Add support for zlib default compression level -1

lib/deflate_compress.c

@@ -3885,6 +3885,13 @@ libdeflate_alloc_compressor_ex(int compression_level,
 	if (options->sizeof_options != sizeof(*options))
 		return NULL;
 
+	/*
+	 * Note: For similarity with zlib's API, -1 is accepted as an alias for
+	 * the default compression level.
+	 */
+	if (compression_level == -1)
+		compression_level = 6;
+
 	if (compression_level < 0 || compression_level > 12)
 		return NULL;
 

libdeflate.h

@@ -43,10 +43,11 @@ struct libdeflate_options;
  * level on a zlib-like scale but with a higher maximum value (1 = fastest, 6 =
  * medium/default, 9 = slow, 12 = slowest).  Level 0 is also supported and means
  * "no compression", specifically "create a valid stream, but only emit
- * uncompressed blocks" (this will expand the data slightly).
+ * uncompressed blocks" (this will expand the data slightly).  Level -1 is an
+ * alias indicating a default level of 6.
  *
  * The return value is a pointer to the new compressor, or NULL if out of memory
- * or if the compression level is invalid (i.e. outside the range [0, 12]).
+ * or if the compression level is invalid (i.e. outside the range [-1, 12]).
  *
  * Note: for compression, the sliding window size is defined at compilation time
  * to 32768, the largest size permissible in the DEFLATE format.  It cannot be

programs/gzip.c

@@ -563,7 +563,7 @@ tmain(int argc, tchar *argv[])
 	options.force = false;
 	options.keep = false;
 	options.test = false;
-	options.compression_level = 6;
+	options.compression_level = -1;
 	options.suffix = T(".gz");
 
 	while ((opt_char = tgetopt(argc, argv, optstring)) != -1) {