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LibTest: Add the RandomnessSource abstraction
This will be a foundational part of bootstrapping generators: this is the way they'll get prerecorded values from / record random values into RandomRuns. (Generators don't get in contact with RandomRuns themselves, they just interact with the RandomnessSource.)
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Userland/Libraries/LibTest/Randomized/RandomnessSource.h
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Userland/Libraries/LibTest/Randomized/RandomnessSource.h
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/*
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* Copyright (c) 2023, Martin Janiczek <martin@janiczek.cz>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Function.h>
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#include <LibTest/Randomized/RandomRun.h>
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#include <LibTest/TestResult.h>
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namespace Test {
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namespace Randomized {
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// RandomnessSource provides random bits to Generators.
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//
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// If it's live, a PRNG will be used and the random values will be recorded into
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// its RandomRun.
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//
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// If it's recorded, its RandomRun will be used to "mock" the PRNG. This allows
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// us to replay the generation of a particular value, and to test out
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// "alternative histories": "what if the PRNG generated 0 instead of 13 here?"
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class RandomnessSource {
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public:
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static RandomnessSource live() { return RandomnessSource(RandomRun(), true); }
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static RandomnessSource recorded(RandomRun const& run) { return RandomnessSource(run, false); }
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RandomRun& run() { return m_run; }
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u32 draw_value(u32 max, Function<u32()> random_generator)
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{
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// Live: use the random generator and remember the value.
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if (m_is_live) {
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u32 value = random_generator();
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m_run.append(value);
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return value;
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}
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// Not live! let's get another prerecorded value.
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auto next = m_run.next();
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if (next.has_value()) {
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return min(next.value(), max);
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}
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// Signal a failure. The value returned doesn't matter at this point but
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// we need to return something.
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set_current_test_result(TestResult::Overrun);
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return 0;
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}
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private:
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explicit RandomnessSource(RandomRun const& run, bool is_live)
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: m_run(run)
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, m_is_live(is_live)
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{
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}
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RandomRun m_run;
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bool m_is_live;
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};
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} // namespace Randomized
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} // namespace Test
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