From 2fd39d6e0c51ca29f1b82be8ba29f8efcebdddfd Mon Sep 17 00:00:00 2001
From: Yujing <yujingzhang2025@gmail.com>
Date: Tue, 15 Oct 2024 22:46:12 +1100
Subject: [PATCH] #7430 implemented multiFactorial

---
 .../google/common/math/BigIntegerMath.java    | 26 ++++++++++++++++++-
 guava/src/com/google/common/math/IntMath.java | 24 +++++++++++++++++
 .../src/com/google/common/math/LongMath.java  | 25 +++++++++++++++++-
 3 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/guava/src/com/google/common/math/BigIntegerMath.java b/guava/src/com/google/common/math/BigIntegerMath.java
index 5f7ce6b57577..17417a4747d4 100644
--- a/guava/src/com/google/common/math/BigIntegerMath.java
+++ b/guava/src/com/google/common/math/BigIntegerMath.java
@@ -520,6 +520,30 @@ public static BigInteger binomial(int n, int k) {
   static boolean fitsInLong(BigInteger x) {
     return x.bitLength() <= Long.SIZE - 1;
   }
-
+  /**
+   * Returns multifactorial of n with step size BigInteger k.
+   * This is used for large values of n.
+   *
+   * @param n the number to compute the multifactorial of. Must be non-negative.
+   * @param k the step size. Must be positive.
+   * @return the multifactorial of n with step size k. If none-zero n is less than k then return n, else return one.
+   * @throws IllegalArgumentException if n is negative or if k is less than 1.
+   */
+  public static BigInteger multiFactorial(int n, int k) {
+    if (n < 0) {
+      throw new IllegalArgumentException("n cannot be negative!");
+    }
+    if (k < 1) {
+      throw new IllegalArgumentException("k must be positive!");
+    }
+    if (n <= k) {
+      return n == 0 ? BigInteger.ONE : BigInteger.valueOf(n);
+    }
+    BigInteger result = BigInteger.valueOf(n);
+    for (int i = n - k; i > 1; i -= k) {
+      result = result.multiply(BigInteger.valueOf(i));
+    }
+    return result;
+  }
   private BigIntegerMath() {}
 }
diff --git a/guava/src/com/google/common/math/IntMath.java b/guava/src/com/google/common/math/IntMath.java
index 7ab7765cf840..c5f1b08a2209 100644
--- a/guava/src/com/google/common/math/IntMath.java
+++ b/guava/src/com/google/common/math/IntMath.java
@@ -721,6 +721,30 @@ public static int mean(int x, int y) {
   public static boolean isPrime(int n) {
     return LongMath.isPrime(n);
   }
+  /**
+   * Returns multifactorial of Int n with step size k.
+   *
+   * @param n the number to compute. Must be non-negative.
+   * @param k the step size for the multifactorial. Must be positive.
+   * @return the multifactorial of n with step size k. If none-zero n is less than k then return n, else return 1.
+   * @throws IllegalArgumentException if n is negative or if k is less than 1.
+   */
+  public static int multiFactorial(int n, int k) {
+    if (n < 0) {
+      throw new IllegalArgumentException("n cannot be negative!");
+    }
+    if (k < 1) {
+      throw new IllegalArgumentException("k must be positive!");
+    }
+    if (n <= k) {
+      return n == 0 ? 1 : n;
+    }
+    int result = n;
+    for (int i = n - k; i > 1; i -= k) {
+      result *= i;
+    }
+    return result;
+  }
 
   private IntMath() {}
 }
diff --git a/guava/src/com/google/common/math/LongMath.java b/guava/src/com/google/common/math/LongMath.java
index 16e0248d99e4..790526969c58 100644
--- a/guava/src/com/google/common/math/LongMath.java
+++ b/guava/src/com/google/common/math/LongMath.java
@@ -1345,6 +1345,29 @@ public static double roundToDouble(long x, RoundingMode mode) {
     }
     throw new AssertionError("impossible");
   }
-
+  /**
+   * Returns multifactorial of int n with step size k.
+   *
+   * @param n the number to compute. Must be non-negative.
+   * @param k the step size must be positive.
+   * @return the long type multifactorial of n with step size k. If none-zero n is less than k then return (long) n, else return 1L.
+   * @throws IllegalArgumentException if n is negative or if k is less than 1.
+   */
+  public static long multiFactorial(int n, int k) {
+    if (n < 0) {
+      throw new IllegalArgumentException("n cannot be negative!");
+    }
+    if (k < 1) {
+      throw new IllegalArgumentException("k must be positive!");
+    }
+    if (n <= k) {
+      return n == 0 ? 1L : (long)n;
+    }
+    long result = n;
+    for (long i = n - k; i > 1; i -= k) {
+      result *= i;
+    }
+    return result;
+  }
   private LongMath() {}
 }