compression: add a few comments, including MS-XCA pointers.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

diff --git a/lib/compression/lzxpress.c b/lib/compression/lzxpress.c
index f010037247424e1d5679bab2ee502dd0f76ada75..71b39c1efb379b5aa94508eeea3ebd320fd19844 100644 (file)
--- a/lib/compression/lzxpress.c
+++ b/lib/compression/lzxpress.c
@@ -58,6 +58,16 @@ ssize_t lzxpress_compress(const uint8_t *uncompressed,
                          uint8_t *compressed,
                          uint32_t max_compressed_size)
 {
+       /*
+        * This is the algorithm in [MS-XCA] 2.3 "Plain LZ77 Compression".
+        *
+        * It avoids Huffman encoding by including literal bytes inline when a
+        * match is not found. Every so often it includes a uint32 bit map
+        * flagging which positions contain matches and which contain
+        * literals. The encoding of matches is of variable size, depending on
+        * the match length; they are always at least 16 bits long, and can
+        * implicitly use unused half-bytes from earlier in the stream.
+        */
        uint32_t uncompressed_pos, compressed_pos;
        uint32_t indic;
        uint32_t indic_pos;
@@ -112,6 +122,11 @@ ssize_t lzxpress_compress(const uint8_t *uncompressed,
                }
 
                if (!found) {
+                       /*
+                        * This is going to literal byte, which we flag by
+                        * setting a bit in an indicator field somewhere
+                        * earlier in the stream.
+                        */
                        CHECK_INPUT_BYTES(sizeof(uint8_t));
                        CHECK_OUTPUT_BYTES(sizeof(uint8_t));
                        compressed[compressed_pos++] = uncompressed[uncompressed_pos++];
@@ -211,6 +226,10 @@ ssize_t lzxpress_decompress(const uint8_t *input,
                            uint8_t *output,
                            uint32_t max_output_size)
 {
+       /*
+        * This is the algorithm in [MS-XCA] 2.4 "Plain LZ77 Decompression
+        * Algorithm Details".
+        */
        uint32_t output_index, input_index;
        uint32_t indicator, indicator_bit;
        uint32_t nibble_index;