I'm looking for a way to get the uncompressed stream size of an LZMA2 / .xz file compressed with the xz utility.
I'm using liblzma from Windows/Linux for this task, so I guess I'm looking for some C/C++ API in liblzma that will do the trick.
I think I've found a solution.
This is a very crude code sample, but seems to work fine.
I'm assuming I have a do_mmap() function that maps the entire file as read-only into memory, and returns the total size mapped. This can naturally be adapted to use read/fread/ReadFile or any other 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;
}
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