1 /*
2 * random.c - random number generator for producing mathlib test cases
3 *
4 * Copyright (c) 1998-2018, Arm Limited.
5 * SPDX-License-Identifier: MIT
6 */
7
8 #include "types.h"
9 #include "random.h"
10
11 static uint32 seedbuf[55];
12 static int seedptr;
13
seed_random(uint32 seed)14 void seed_random(uint32 seed) {
15 int i;
16
17 seedptr = 0;
18 for (i = 0; i < 55; i++) {
19 seed = seed % 44488 * 48271 - seed / 44488 * 3399;
20 seedbuf[i] = seed - 1;
21 }
22 }
23
base_random(void)24 uint32 base_random(void) {
25 seedptr %= 55;
26 seedbuf[seedptr] += seedbuf[(seedptr+31)%55];
27 return seedbuf[seedptr++];
28 }
29
random32(void)30 uint32 random32(void) {
31 uint32 a, b, b1, b2;
32 a = base_random();
33 b = base_random();
34 for (b1 = 0x80000000, b2 = 1; b1 > b2; b1 >>= 1, b2 <<= 1) {
35 uint32 b3 = b1 | b2;
36 if ((b & b3) != 0 && (b & b3) != b3)
37 b ^= b3;
38 }
39 return a ^ b;
40 }
41
42 /*
43 * random_upto: generate a uniformly randomised number in the range
44 * 0,...,limit-1. (Precondition: limit > 0.)
45 *
46 * random_upto_biased: generate a number in the same range, but with
47 * the probability skewed towards the high end by means of taking the
48 * maximum of 8*bias+1 samples from the uniform distribution on the
49 * same range. (I don't know why bias is given in that curious way -
50 * historical reasons, I expect.)
51 *
52 * For speed, I separate the implementation of random_upto into the
53 * two stages of (a) generate a bitmask which reduces a 32-bit random
54 * number to within a factor of two of the right range, (b) repeatedly
55 * generate numbers in that range until one is small enough. Splitting
56 * it up like that means that random_upto_biased can do (a) only once
57 * even when it does (b) lots of times.
58 */
59
random_upto_makemask(uint32 limit)60 static uint32 random_upto_makemask(uint32 limit) {
61 uint32 mask = 0xFFFFFFFF;
62 int i;
63 for (i = 16; i > 0; i >>= 1)
64 if ((limit & (mask >> i)) == limit)
65 mask >>= i;
66 return mask;
67 }
68
random_upto_internal(uint32 limit,uint32 mask)69 static uint32 random_upto_internal(uint32 limit, uint32 mask) {
70 uint32 ret;
71 do {
72 ret = random32() & mask;
73 } while (ret > limit);
74 return ret;
75 }
76
random_upto(uint32 limit)77 uint32 random_upto(uint32 limit) {
78 uint32 mask = random_upto_makemask(limit);
79 return random_upto_internal(limit, mask);
80 }
81
random_upto_biased(uint32 limit,int bias)82 uint32 random_upto_biased(uint32 limit, int bias) {
83 uint32 mask = random_upto_makemask(limit);
84
85 uint32 ret = random_upto_internal(limit, mask);
86 while (bias--) {
87 uint32 tmp;
88 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
89 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
90 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
91 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
92 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
93 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
94 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
95 tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
96 }
97
98 return ret;
99 }
100