如何获取LZMA2文件的未压缩大小(.xz/liblzma)

Question

我正在寻找一种方法来获取使用xz实用程序压缩的LZMA2/.xz文件的未压缩流大小.

我正在使用Windows/Linux的liblzma来完成这项任务,所以我想我正在寻找liblzma中的一些C/C++ API来实现这个目的.

Answer 1

我想我找到了解决方案.

这是一个非常粗糙的代码示例,但似乎工作正常.

我假设我有一个do_mmap()函数,它将整个文件作为只读映射到内存中,并返回映射的总大小.这自然可以适用于使用read/fread/ReadFile或任何其他File API.

extern size_t get_uncompressed_size(const char *filename)
{
   lzma_stream_flags stream_flags;
   int file_size;

   const uint8_t *data = (uint8_t *) do_mmap(filename, &file_size);

   // 12 is the size of the footer per the file-spec...
   const uint8_t *footer_ptr = data + file_size - 12;

   // Something is terribly wrong
   if (footer_ptr < data) {
     do_unmap((void *)data, file_size);
     return -1;
   }

   // Decode the footer, so we have the backward_size pointing to the index
   lzma_stream_footer_decode(&stream_flags, (const uint8_t *)footer_ptr);
   // This is the index pointer, where the size is ultimately stored...
   const uint8_t *index_ptr = footer_ptr - stream_flags.backward_size;
   // Allocate an index
   lzma_index *index = lzma_index_init(NULL);
   uint64_t memlimit;
   size_t in_pos = 0;
   // decode the index we calculated
   lzma_index_buffer_decode(&index, &memlimit, NULL, index_ptr, &in_pos, footer_ptr - index_ptr);
   // Just make sure the whole index was decoded, otherwise, we might be
   // dealing with something utterly corrupt
   if (in_pos != stream_flags.backward_size) {
     do_unmap((void *)data, file_size);
     lzma_index_end(index, NULL);
     return -1;
   }
   // Finally get the size
   lzma_vli uSize = lzma_index_uncompressed_size(index);
   lzma_index_end(index, NULL);
   return (size_t) uSize;
}