ZlibBufferWriter.hpp

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#ifndef BYTEME_ZLIB_BUFFER_WRITER_HPP
#define BYTEME_ZLIB_BUFFER_WRITER_HPP
 
#include <stdexcept>
#include <vector>
#include <cstddef>
 
#include "zlib.h"
#include "sanisizer/sanisizer.hpp"
 
#include "Writer.hpp"
#include "magic_numbers.hpp"
#include "utils.hpp"
 
namespace byteme {
 
struct ZlibBufferWriterOptions {
    ZlibCompressionMode mode = ZlibCompressionMode::GZIP;
 
    int compression_level = 6;
 
    std::size_t buffer_size = sanisizer::cap<std::size_t>(65536);
};
 
class ZlibBufferWriter final : public Writer {
private:
    struct ZStream {
        ZStream(ZlibCompressionMode mode, int level) {
            strm.zalloc = Z_NULL;
            strm.zfree = Z_NULL;
            strm.opaque = Z_NULL;
            strm.avail_in = 0;
            strm.next_in = Z_NULL;
 
            // Check out https://zlib.net/manual.html for the constants.
            int windowBits;
            switch (mode) {
                case ZlibCompressionMode::DEFLATE:
                    windowBits = -15;
                    break;
                case ZlibCompressionMode::ZLIB:
                    windowBits = 15;
                    break;
                case ZlibCompressionMode::GZIP:
                    windowBits = 16 + 15;
                    break;
                default:
                    throw std::runtime_error("unknown Zlib compression mode");
            } 
 
            int ret = deflateInit2(&strm, level, Z_DEFLATED, windowBits, 8, Z_DEFAULT_STRATEGY);
            if (ret != Z_OK) {
                throw std::runtime_error("failed to initialize Zlib buffer compression");
            }
        }
 
        ~ZStream() {
            deflateEnd(&strm);
            return;
        }
 
    public:
        // Delete the remaining constructors.
        ZStream(const ZStream&) = delete;
        ZStream(ZStream&&) = delete;
        ZStream& operator=(const ZStream&) = delete;
        ZStream& operator=(ZStream&&) = delete;
 
    public:
        z_stream strm;
    };
    typedef I<decltype(I<decltype(ZStream::strm)>::avail_out)> ZoutSize;
 
public:
    ZlibBufferWriter(const ZlibBufferWriterOptions& options) : 
        my_zstr(options.mode, options.compression_level),
        // Cap it for both allocation safety and to avoid overflow of the avail_out member.
        my_holding(sanisizer::cap<I<decltype(my_holding.size())> >(sanisizer::cap<ZoutSize>(options.buffer_size))) 
    {}
 
    // One might think that we need to call finish() in the destructor, but in fact we don't.
    // This is because the only observable effect of finish() is to flush all remaining content to the output buffer;
    // but if we're destroying the object, the contents of the output buffer don't matter, as it'll be destroyed as well.
    // So in fact, it's totally acceptable to just not do anything on destruction.
 
public:
    using Writer::write;
 
    void write(const unsigned char* buffer, std::size_t n) {
        typedef I<decltype(I<decltype(ZStream::strm)>::avail_in)> Size; // constrained for the z_stream interface.
        safe_write<Size, false>(
            buffer,
            n,
            [&](const unsigned char* buffer0, Size n0) -> void {
                my_zstr.strm.next_in = const_cast<unsigned char*>(buffer0); // for C compatibility.
                my_zstr.strm.avail_in = n0;
                dump(Z_NO_FLUSH);
            }
        );
    }
 
    void finish() {
        my_zstr.strm.next_in = nullptr;
        my_zstr.strm.avail_in = 0;
        dump(Z_FINISH);
    }
 
private:
    ZStream my_zstr;
    std::vector<unsigned char> my_holding;
 
    void dump(int flag) {
        do {
            my_zstr.strm.avail_out = my_holding.size(); // no need to worry about overflow, we capped the size in the constructor.
            my_zstr.strm.next_out = my_holding.data();
            deflate(&(my_zstr.strm), flag); // no need to check, see https://zlib.net/zlib_how.html.
            std::size_t compressed = my_holding.size() - my_zstr.strm.avail_out;
            output.insert(output.end(), my_holding.begin(), my_holding.begin() + compressed);
        } while (my_zstr.strm.avail_out == 0);
    }
 
public:
    // Exposed for back-compatibility only.
    std::vector<unsigned char> output;
    std::vector<unsigned char>& get_output() {
        return output;
    }
};
 
}
 
#endif