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AK: Introduce get_random_uniform()

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This is arc4random_uniform(), but inside AK.
This commit is contained in:
Jean-Baptiste Boric 2021-05-14 17:16:06 +02:00 committed by Andreas Kling
parent ad7cd05fc1
commit 069bf988ed
Notes: sideshowbarker 2024-07-18 22:57:59 +09:00 
Author: https://github.com/boricj
Commit: https://github.com/SerenityOS/serenity/commit/069bf988ed9
Pull-request: https://github.com/SerenityOS/serenity/pull/7105
3 changed files with 36 additions and 16 deletions
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31
AK/Random.cpp Normal file
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@ -0,0 +1,31 @@
/*
* Copyright (c) 2021, the SerenityOS developers.
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <AK/Random.h>
namespace AK {
u32 get_random_uniform(u32 max_bounds)
{
// If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
// with a group around 2**32-1 that is a bit too small. For this reason, the implementation
// `arc4random() % max_bounds` would be insufficient. Here we compute the last number of the
// last "full group". Note that if max_bounds is a divisor of UINT32_MAX,
// then we end up with UINT32_MAX:
const u32 max_usable = UINT32_MAX - (static_cast<u64>(UINT32_MAX) + 1) % max_bounds;
auto random_value = get_random<u32>();
for (int i = 0; i < 20 && random_value > max_usable; ++i) {
// By chance we picked a value from the incomplete group. Note that this group has size at
// most 2**31-1, so picking this group has a chance of less than 50%.
// In practice, this means that for the worst possible input, there is still only a
// once-in-a-million chance to get to iteration 20. In theory we should be able to loop
// forever. Here we prefer marginally imperfect random numbers over weird runtime behavior.
random_value = get_random<u32>();
}
return random_value % max_bounds;
}
}

3
AK/Random.h
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@ -41,7 +41,10 @@ inline T get_random()
return t;
}
u32 get_random_uniform(u32 max_bounds);
}
using AK::fill_with_random;
using AK::get_random;
using AK::get_random_uniform;

18
Userland/Libraries/LibC/stdlib.cpp
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@ -7,6 +7,7 @@
#include <AK/Assertions.h>
#include <AK/HashMap.h>
#include <AK/Noncopyable.h>
#include <AK/Random.h>
#include <AK/StdLibExtras.h>
#include <AK/Types.h>
#include <AK/Utf8View.h>
@ -1093,22 +1094,7 @@ void arc4random_buf(void* buffer, size_t buffer_size)
uint32_t arc4random_uniform(uint32_t max_bounds)
{
// If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
// with a group around 2**32-1 that is a bit too small. For this reason, the implementation
// `arc4random() % max_bounds` would be insufficient. Here we compute the last number of the
// last "full group". Note that if max_bounds is a divisor of UINT32_MAX,
// then we end up with UINT32_MAX:
const uint32_t max_usable = UINT32_MAX - (static_cast<uint64_t>(UINT32_MAX) + 1) % max_bounds;
uint32_t random_value = arc4random();
for (int i = 0; i < 20 && random_value > max_usable; ++i) {
// By chance we picked a value from the incomplete group. Note that this group has size at
// most 2**31-1, so picking this group has a chance of less than 50%.
// In practice, this means that for the worst possible input, there is still only a
// once-in-a-million chance to get to iteration 20. In theory we should be able to loop
// forever. Here we prefer marginally imperfect random numbers over weird runtime behavior.
random_value = arc4random();
}
return random_value % max_bounds;
return AK::get_random_uniform(max_bounds);
}
char* realpath(const char* pathname, char* buffer)