unlzma: expand comments, no code changes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/archival/libarchive/decompress_unlzma.c b/archival/libarchive/decompress_unlzma.c
index 90a428583..a9040877e 100644
--- a/archival/libarchive/decompress_unlzma.c
+++ b/archival/libarchive/decompress_unlzma.c
@@ -432,6 +432,21 @@ unpack_lzma_stream(transformer_state_t *xstate)
                         }
 
                         len += LZMA_MATCH_MIN_LEN;
+                        /*
+                         * LZMA SDK has this optimized:
+                         * it precalculates size and copies many bytes
+                         * in a loop with simpler checks, a-la:
+                         *      do
+                         *              *(dest) = *(dest + ofs);
+                         *      while (++dest != lim);
+                         * and
+                         *      do {
+                         *              buffer[buffer_pos++] = buffer[pos];
+                         *              if (++pos == header.dict_size)
+                         *                      pos = 0;
+                         *      } while (--cur_len != 0);
+                         * Our code is slower (more checks per byte copy):
+                         */
  IF_NOT_FEATURE_LZMA_FAST(string:)
                         do {
                                 uint32_t pos = buffer_pos - rep0;
@@ -451,6 +466,9 @@ unpack_lzma_stream(transformer_state_t *xstate)
                         } while (len != 0 && buffer_pos < header.dst_size);
                         /* FIXME: ...........^^^^^
                          * shouldn't it be "global_pos + buffer_pos < header.dst_size"?
+                         * It probably should, but it is a "do we accidentally
+                         * unpack more bytes than expected?" check - which
+                         * never happens for well-formed compression data...
                          */
                 }
         }