bzip2-1.0.3bzip2-1.0.3

1 files changed, 20 insertions, 3 deletions

diff --git a/huffman.c b/huffman.c
index 293095c..5bf190b 100644
--- a/huffman.c
+++ b/huffman.c
@@ -8,7 +8,7 @@
   This file is a part of bzip2 and/or libbzip2, a program and
   library for lossless, block-sorting data compression.
 
-  Copyright (C) 1996-2002 Julian R Seward.  All rights reserved.
+  Copyright (C) 1996-2005 Julian R Seward.  All rights reserved.
 
   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
@@ -42,7 +42,7 @@
   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
   Julian Seward, Cambridge, UK.
-  jseward@acm.org
+  jseward@bzip.org
   bzip2/libbzip2 version 1.0 of 21 March 2000
 
   This program is based on (at least) the work of:
@@ -162,7 +162,24 @@ void BZ2_hbMakeCodeLengths ( UChar *len,
       
       if (! tooLong) break;
 
-      for (i = 1; i < alphaSize; i++) {
+      /* 17 Oct 04: keep-going condition for the following loop used
+         to be 'i < alphaSize', which missed the last element,
+         theoretically leading to the possibility of the compressor
+         looping.  However, this count-scaling step is only needed if
+         one of the generated Huffman code words is longer than
+         maxLen, which up to and including version 1.0.2 was 20 bits,
+         which is extremely unlikely.  In version 1.0.3 maxLen was
+         changed to 17 bits, which has minimal effect on compression
+         ratio, but does mean this scaling step is used from time to
+         time, enough to verify that it works.
+
+         This means that bzip2-1.0.3 and later will only produce
+         Huffman codes with a maximum length of 17 bits.  However, in
+         order to preserve backwards compatibility with bitstreams
+         produced by versions pre-1.0.3, the decompressor must still
+         handle lengths of up to 20. */
+
+      for (i = 1; i <= alphaSize; i++) {
          j = weight[i] >> 8;
          j = 1 + (j / 2);
          weight[i] = j << 8;