Fix deflateBound and compressBound returning very small size estimates.

compress.c

@@ -91,10 +91,11 @@ z_size_t Z_EXPORT PREFIX(compressBound)(z_size_t sourceLen) {
         return complen + ZLIB_WRAPLEN;
 
 #ifndef NO_QUICK_STRATEGY
-    /* Quick deflate strategy worse case is 9 bits per literal, rounded to nearest byte,
-       plus the size of block & gzip headers and footers */
-    return sourceLen + ((sourceLen + 13 + 7) >> 3) + 18;
+    return sourceLen                       /* The source size itself */
+      + DEFLATE_QUICK_OVERHEAD(sourceLen)  /* Source encoding overhead, padded to next full byte */
+      + DEFLATE_BLOCK_OVERHEAD             /* Deflate block overhead bytes */
+      + ZLIB_WRAPLEN;                      /* zlib wrapper */
 #else
-    return sourceLen + (sourceLen >> 12) + (sourceLen >> 14) + (sourceLen >> 25) + 13;
+    return sourceLen + (sourceLen >> 4) + 7 + ZLIB_WRAPLEN;
 #endif
 }

deflate.c

@@ -611,11 +611,11 @@ unsigned long Z_EXPORT PREFIX(deflateBound)(PREFIX3(stream) *strm, unsigned long
         wraplen = 0;
         break;
     case 1:                                 /* zlib wrapper */
-        wraplen = 6 + (s->strstart ? 4 : 0);
+        wraplen = ZLIB_WRAPLEN + (s->strstart ? 4 : 0);
         break;
 #ifdef GZIP
     case 2:                                 /* gzip wrapper */
-        wraplen = 18;
+        wraplen = GZIP_WRAPLEN;
         if (s->gzhead != NULL) {            /* user-supplied gzip header */
             unsigned char *str;
             if (s->gzhead->extra != NULL) {
@@ -639,16 +639,22 @@ unsigned long Z_EXPORT PREFIX(deflateBound)(PREFIX3(stream) *strm, unsigned long
         break;
 #endif
     default:                                /* for compiler happiness */
-        wraplen = 6;
+        wraplen = ZLIB_WRAPLEN;
     }
 
     /* if not default parameters, return conservative bound */
     if (DEFLATE_NEED_CONSERVATIVE_BOUND(strm) ||  /* hook for IBM Z DFLTCC */
             s->w_bits != 15 || HASH_BITS < 15)
         return complen + wraplen;
 
-    /* default settings: return tight bound for that case */
-    return sourceLen + (sourceLen >> 12) + (sourceLen >> 14) + (sourceLen >> 25) + 13 - 6 + wraplen;
+#ifndef NO_QUICK_STRATEGY
+    return sourceLen                       /* The source size itself */
+      + DEFLATE_QUICK_OVERHEAD(sourceLen)  /* Source encoding overhead, padded to next full byte */
+      + DEFLATE_BLOCK_OVERHEAD             /* Deflate block overhead bytes */
+      + wraplen;                           /* none, zlib or gzip wrapper */
+#else
+    return sourceLen + (sourceLen >> 4) + 7 + wraplen;
+#endif
 }
 
 /* =========================================================================

test/switchlevels.c

@@ -68,7 +68,7 @@ static int compress_chunk(PREFIX3(stream) *strm, int level, int size, int last)
         goto done;
     }
 
-    compsize = 100 + 2 * PREFIX(deflateBound)(strm, size);
+    compsize = PREFIX(deflateBound)(strm, size);
     buf = malloc(size + compsize);
     if (buf == NULL) {
         fprintf(stderr, "Out of memory\n");

zutil.h

@@ -78,7 +78,19 @@ extern z_const char * const PREFIX(z_errmsg)[10]; /* indexed by 2-zlib_error */
 #define ADLER32_INITIAL_VALUE 1 /* initial adler-32 hash value */
 #define CRC32_INITIAL_VALUE   0 /* initial crc-32 hash value */
 
-#define ZLIB_WRAPLEN 6 /* zlib format overhead */
+#define ZLIB_WRAPLEN 6      /* zlib format overhead */
+#define GZIP_WRAPLEN 18     /* gzip format overhead */
+
+#define DEFLATE_HEADER_BITS 3
+#define DEFLATE_EOBS_BITS   15
+#define DEFLATE_PAD_BITS    6
+#define DEFLATE_BLOCK_OVERHEAD ((DEFLATE_HEADER_BITS + DEFLATE_EOBS_BITS + DEFLATE_PAD_BITS) >> 3)
+/* deflate block overhead: 3 bits for block start + 15 bits for block end + padding to nearest byte */
+
+#define DEFLATE_QUICK_LIT_MAX_BITS 9
+#define DEFLATE_QUICK_OVERHEAD(x) ((x * (DEFLATE_QUICK_LIT_MAX_BITS - 8) + 7) >> 3)
+/* deflate_quick worst-case overhead: 9 bits per literal, round up to next byte (+7) */
+
 
         /* target dependencies */