LibCompress: Add a PackBits decoder

This compression scheme was quite popular during the 80's, and we can
still find it in use inside file formats such as TIFF or PDF.

Userland/Libraries/LibCompress/PackBitsDecoder.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2023, Lucas Chollet <lucas.chollet@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "PackBitsDecoder.h"
+#include <AK/MemoryStream.h>
+
+namespace Compress::PackBits {
+
+ErrorOr<ByteBuffer> decode_all(ReadonlyBytes bytes, Optional<u64> expected_output_size, CompatibilityMode mode)
+{
+    // This implementation uses unsigned values for the selector, as described in the PDF spec.
+    // Note that this remains compatible with other implementations based on signed numbers.
+
+    auto memory_stream = make<FixedMemoryStream>(bytes);
+
+    ByteBuffer decoded_bytes;
+
+    if (expected_output_size.has_value())
+        TRY(decoded_bytes.try_ensure_capacity(*expected_output_size));
+
+    while (memory_stream->remaining() > 0 && decoded_bytes.size() < expected_output_size.value_or(NumericLimits<u64>::max())) {
+        auto const length = TRY(memory_stream->read_value<u8>());
+
+        if (length < 128) {
+            for (u8 i = 0; i <= length; ++i)
+                TRY(decoded_bytes.try_append(TRY(memory_stream->read_value<u8>())));
+        } else if (length > 128) {
+            auto const next_byte = TRY(memory_stream->read_value<u8>());
+
+            for (u8 i = 0; i < 257 - length; ++i)
+                TRY(decoded_bytes.try_append(next_byte));
+        } else {
+            VERIFY(length == 128);
+            if (mode == CompatibilityMode::PDF)
+                break;
+        }
+    }
+
+    return decoded_bytes;
+}
+
+}