liblzma: Exports lzma_mt_block_size() as an API function.

The lzma_mt_block_size() was previously just an internal function for
the multithreaded .xz encoder. It is used to provide a recommended Block
size for a given filter chain.

This function is helpful to determine the maximum Block size for the
multithreaded .xz encoder when one wants to change the filters between
blocks. Then, this determined Block size can be provided to
lzma_stream_encoder_mt() in the lzma_mt options parameter when
intializing the coder. This requires one to know all the filter chains
they are using before starting to encode (or at least the filter chain
that will need the largest Block size), but that isn't a bad limitation.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/liblzma/api/lzma/container.h b/src/liblzma/api/lzma/container.h
index 48a64365..7e4ca3bf 100644
--- a/src/liblzma/api/lzma/container.h
+++ b/src/liblzma/api/lzma/container.h
@@ -436,6 +436,34 @@ extern LZMA_API(lzma_ret) lzma_stream_encoder_mt(
 
 
 /**
+ * \brief       Calculate recommended Block size for multithreaded .xz encoder
+ *
+ * This calculates a recommended Block size for multithreaded encoding given
+ * a filter chain. This is used internally by lzma_stream_encoder_mt() to
+ * determine the Block size if the block_size member is not set to the
+ * special value of 0 in the lzma_mt options struct.
+ *
+ * If one wishes to change the filters between Blocks, this function is
+ * helpful to set the block_size member of the lzma_mt struct before calling
+ * lzma_stream_encoder_mt(). Since the block_size member represents the
+ * maximum possible Block size for the multithreaded .xz encoder, one can
+ * use this function to find the maximum recommended Block size based on
+ * all planned filter chains. Otherwise, the multithreaded encoder will
+ * base its maximum Block size on the first filter chain used (if the
+ * block_size member is not set), which may unnecessarily limit the Block
+ * size for a later filter chain.
+ *
+ * \param       filters   Array of filters terminated with
+ *                        .id == LZMA_VLI_UNKNOWN.
+ *
+ * \return      Recommended Block size in bytes, or UINT64_MAX if
+ *              an error occurred.
+ */
+extern LZMA_API(uint64_t) lzma_mt_block_size(const lzma_filter *filters)
+		lzma_nothrow;
+
+
+/**
  * \brief       Initialize .lzma encoder (legacy file format)
  *
  * The .lzma format is sometimes called the LZMA_Alone format, which is the