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.)

Userland/Libraries/LibTest/Randomized/RandomnessSource.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2023, Martin Janiczek <martin@janiczek.cz>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibTest/Randomized/RandomRun.h>
+#include <LibTest/TestResult.h>
+
+namespace Test {
+namespace Randomized {
+
+// RandomnessSource provides random bits to Generators.
+//
+// If it's live, a PRNG will be used and the random values will be recorded into
+// its RandomRun.
+//
+// If it's recorded, its RandomRun will be used to "mock" the PRNG. This allows
+// us to replay the generation of a particular value, and to test out
+// "alternative histories": "what if the PRNG generated 0 instead of 13 here?"
+class RandomnessSource {
+public:
+    static RandomnessSource live() { return RandomnessSource(RandomRun(), true); }
+    static RandomnessSource recorded(RandomRun const& run) { return RandomnessSource(run, false); }
+    RandomRun& run() { return m_run; }
+    u32 draw_value(u32 max, Function<u32()> random_generator)
+    {
+        // Live: use the random generator and remember the value.
+        if (m_is_live) {
+            u32 value = random_generator();
+            m_run.append(value);
+            return value;
+        }
+
+        // Not live! let's get another prerecorded value.
+        auto next = m_run.next();
+        if (next.has_value()) {
+            return min(next.value(), max);
+        }
+
+        // Signal a failure. The value returned doesn't matter at this point but
+        // we need to return something.
+        set_current_test_result(TestResult::Overrun);
+        return 0;
+    }
+
+private:
+    explicit RandomnessSource(RandomRun const& run, bool is_live)
+        : m_run(run)
+        , m_is_live(is_live)
+    {
+    }
+    RandomRun m_run;
+    bool m_is_live;
+};
+
+} // namespace Randomized
+} // namespace Test