[erlang-patches] Patch to add zlib:crc32_combine/4 and zlib:adler32_combine/4

[Matthew Dempsky]

I wrote an Erlang module to concatenate zlib compressed data, but to
generate proper zlib and gzip trailers, I need to compute the adler32
and crc32 checksums.  adler32 has a simple algebraic structure that
makes it easy to implement in just a few lines of Erlang code (give it
a shot---it's fun :-), but combining crc32 checksums involves
computing x*2^n in a Galois field (which isn't so fun in Erlang).

Anyways, I noticed the zlib C library already provides functions for
these, but they're not exposed by the Erlang interface... so I wrote
the patch below.  (I'm mainly interested in crc32_combine, but I
thought it would be nice to have adler32_combine for completeness.)

--- erts/emulator/drivers/common/zlib_drv.c.orig	2007-11-06
11:44:12.000000000 -0800
+++ erts/emulator/drivers/common/zlib_drv.c	2007-11-06 11:30:11.000000000 -0800
@@ -55,6 +55,9 @@
 #define ADLER32_1         21
 #define ADLER32_2         22

+#define CRC32_COMBINE   23
+#define ADLER32_COMBINE 24
+
 #define DEFAULT_BUFSZ   4000

 static int zlib_init(void);
@@ -597,6 +600,26 @@
 	adler = adler32(adler, buf+4, len-4);
 	return zlib_value(adler, rbuf, rlen);
     }
+
+    case CRC32_COMBINE: {
+	uLong crc1, crc2, len2;
+	if (len != 12) goto badarg;
+	crc1 = i32(buf);
+	crc2 = i32(buf+4);
+	len2 = i32(buf+8);
+	crc1 = crc32_combine(crc1, crc2, len2);
+	return zlib_value(crc1, rbuf, rlen);
+    }
+
+    case ADLER32_COMBINE: {
+	uLong adler1, adler2, len2;
+	if (len != 12) goto badarg;
+	adler1 = i32(buf);
+	adler2 = i32(buf+4);
+	len2 = i32(buf+8);
+	adler1 = adler32_combine(adler1, adler2, len2);
+	return zlib_value(adler1, rbuf, rlen);
+    }
     }

  badarg:
--- lib/kernel/src/zlib.erl.orig	2007-11-06 11:20:14.000000000 -0800
+++ lib/kernel/src/zlib.erl	2007-11-06 11:43:28.000000000 -0800
@@ -25,6 +25,7 @@
 	 inflateSync/1,inflateReset/1,inflate/2,inflateEnd/1,
 	 setBufSize/2,getBufSize/1,
 	 crc32/1,crc32/2,crc32/3,adler32/2,adler32/3,getQSize/1,
+	 crc32_combine/4,adler32_combine/4,
 	 compress/1,uncompress/1,zip/1,unzip/1,
 	 gzip/1,gunzip/1]).

@@ -108,6 +109,9 @@
 -define(ADLER32_1,       21).
 -define(ADLER32_2,       22).

+-define(CRC32_COMBINE,   23).
+-define(ADLER32_COMBINE, 24).
+
 %% open a z_stream
 open() ->
     open_port({spawn, zlib_drv}, [binary]).
@@ -213,6 +217,16 @@
 adler32(_Z, _Adler, _Binary)  ->
     erlang:error(badarg).

+crc32_combine(Z, CRC1, CRC2, Len2) when is_integer(CRC1),
is_integer(CRC2), is_integer(Len2) ->
+    call(Z, ?CRC32_COMBINE, <<CRC1:32, CRC2:32, Len2:32>>);
+crc32_combine(_Z, _CRC1, _CRC2, _Len2) ->
+    erlang:error(badarg).
+
+adler32_combine(Z, Adler1, Adler2, Len2) when is_integer(Adler1),
is_integer(Adler2), is_integer(Len2) ->
+    call(Z, ?ADLER32_COMBINE, <<Adler1:32, Adler2:32, Len2:32>>);
+adler32_combine(_Z, _Adler1, _Adler2, _Len2) ->
+    erlang:error(badarg).
+
 getQSize(Z) ->
     call(Z, ?GET_QSIZE, []).