me.hatter.math.util;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.math.BigDecimal;
+
+public class MathUtilTest {
+
+    @Test
+    public void testFactorial() {
+        Assert.assertEquals(1, MathUtil.factorial(0).longValue());
+        Assert.assertEquals(1, MathUtil.factorial(1).longValue());
+        Assert.assertEquals(2, MathUtil.factorial(2).longValue());
+        Assert.assertEquals(6, MathUtil.factorial(3).longValue());
+        Assert.assertEquals(24, MathUtil.factorial(4).longValue());
+        Assert.assertEquals(120, MathUtil.factorial(5).longValue());
+    }
+
+    @Test
+    public void testSimpleContinuedFraction() {
+        // 3.245 = [3; 4, 12, 4]
+        Assert.assertTrue(MathUtil.bigDecimalEquals(
+                new BigDecimal("3.245"),
+                MathUtil.simpleContinuedFraction(new long[]{3, 4, 12, 4})
+        ));
+        // 1 = [0; 1]
+        Assert.assertTrue(MathUtil.bigDecimalEquals(
+                new BigDecimal("1"),
+                MathUtil.simpleContinuedFraction(new long[]{0, 1})
+        ));
+        // 0.8 = [0; 1, 4]
+        Assert.assertTrue(MathUtil.bigDecimalEquals(
+                new BigDecimal("0.8"),
+                MathUtil.simpleContinuedFraction(new long[]{0, 1, 4})
+        ));
+        // 3.14155 = [3; 7, 15, 2, 7, 1, 4, 1, 1]
+        Assert.assertTrue(MathUtil.bigDecimalEquals(
+                new BigDecimal("3.14155"),
+                MathUtil.simpleContinuedFraction(new long[]{3, 7, 15, 2, 7, 1, 4, 1, 1})
+        ));
+        // 3.14165 = [3; 7, 16, 1, 3, 4, 2, 3, 1]
+        Assert.assertTrue(MathUtil.bigDecimalEquals(
+                new BigDecimal("3.14165"),
+                MathUtil.simpleContinuedFraction(new long[]{3, 7, 16, 1, 3, 4, 2, 3, 1})
+        ));
+    }
+}