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43 lines
1.7 KiB
C++
43 lines
1.7 KiB
C++
/*
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* Copyright (c) 2021, the SerenityOS developers.
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Random.h>
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namespace AK {
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u32 get_random_uniform(u32 max_bounds)
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{
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// If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
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// with one group that is a bit too small. For this reason, the implementation
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// `arc4random() % max_bounds` would be insufficient. Here we compute the first number of
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// the first "full group" as 2**32 % max_bounds, and rely on the equivalence
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// `2**32 % x == (2**32 - x) % x` to keep calculations in u32 domain:
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u32 const threshold = static_cast<u32>(-max_bounds) % max_bounds;
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auto random_value = get_random<u32>();
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for (int i = 0; i < 20 && random_value < threshold; ++i) {
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// By chance we picked a value from the incomplete group. Note that this group has size at
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// most 2**31-1, so picking this group has a chance of less than 50%.
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// In practice, this means that for the worst possible input, there is still only a
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// once-in-a-million chance to get to iteration 20. In theory we should be able to loop
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// forever. Here we prefer marginally imperfect random numbers over weird runtime behavior.
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random_value = get_random<u32>();
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}
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return random_value % max_bounds;
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}
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u64 get_random_uniform_64(u64 max_bounds)
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{
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// Uses the same algorithm as `get_random_uniform`,
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// by replacing u32 with u64.
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u64 const threshold = static_cast<u64>(-max_bounds) % max_bounds;
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auto random_value = get_random<u64>();
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for (int i = 0; i < 20 && random_value < threshold; ++i) {
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random_value = get_random<u64>();
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}
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return random_value % max_bounds;
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}
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}
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