Add deflateGetDictionary() function.

Per request, but its utility is likely to be very limited. See the
comments in zlib.h.

Based on upstream ee7d7b5dda25c111e61e19ac7b476c26aa6f3020

diff --git a/deflate.c b/deflate.c
index e28a9a80639b87a4073506229dc9ee56281e495e..90415d35a646a05fd32fd017901c0832d6b3a0d0 100644 (file)
--- a/deflate.c
+++ b/deflate.c
@@ -415,6 +415,23 @@ int ZEXPORT deflateSetDictionary(z_stream *strm, const unsigned char *dictionary
     return Z_OK;
 }
 
+/* ========================================================================= */
+int ZEXPORT deflateGetDictionary (z_stream *strm, unsigned char *dictionary, unsigned int  *dictLength) {
+    deflate_state *s;
+
+    if (deflateStateCheck(strm))
+        return Z_STREAM_ERROR;
+    s = strm->state;
+    unsigned int len = s->strstart + s->lookahead;
+    if (len > s->w_size)
+        len = s->w_size;
+    if (dictionary != NULL && len)
+        memcpy(dictionary, s->window + s->strstart + s->lookahead - len, len);
+    if (dictLength != NULL)
+        *dictLength = len;
+    return Z_OK;
+}
+
 /* ========================================================================= */
 int ZEXPORT deflateResetKeep(z_stream *strm) {
     deflate_state *s;

diff --git a/zlib.h b/zlib.h
index ff7441415e9623adb638d94ace4ad30bed8b1acc..1cec7a5d92066eb4b60e07d39775a9466b8d5a6b 100644 (file)
--- a/zlib.h
+++ b/zlib.h
@@ -648,6 +648,26 @@ ZEXTERN int ZEXPORT deflateSetDictionary(z_stream *strm,
    not perform any compression: this will be done by deflate().
 */
 
+ZEXTERN int ZEXPORT deflateGetDictionary (z_stream *strm, unsigned char *dictionary, unsigned int  *dictLength) {
+/*
+     Returns the sliding dictionary being maintained by deflate.  dictLength is
+   set to the number of bytes in the dictionary, and that many bytes are copied
+   to dictionary.  dictionary must have enough space, where 32768 bytes is
+   always enough.  If deflateGetDictionary() is called with dictionary equal to
+   Z_NULL, then only the dictionary length is returned, and nothing is copied.
+   Similary, if dictLength is Z_NULL, then it is not set.
+
+     deflateGetDictionary() may return a length less than the window size, even
+   when more than the window size in input has been provided. It may return up
+   to 258 bytes less in that case, due to how zlib's implementation of deflate
+   manages the sliding window and lookahead for matches, where matches can be
+   up to 258 bytes long. If the application needs the last window-size bytes of
+   input, then that would need to be saved by the application outside of zlib.
+
+     deflateGetDictionary returns Z_OK on success, or Z_STREAM_ERROR if the
+   stream state is inconsistent.
+*/
+
 ZEXTERN int ZEXPORT deflateCopy(z_stream *dest, z_stream *source);
 /*
      Sets the destination stream as a complete copy of the source stream.