LibCompress: Add a method that describes a Gzip Header

This can be used to display information about a gzip file that is
stored in it's header.

Userland/Libraries/LibCompress/Gzip.cpp

@@ -9,6 +9,7 @@
 
 #include <AK/MemoryStream.h>
 #include <AK/String.h>
+#include <LibCore/DateTime.h>
 
 namespace Compress {
 
@@ -140,6 +141,19 @@ size_t GzipDecompressor::read(Bytes bytes)
     return total_read;
 }
 
+Optional<String> GzipDecompressor::describe_header(ReadonlyBytes bytes)
+{
+    if (bytes.size() < sizeof(BlockHeader))
+        return {};
+
+    auto& header = *(reinterpret_cast<const BlockHeader*>(bytes.data()));
+    if (!header.valid_magic_number() || !header.supported_by_implementation())
+        return {};
+
+    LittleEndian<u32> original_size = *reinterpret_cast<const u32*>(bytes.offset(bytes.size() - sizeof(u32)));
+    return String::formatted("last modified: {}, original size {}", Core::DateTime::from_timestamp(header.modification_time).to_string(), (u32)original_size);
+}
+
 bool GzipDecompressor::read_or_error(Bytes bytes)
 {
     if (read(bytes) < bytes.size()) {

Userland/Libraries/LibCompress/Gzip.h

@@ -50,6 +50,7 @@ public:
     bool handle_any_error() override;
 
     static Optional<ByteBuffer> decompress_all(ReadonlyBytes);
+    static Optional<String> describe_header(ReadonlyBytes);
     static bool is_likely_compressed(ReadonlyBytes bytes);
 
 private: