1 /*
2 * Copyright (c) 2020-2024 Stefan Krah. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 *
8 * 1. Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 */
26
27
28 #include <cassert>
29 #include <climits>
30
31 #include <iostream>
32 #include <stdexcept>
33 #include <string>
34
35 #include "decimal.hh"
36
37
38 using decimal::Decimal;
39 using decimal::context;
40
41
42 /* The algorithm is not optimized for bignum arithmetic */
43 Decimal
pi(int prec)44 pi(int prec)
45 {
46 assert(1 <= prec && prec <= INT_MAX-2);
47 context.prec(prec + 2);
48
49 Decimal lasts = 0;
50 Decimal t = 3;
51 Decimal s = 3;
52 Decimal n = 1;
53 Decimal na = 0;
54 Decimal d = 0;
55 Decimal da = 24;
56
57 while (s != lasts) {
58 lasts = s;
59 n += na;
60 na += 8;
61 d += da;
62 da += 32;
63 t = (t * n) / d;
64 s += t;
65 }
66
67 context.prec(prec);
68 return +s;
69 }
70
71 void
err_exit(const std::string & msg)72 err_exit(const std::string& msg)
73 {
74 std::cerr << msg << std::endl;
75 std::exit(1);
76 }
77
78
79 int
main(int argc,char * argv[])80 main(int argc, char *argv[])
81 {
82 size_t pos = 0;
83 int prec = 0;
84
85 if (argc != 2) {
86 err_exit("usage: ./pi prec");
87 }
88 std::string s = argv[1];
89
90 try {
91 prec = std::stoi(s, &pos);
92 } catch (const std::invalid_argument&) {
93 err_exit("not a number");
94 } catch (const std::out_of_range &) {
95 err_exit("out of range");
96 }
97
98 if (pos < s.size()) {
99 err_exit("trailing characters");
100 }
101 if (prec <= 0 || prec > 999999) {
102 err_exit("prec must be in [1, 999999]");
103 }
104
105 std::cout << pi(prec) << std::endl;
106
107 return 0;
108 }
109