LibCompress: Add a utility to GZIP compress an entire file

This is copy-pasted from the gzip utility, along with its existing TODO.
This is currently only needed by that utility, but this gives us API
symmetry with GzipDecompressor, and helps ensure we won't end up in a
situation where only one utility receives optimizations that should be
received by all interested parties.

Userland/Libraries/LibCompress/Gzip.cpp

@@ -12,6 +12,8 @@
 #include <AK/MemoryStream.h>
 #include <LibCore/DateTime.h>
 #include <LibCore/File.h>
+#include <LibCore/MappedFile.h>
+#include <LibCore/System.h>
 
 namespace Compress {
 
@@ -262,4 +264,22 @@ ErrorOr<ByteBuffer> GzipCompressor::compress_all(ReadonlyBytes bytes)
     return buffer;
 }
 
+ErrorOr<void> GzipCompressor::compress_file(StringView input_filename, NonnullOwnPtr<Stream> output_stream)
+{
+    // We map the whole file instead of streaming to reduce size overhead (gzip header) and increase the deflate block size (better compression)
+    // TODO: automatically fallback to buffered streaming for very large files
+    RefPtr<Core::MappedFile> file;
+    ReadonlyBytes input_bytes;
+
+    if (TRY(Core::System::stat(input_filename)).st_size > 0) {
+        file = TRY(Core::MappedFile::map(input_filename));
+        input_bytes = file->bytes();
+    }
+
+    auto output_bytes = TRY(Compress::GzipCompressor::compress_all(input_bytes));
+    TRY(output_stream->write_until_depleted(output_bytes));
+
+    return {};
+}
+
 }

Userland/Libraries/LibCompress/Gzip.h

@@ -93,6 +93,7 @@ public:
     virtual void close() override;
 
     static ErrorOr<ByteBuffer> compress_all(ReadonlyBytes bytes);
+    static ErrorOr<void> compress_file(StringView input_file, NonnullOwnPtr<Stream> output_stream);
 
 private:
     MaybeOwned<Stream> m_output_stream;