Package org.apache.commons.numbers.combinatorics

Class LogFactorial

java.lang.Object
org.apache.commons.numbers.combinatorics.LogFactorial
public final class LogFactorial extends Object
Class for computing the natural logarithm of the factorial of a number. It allows to allocate a cache of precomputed values. In case of cache miss, computation is performed by a call to LogGamma.value(double).

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    private static final int
    FACTORIALS_CACHE_SIZE
    Size of precomputed factorials.
    private final double[]
    logFactorials
    Precomputed values of the function: logFactorials[i] = Math.log(i!).

  • Constructor Summary

    Constructors
    Modifier
    Constructor
    Description
    private
    LogFactorial(int numValues, double[] cache)
    Creates an instance, reusing the already computed values if available.

  • Method Summary

    Modifier and Type
    Method
    Description
    static LogFactorial
    create()
    Creates an instance with no precomputed values.
    double
    value(int n)
    Computes \( log_e(n!) \).
    LogFactorial
    withCache(int cacheSize)
    Creates an instance with the specified cache size.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Field Details

    • FACTORIALS_CACHE_SIZE

      private static final int FACTORIALS_CACHE_SIZE
      Size of precomputed factorials.
      See Also:

    • logFactorials

      private final double[] logFactorials
      Precomputed values of the function: logFactorials[i] = Math.log(i!).

  • Constructor Details

    • LogFactorial

      private LogFactorial(int numValues, double[] cache)
      Creates an instance, reusing the already computed values if available.
      Parameters:
      numValues - Number of values of the function to compute.
      cache - Cached values.
      Throws:
      IllegalArgumentException - if n < 0.

  • Method Details

    • create

      public static LogFactorial create()
      Creates an instance with no precomputed values.
      Returns:
      instance with no precomputed values

    • withCache

      public LogFactorial withCache(int cacheSize)
      Creates an instance with the specified cache size.
      Parameters:
      cacheSize - Number of precomputed values of the function.
      Returns:
      a new instance where cacheSize values have been precomputed.
      Throws:
      IllegalArgumentException - if cacheSize < 0.

    • value

      public double value(int n)
      Computes \( log_e(n!) \).
      Parameters:
      n - Argument.
      Returns:
      log(n!).
      Throws:
      IllegalArgumentException - if n < 0.