LibTest: Add the Chunk abstraction

A prerequisite for shrinking the failing RandomRuns in randomized tests.

Userland/Libraries/LibTest/Randomized/Chunk.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023, Martin Janiczek <martin@janiczek.cz>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Format.h>
+#include <AK/String.h>
+
+namespace Test {
+namespace Randomized {
+
+// Chunk is a description of a RandomRun slice.
+// Used to say which part of a given RandomRun will be shrunk by some
+// ShrinkCommand.
+//
+// For a RandomRun [0,1,2,3,4,5,6,7,8], the Chunk{size=4, index=2} means this:
+//                 [_,_,X,X,X,X,_,_,_]
+//
+// Different ShrinkCommands will use the Chunk in different ways.
+// A few examples:
+//
+//     Original RandomRun:             [5,1,3,9,4,2,3,0]
+//     Chunk we'll show off:           [_,_,X,X,X,X,_,_]
+//
+//     ZeroChunk:                      [5,1,0,0,0,0,3,0]
+//     SortChunk:                      [5,1,2,3,4,9,3,0]
+//     DeleteChunkAndMaybeDecPrevious: [5,1,        3,0]
+struct Chunk {
+    // Possible sizes: 1,2,3,4,8
+    u8 size = 0;
+    size_t index = 0;
+};
+
+} // namespace Randomized
+} // namespace Test
+
+template<>
+struct AK::Formatter<Test::Randomized::Chunk> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder& builder, Test::Randomized::Chunk chunk)
+    {
+        return Formatter<StringView>::format(builder, TRY(String::formatted("Chunk<size={}, i={}>", chunk.size, chunk.index)));
+    }
+};