Fix PNG compression

aaaaaa123456789 · Jan 22, 2022 · d15765a · d15765a
1 parent 121d9be
commit d15765a
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diff --git a/docs/changelog.md b/docs/changelog.md
@@ -14,6 +14,7 @@ Note: releases are listed from latest to oldest.
 - Added some warning flags for debug builds, and cleared some warnings that would be raised by them
 - Added and improved some safety checks that detect maliciously-crafted and other pathological files
 - Added detection for empty GIF files (erroring out with `PLUM_ERR_NO_DATA` instead of `PLUM_ERR_INVALID_FILE_FORMAT`)
+- Improved PNG compression by fixing a lookback bug in the compressor
 - Some minor documentation updates and code cleanup
 
 ## Version 0.3 (10 January 2022)

diff --git a/src/pngcompress.c b/src/pngcompress.c
@@ -140,7 +140,7 @@ unsigned find_PNG_reference (const unsigned char * data, const uint16_t * refere
 
 void append_PNG_reference (const unsigned char * restrict data, size_t offset, uint16_t * restrict references) {
   uint_fast32_t key = compute_PNG_reference_key(data + offset) * (uint_fast32_t) PNG_MAX_LOOKBACK_COUNT;
-  memmove(references + key + 1, references + key, 7 * sizeof *references);
+  memmove(references + key + 1, references + key, (PNG_MAX_LOOKBACK_COUNT - 1) * sizeof *references);
   references[key] = offset & 0x7fff;
 }