From e6e1c19badac420e86797c3bc351a6b1c59c7107 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C5=99emysl=20Janouch?= <p.janouch@gmail.com>
Date: Sun, 21 May 2017 21:08:33 +0200
Subject: Add a partial decoder for RFC 1950: ZLIB

---
 plugins/zlib.lua | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 plugins/zlib.lua

diff --git a/plugins/zlib.lua b/plugins/zlib.lua
new file mode 100644
index 0000000..0290194
--- /dev/null
+++ b/plugins/zlib.lua
@@ -0,0 +1,57 @@
+--
+-- zlib.lua: ZLIB Compressed Data Format
+--
+-- Copyright (c) 2017, Přemysl Janouch <p.janouch@gmail.com>
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+-- WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+-- MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+-- SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+-- WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+-- OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+-- CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+--
+
+-- Based on RFC 1950, this format isn't very widely used
+local decode = function (c)
+	c.endianity = 'be'
+
+	local deflate_levels = { "fastest", "fast", "default", "slowest" }
+	local deflate
+	local cmf = c:u8 ("compression method and flags: %s", function (u8)
+		local cm,    cm_name    = u8 & 0xf, "unknown"
+		local cinfo, cinfo_name = u8 >> 4,  "unknown"
+		if cm == 8 then
+			deflate = true
+			cm_name = "deflate"
+			cinfo_name = ("window size = %d"):format (2 ^ (cinfo + 8))
+		end
+		return "%s (%d), %s (%d)", cm_name, cm, cinfo_name, cinfo
+	end)
+	local have_dict
+	local flags = c:u8 ("flags: %s", function (u8)
+		local check = (cmf * 256 + u8) % 31 == 0
+		if check then check = "ok" else check = "failed" end
+
+		local level, level_name = (u8 >> 6) & 3, "unknown"
+		if deflate then level_name = deflate_levels[level + 1] end
+		have_dict = (u8 >> 5) & 1 == 1
+
+		local info = "preset dictionary"
+		if not have_dict then info = "no " .. info end
+		return "level %s (%d), check %s, %s", level_name, level, check, info
+	end)
+	if have_dict then
+		c:u16 ("dictionary Adler-32 checksum: %04x")
+	end
+
+	-- Compressed data follows immediately
+	-- Not going to decompress it, so we don't know where the final checksum is
+end
+
+hex.register { type="zlib", detect=nil, decode=decode }
+
-- 
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