split the hash slide out of fill window

as a preparation to optimize it.

"Sliding" happens when the compression window/2 gets moved.
In this case the hash values are adapted, which means
basically an unsigned subtration with saturation (no wrap
at zero).

Many processor have an instruction for this, but the
compiler, trapped in the C-type systems, is seldomly
capable to exploit this.

INDEX

@@ -59,6 +59,7 @@ inflate.c
 inflate.h
 inftrees.c
 inftrees.h
+slhash.c
 trees.c
 trees.h
 uncompr.c

Makefile.in

@@ -53,10 +53,10 @@ mandir = ${prefix}/share/man
 man3dir = ${mandir}/man3
 pkgconfigdir = ${libdir}/pkgconfig
 
-OBJC = adler32.o compress.o crc32.o deflate.o gzclose.o gzlib.o gzread.o \
+OBJC = adler32.o compress.o crc32.o deflate.o slhash.o gzclose.o gzlib.o gzread.o \
 	gzwrite.o infback.o inffast.o inflate.o inftrees.o trees.o uncompr.o zutil.o
 
-PIC_OBJC = adler32.lo compress.lo crc32.lo deflate.lo gzclose.lo gzlib.lo gzread.lo \
+PIC_OBJC = adler32.lo compress.lo crc32.lo deflate.lo slhash.lo gzclose.lo gzlib.lo gzread.lo \
 	gzwrite.lo infback.lo inffast.lo inflate.lo inftrees.lo trees.lo uncompr.lo zutil.lo
 
 # to use the asm code: make OBJA=match.o, PIC_OBJA=match.lo
@@ -244,6 +244,7 @@ gzclose.o gzlib.o gzread.o gzwrite.o: zlib.h zconf.h gzguts.h
 compress.o example.o minigzip.o uncompr.o: zlib.h zconf.h
 crc32.o: zutil.h zlib.h zconf.h crc32.h
 deflate.o: deflate.h zutil.h zlib.h zconf.h
+slhash.o: deflate.h zutil.h zlib.h zconf.h
 infback.o inflate.o: zutil.h zlib.h zconf.h inftrees.h inflate.h inffast.h inffixed.h
 inffast.o: zutil.h zlib.h zconf.h inftrees.h inflate.h inffast.h
 inftrees.o: zutil.h zlib.h zconf.h inftrees.h
@@ -255,6 +256,7 @@ gzclose.lo gzlib.lo gzread.lo gzwrite.lo: zlib.h zconf.h gzguts.h
 compress.lo example.lo minigzip.lo uncompr.lo: zlib.h zconf.h
 crc32.lo: zutil.h zlib.h zconf.h crc32.h
 deflate.lo: deflate.h zutil.h zlib.h zconf.h
+slhash.lo: deflate.h zutil.h zlib.h zconf.h
 infback.lo inflate.lo: zutil.h zlib.h zconf.h inftrees.h inflate.h inffast.h inffixed.h
 inffast.lo: zutil.h zlib.h zconf.h inftrees.h inflate.h inffast.h
 inftrees.lo: zutil.h zlib.h zconf.h inftrees.h

deflate.c

@@ -1317,8 +1317,7 @@ local void check_match(s, start, match, length)
 local void fill_window(s)
     deflate_state *s;
 {
-    register unsigned n, m;
-    register Posf *p;
+    register unsigned n;
     unsigned more;    /* Amount of free space at the end of the window. */
     uInt wsize = s->w_size;
 
@@ -1354,24 +1353,7 @@ local void fill_window(s)
                later. (Using level 0 permanently is not an optimal usage of
                zlib, so we don't care about this pathological case.)
              */
-            n = s->hash_size;
-            p = &s->head[n];
-            do {
-                m = *--p;
-                *p = (Pos)(m >= wsize ? m-wsize : NIL);
-            } while (--n);
-
-            n = wsize;
-#ifndef FASTEST
-            p = &s->prev[n];
-            do {
-                m = *--p;
-                *p = (Pos)(m >= wsize ? m-wsize : NIL);
-                /* If n is not on any hash chain, prev[n] is garbage but
-                 * its value will never be used.
-                 */
-            } while (--n);
-#endif
+            _sh_slide(s->head, s->prev, wsize, s->hash_size);
             more += wsize;
         }
         if (s->strm->avail_in == 0) return;

deflate.h

@@ -297,6 +297,8 @@ void ZLIB_INTERNAL _tr_flush_block OF((deflate_state *s, charf *buf,
 void ZLIB_INTERNAL _tr_align OF((deflate_state *s));
 void ZLIB_INTERNAL _tr_stored_block OF((deflate_state *s, charf *buf,
                         ulg stored_len, int last));
+        /* in slhash.c */
+void ZLIB_INTERNAL _sh_slide OF((Posf *p, Posf *q, uInt wsize, unsigned n));
 
 #define d_code(dist) \
    ((dist) < 256 ? _dist_code[dist] : _dist_code[256+((dist)>>7)])

slhash.c

@@ -0,0 +1,30 @@
+/* slhash.c -- slide the hash table during fill_window()
+ * Copyright (C) 1995-2010 Jean-loup Gailly and Mark Adler
+ * For conditions of distribution and use, see copyright notice in zlib.h
+ */
+#include "deflate.h"
+#define NIL 0
+
+local void update_hoffset(Posf *p, uInt wsize, unsigned n)
+{
+    register unsigned m;
+    do {
+        m = *p;
+        *p++ = (Pos)(m >= wsize ? m-wsize : NIL);
+    } while (--n);
+}
+
+void ZLIB_INTERNAL _sh_slide (p, q, wsize, n)
+    Posf *p;
+    Posf *q;
+    uInt wsize;
+    unsigned n;
+{
+    update_hoffset(p, wsize, n);
+#ifndef FASTEST
+    /* If n is not on any hash chain, prev[n] is garbage but
+     * its value will never be used.
+     */
+    update_hoffset(q, wsize, wsize);
+#endif
+}