1 /*
2 * Copyright (C) 2023 Alberto Irurueta Carro (alberto@irurueta.com)
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16 package com.irurueta.numerical;
17
18 /**
19 * Estimates factorial values as double precision floating point values.
20 */
21 public class DoubleFactorialEstimator {
22
23 /**
24 * Cache size for factorial values that can be represented without overflowing with double
25 * precision.
26 * Values up to 170! are representable, 171! overflows.
27 */
28 public static final int CACHE_SIZE = 171;
29
30 /**
31 * Cache of values.
32 */
33 private final double[] cache;
34
35 /**
36 * Constructor with default cache size.
37 */
38 public DoubleFactorialEstimator() {
39 this(CACHE_SIZE);
40 }
41
42 /**
43 * Constructor.
44 *
45 * @param cacheSize size of cache.
46 * @throws IllegalArgumentException if provided value is less than 1 or exceeds 171.
47 */
48 public DoubleFactorialEstimator(final int cacheSize) {
49 if (cacheSize < 1) {
50 throw new IllegalArgumentException("Cache size must be at least 1");
51 }
52 if (cacheSize > CACHE_SIZE) {
53 throw new IllegalArgumentException("Maximum allowed cache size is 171");
54 }
55
56 // initialize cache
57 cache = new double[cacheSize];
58 cache[0] = 1.0;
59 for (var i = 1; i < cacheSize; i++) {
60 cache[i] = i * cache[i - 1];
61 }
62 }
63
64 /**
65 * Gets current cache size.
66 *
67 * @return cache size.
68 */
69 public int getCacheSize() {
70 return cache.length;
71 }
72
73 /**
74 * Gets factorial of provided value represented with double precision.
75 *
76 * @param value value to compute factorial for.
77 * @return computed factorial value.
78 *
79 * @throws IllegalArgumentException if provided value exceeds cache size.
80 */
81 public double factorial(final int value) {
82 if (value < 0) {
83 throw new IllegalArgumentException("Minimum supported value is 0");
84 }
85 if (value >= cache.length) {
86 throw new IllegalArgumentException("Maximum supported value is " + (cache.length - 1));
87 }
88
89 return cache[value];
90 }
91 }