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100 lines
2.9 KiB
100 lines
2.9 KiB
4 months ago
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/*
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* random.c - random number generator for producing mathlib test cases
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*
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* Copyright (c) 1998-2019, Arm Limited.
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* SPDX-License-Identifier: MIT
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*/
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#include "types.h"
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#include "random.h"
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static uint32 seedbuf[55];
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static int seedptr;
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void seed_random(uint32 seed) {
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int i;
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seedptr = 0;
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for (i = 0; i < 55; i++) {
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seed = seed % 44488 * 48271 - seed / 44488 * 3399;
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seedbuf[i] = seed - 1;
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}
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}
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uint32 base_random(void) {
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seedptr %= 55;
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seedbuf[seedptr] += seedbuf[(seedptr+31)%55];
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return seedbuf[seedptr++];
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}
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uint32 random32(void) {
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uint32 a, b, b1, b2;
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a = base_random();
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b = base_random();
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for (b1 = 0x80000000, b2 = 1; b1 > b2; b1 >>= 1, b2 <<= 1) {
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uint32 b3 = b1 | b2;
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if ((b & b3) != 0 && (b & b3) != b3)
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b ^= b3;
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}
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return a ^ b;
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}
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/*
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* random_upto: generate a uniformly randomised number in the range
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* 0,...,limit-1. (Precondition: limit > 0.)
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*
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* random_upto_biased: generate a number in the same range, but with
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* the probability skewed towards the high end by means of taking the
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* maximum of 8*bias+1 samples from the uniform distribution on the
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* same range. (I don't know why bias is given in that curious way -
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* historical reasons, I expect.)
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*
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* For speed, I separate the implementation of random_upto into the
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* two stages of (a) generate a bitmask which reduces a 32-bit random
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* number to within a factor of two of the right range, (b) repeatedly
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* generate numbers in that range until one is small enough. Splitting
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* it up like that means that random_upto_biased can do (a) only once
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* even when it does (b) lots of times.
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*/
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static uint32 random_upto_makemask(uint32 limit) {
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uint32 mask = 0xFFFFFFFF;
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int i;
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for (i = 16; i > 0; i >>= 1)
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if ((limit & (mask >> i)) == limit)
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mask >>= i;
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return mask;
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}
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static uint32 random_upto_internal(uint32 limit, uint32 mask) {
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uint32 ret;
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do {
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ret = random32() & mask;
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} while (ret > limit);
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return ret;
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}
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uint32 random_upto(uint32 limit) {
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uint32 mask = random_upto_makemask(limit);
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return random_upto_internal(limit, mask);
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}
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uint32 random_upto_biased(uint32 limit, int bias) {
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uint32 mask = random_upto_makemask(limit);
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uint32 ret = random_upto_internal(limit, mask);
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while (bias--) {
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uint32 tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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tmp = random_upto_internal(limit, mask); if (tmp < ret) ret = tmp;
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}
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return ret;
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}
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