Increasing the expected compressed output size

Usage of CRC32 Castagnoli as a hash function for the hash table of symbols
used for compression has a side effect where for compression level [4, 5]
it will increase the required output buffer size by 0.1% (i.e. less than 1%)
for a high entropy input (i.e. random data).

To avoid a scenario where client code would fail while compressing data,
this patch will increase the compressBound by 0.8% (i.e. 8x than worst
case scenario).

Validated using zlib_bench with the data from the 'Random Compression
Challenge' on both Intel and Arm.

Bug: 990489
Change-Id: I86c6ab09fce6ab09b45c502221864400b86a7d80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733790
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#684170}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6c37c10f93e9d3ac37fd3018f15ec51340d58369

1 files changed, 12 insertions, 2 deletions

diff --git a/compress.c b/compress.c
index e2db404..6e85173 100644
--- a/compress.c
+++ b/compress.c
@@ -81,6 +81,16 @@ int ZEXPORT compress (dest, destLen, source, sourceLen)
 uLong ZEXPORT compressBound (sourceLen)
     uLong sourceLen;
 {
-    return sourceLen + (sourceLen >> 12) + (sourceLen >> 14) +
-           (sourceLen >> 25) + 13;
+    sourceLen = sourceLen + (sourceLen >> 12) + (sourceLen >> 14) +
+                (sourceLen >> 25) + 13;
+    /* FIXME(cavalcantii): usage of CRC32 Castagnoli as a hash function
+     * for the hash table of symbols used for compression has a side effect
+     * where for compression level [4, 5] it will increase the output buffer size
+     * by 0.1% (i.e. less than 1%) for a high entropy input (i.e. random data).
+     * To avoid a scenario where client code would fail, for safety we increase
+     * the expected output size by 0.8% (i.e. 8x more than the worst scenario).
+     * See: http://crbug.com/990489
+     */
+    sourceLen += sourceLen >> 7; // Equivalent to 1.0078125
+    return sourceLen;
 }