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c++ - zlib 可以在不压缩文件其余部分的情况下删除填充吗？

问题描述

我正在寻找一种使用 zlib-1.2.5 应用特定压缩策略的方法。我需要强制 zlib 打包文件而不实际压缩它。我要压缩的唯一部分是文件末尾的 0 填充，以便输出文件没有它。输入文件的大小范围从 1MB 到 1GB，填充为 512B。这可以用zlib实现吗？

编辑：

我正在处理的代码基于 Unreal Engine 4： https ://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Runtime/Core/Private/Misc/Compression.cpp

static bool appCompressMemoryZLIB(void* CompressedBuffer, int32& CompressedSize, const void* UncompressedBuffer, int32 UncompressedSize, int32 BitWindow, int32 CompLevel)
{
    DECLARE_SCOPE_CYCLE_COUNTER(TEXT("Compress Memory ZLIB"), STAT_appCompressMemoryZLIB, STATGROUP_Compression);

    ensureMsgf(CompLevel >= Z_DEFAULT_COMPRESSION, TEXT("CompLevel must be >= Z_DEFAULT_COMPRESSION"));
    ensureMsgf(CompLevel <= Z_BEST_COMPRESSION, TEXT("CompLevel must be <= Z_BEST_COMPRESSION"));

    CompLevel = FMath::Clamp(CompLevel, Z_DEFAULT_COMPRESSION, Z_BEST_COMPRESSION);

    // Zlib wants to use unsigned long.
    unsigned long ZCompressedSize = CompressedSize;
    unsigned long ZUncompressedSize = UncompressedSize;
    bool bOperationSucceeded = false;

    // Compress data
    // If using the default Zlib bit window, use the zlib routines, otherwise go manual with deflate2
    if (BitWindow == 0 || BitWindow == DEFAULT_ZLIB_BIT_WINDOW)
    {
        bOperationSucceeded = compress2((uint8*)CompressedBuffer, &ZCompressedSize, (const uint8*)UncompressedBuffer, ZUncompressedSize, CompLevel) == Z_OK ? true : false;
    }
    else
    {
        z_stream stream;
        stream.next_in = (Bytef*)UncompressedBuffer;
        stream.avail_in = (uInt)ZUncompressedSize;
        stream.next_out = (Bytef*)CompressedBuffer;
        stream.avail_out = (uInt)ZCompressedSize;
        stream.zalloc = &zalloc;
        stream.zfree = &zfree;
        stream.opaque = Z_NULL;

        if (ensure(Z_OK == deflateInit2(&stream, CompLevel, Z_DEFLATED, BitWindow, MAX_MEM_LEVEL, Z_DEFAULT_STRATEGY)))
        {
            if (ensure(Z_STREAM_END == deflate(&stream, Z_FINISH)))
            {
                ZCompressedSize = stream.total_out;
                if (ensure(Z_OK == deflateEnd(&stream)))
                {
                    bOperationSucceeded = true;
                }
            }
            else
            {
                deflateEnd(&stream);
            }
        }
    }

    // Propagate compressed size from intermediate variable back into out variable.
    CompressedSize = ZCompressedSize;
    return bOperationSucceeded;
}

带参数：输入缓冲区大小 = 65kB，CompLevel=Z_DEFAULT_COMPRESSION，MAX_MEM_LEVEL=9，BitWindow=15

标签： c++compressionzlib

解决方案

不，那里没有。

如果您只想在最后去掉零，那是微不足道的。这是几行代码。你不需要 zlib 的 18,000 行代码来做到这一点。

如果您还想将这些零恢复到另一端的文件（“解压缩”），那也很简单。只需发送已删除的零的计数，并将该计数附加到末尾的几个字节中。另一方面，用那么多零字节替换该计数。还有几行代码。

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