From 578d5045754d67f900b8edc2ee63da8efc968022 Mon Sep 17 00:00:00 2001
From: Lars Mueller <appgurulars@gmx.de>
Date: Thu, 21 Oct 2021 15:40:04 +0200
Subject: [PATCH] Add modlib.minetest.encode_png

---
 minetest.lua            |   3 +-
 minetest/encode_png.lua | 118 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 120 insertions(+), 1 deletion(-)
 create mode 100644 minetest/encode_png.lua

diff --git a/minetest.lua b/minetest.lua
index 7966627..4026568 100644
--- a/minetest.lua
+++ b/minetest.lua
@@ -6,7 +6,8 @@ for _, value in pairs{
 	"raycast",
 	"schematic",
 	"colorspec",
-	"media"
+	"media",
+	"encode_png",
 } do
 	components[value] = value
 end
diff --git a/minetest/encode_png.lua b/minetest/encode_png.lua
new file mode 100644
index 0000000..1e9d940
--- /dev/null
+++ b/minetest/encode_png.lua
@@ -0,0 +1,118 @@
+if minetest.encode_png then
+	return minetest.encode_png
+end
+
+local assert, char, ipairs, insert, concat, floor = assert, string.char, ipairs, table.insert, table.concat, math.floor
+
+-- TODO move to modlib.bit eventually
+local bit_xor = bit.xor or function(a, b)
+	local res = 0
+	local bit = 1
+	for _ = 1, 32 do
+		if a % 2 ~= b % 2 then
+			res = res + bit
+		end
+		a = floor(a / 2)
+		b = floor(b / 2)
+		bit = bit * 2
+	end
+	return res
+end
+
+local crc_table = {}
+for i = 0, 255 do
+	local c = i
+	for _ = 0, 7 do
+		if c % 2 > 0 then
+			c = bit_xor(0xEDB88320, floor(c / 2))
+		else
+			c = floor(c / 2)
+		end
+	end
+	crc_table[i] = c
+end
+
+local function encode_png(width, height, data, compression, raw_write)
+	local write = raw_write
+	local function byte(value)
+		write(char(value))
+	end
+	local function _uint(value)
+		local div = 0x1000000
+		for _ = 1, 4 do
+			byte(floor(value / div) % 0x100)
+			div = div / 0x100
+		end
+	end
+	local function uint(value)
+		assert(value < 2^31)
+		_uint(value)
+	end
+	local chunk_content
+	local function chunk_write(text)
+		insert(chunk_content, text)
+	end
+	local function chunk(type)
+		chunk_content = {}
+		write = chunk_write
+		write(type)
+	end
+	local function end_chunk()
+		write = raw_write
+		local chunk_len = 0
+		for i = 2, #chunk_content do
+			chunk_len = chunk_len + #chunk_content[i]
+		end
+		uint(chunk_len)
+		write(concat(chunk_content))
+		local chunk_crc = 0xFFFFFFFF
+		for _, text in ipairs(chunk_content) do
+			for i = 1, #text do
+				chunk_crc = bit_xor(crc_table[bit_xor(chunk_crc % 0x100, text:byte(i))], floor(chunk_crc / 0x100))
+			end
+		end
+		_uint(bit_xor(chunk_crc, 0xFFFFFFFF))
+	end
+	-- Signature
+	write"\137\80\78\71\13\10\26\10"
+	chunk"IHDR"
+	uint(width)
+	uint(height)
+	-- Always use bit depth 8
+	byte(8)
+	-- Always use color type "truecolor with alpha"
+	byte(6)
+	-- Compression method: deflate
+	byte(0)
+	-- Filter method: PNG filters
+	byte(0)
+	-- No interlace
+	byte(0)
+	end_chunk()
+	chunk"IDAT"
+	local data_bytestring = {}
+	for y = 0, height - 1 do
+		local base = y * width
+		insert(data_bytestring, "\0")
+		for x = 1, width do
+			local color_int = data[base + x]
+			local a = floor(color_int / 0x1000000) % 0x100
+			local r = floor(color_int / 0x10000) % 0x100
+			local g = floor(color_int / 0x100) % 0x100
+			local b = color_int % 0x100
+			insert(data_bytestring, char(r, g, b, a))
+		end
+	end
+	write(minetest.compress(concat(data_bytestring), "deflate", compression))
+	end_chunk()
+	chunk"IEND"
+	end_chunk()
+end
+
+(...).encode_png = function(width, height, data, compression)
+	local rope = {}
+	encode_png(width, height, data, compression or 9, function(text)
+		insert(rope, text)
+	end)
+	return concat(rope)
+end
\ No newline at end of file