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This compresses files as described in MS-XCA 2.2, and as decompressed
by the decompressor in the previous commit.
As with the decompressor, there are two public functions -- one that
uses a talloc context, and one that uses pre-allocated memory. The
compressor requires a tightly bound amount of auxillary memory
(>220kB) in a few different buffers, which is all gathered together in
the public struct lzxhuff_compressor_mem. An instantiated but not
initialised copy of this struct is required by the non-talloc
function; it can be used over and over again.
Our compression speed is about the same as the decompression speed
(between 20 and 500 MB/s on this laptop, depending on the data), and
our compression ratio is very similar to that of Windows.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
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This format is described in [MS-XCA] 2.1 and 2.2, with exegesis in
many posts on the cifs-protocol list[1].
The two public functions are:
ssize_t lzxpress_huffman_decompress(const uint8_t *input,
size_t input_size,
uint8_t *output,
size_t output_size);
uint8_t *lzxpress_huffman_decompress_talloc(TALLOC_CTX *mem_ctx,
const uint8_t *input_bytes,
size_t input_size,
size_t output_size);
In both cases the caller needs to know the *exact* decompressed size,
which is essential for decompression. The _talloc version allocates
the buffer for you, and uses the talloc context to allocate a 128k
working buffer. THe non-talloc function will allocate the working
buffer on the stack.
This compression format gives better compression for messages of
several kilobytes than the "plain" LXZPRESS compression, but is
probably a bit slower to decompress and is certainly worse for very
short messages, having a fixed 256 byte overhead for the first Huffman
table.
Experiments show decompression rates between 20 and 500 MB per second,
depending on the compression ratio and data size, on an i5-1135G7 with
no compiler optimisations.
This compression format is used in AD claims and in SMB, but that
doesn't happen with this commit.
I will not try to describe LZ77 or Huffman encoding here. Don't expect
an answer in MS-XCA either; instead read the code and/or Wikipedia.
[1] Much of that starts here:
https://lists.samba.org/archive/cifs-protocol/2022-October/
but there's more earlier, particularly in June/July 2020, when
Aurélien Aptel was working on an implementation that ended up in
Wireshark.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
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