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Make deflateBound() account for a preset dictionary set after the call #1308

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Make deflateBound() account for a preset dictionary set after the call #1308

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4 changes: 3 additions & 1 deletion ChangeLog
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,9 @@
ChangeLog file for zlib

Changes in 1.3.2.1 (xx Feb 2026)
-
- deflateBound() now includes the preset-dictionary Adler-32 when the stream
header has not yet been emitted, so the bound holds when a dictionary is
provided after deflateBound() and before the first deflate()

Changes in 1.3.2 (17 Feb 2026)
- Continued rewrite of CMake build [Vollstrecker]
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10 changes: 9 additions & 1 deletion deflate.c
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Expand Up @@ -885,7 +885,15 @@ z_size_t ZEXPORT deflateBound_z(z_streamp strm, z_size_t sourceLen) {
wraplen = 0;
break;
case 1: /* zlib wrapper */
wraplen = 6 + (s->strstart ? 4 : 0);
/* A preset dictionary adds four bytes of its Adler-32 right after
the two-byte header. This can happen if a dictionary was already
provided (strstart is then non-zero), or if deflateBound() was
called before deflateSetDictionary() and the header has not yet
been emitted (status is still INIT_STATE, as no data has been
compressed). Include the four bytes in the latter case as well,
so the returned bound covers the documented use of allocating an
output buffer for a single-pass deflate(). */
wraplen = 6 + (s->strstart || s->status == INIT_STATE ? 4 : 0);
break;
#ifdef GZIP
case 2: /* gzip wrapper */
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