=begin
= Description
Can read/write a PNG file, if it is 8bpp with a palette (optionnally with the 1st entry transparent)

Unsupported:
* Adam7 interlaced
* non-paletted
* no 8bpp
* transparency other that only 1st palette fully transparent

= Usage:
(({
require 'libpng'

# print debug messages during loading
$DEBUG = true

img = Png.read('picture.png')
# read a pixel color
if img.px(30, 60) == [255, 0, 0]
	puts "oh, it's red !"
end

# change a palette entry
img.palette[4] = [0, 255, 0]
# change a pixel
img.lines[50][img.width-2] = 4
# save the result
img.write('mypicture.png')
}))

= Methods
--- Png.new(string, offset=0)
Reads a png stream from string, starting at offset.
Raises RuntimeError on error (invalid/unsupported png feature).

--- Png.px(x, y)
Returns the [r, g, b] array for the specified pixel, or nil if it is transparent.
Raises RuntimeError on error (invalid x/y/palette entry)

--- Png.write(filename, overwrite = false)
Writes the current image to a new file

--- Png#read(filename)
Reads a PNG file

= Author
Yoann Guillot, 2005

=end

require 'zlib'

class Png
	def Png.read(file)
		Png.new(File.read(file))
	end

	attr_reader :width, :height, :palette, :lines
	attr_accessor :trans
	
	def rd(raw, len)
		@off += len
		raw[(@off-len)...(@off)]
	end
	
	def decodeint(txt)
		len = 0
		txt.split(//).map{ |i| len <<= 8 ; len += i[0] }
		len
	end
	private :decodeint, :rd
	
	def initialize(raw, off = 0)
		@off = off
		sig = rd(raw, 8).split(//).map{ |i| i[0] }
		if sig != [137, 80, 78, 71, 13, 10, 26, 10]
			raise "Invalid signature: #{raw[off...(off+8)].inspect}"
		end

		compresseddata = ''
		loop do
			len = rd(raw, 4)
			raise "Premature end of file" if not len
			len = decodeint(len)
			chunktype = rd(raw, 4)
			chunk = rd(raw, len)
			crc = decodeint(rd(raw, 4))

			puts "Chunk #{chunktype}: #{len}o" if $DEBUG

			raise "Bad crc %.8X for #{chunktype}" % crc if crc != Zlib.crc32(chunktype+chunk)
			
			# http://www.w3.org/TR/PNG-Chunks.html
			case chunktype
			when 'IHDR'
				@width = decodeint(chunk[0..3])
				@height = decodeint(chunk[4..7])
				bitdepth = chunk[8]
				colourtype = chunk[9]	# bitmask 1: palette, 2: color, 4: alpha
				compression = chunk[10]
				filter = chunk[11]
				interlace = chunk[12]

				puts "#{@width}x#{@height}x#{bitdepth}bpp, colourtype #{colourtype}, compression #{compression}, filter #{filter}, interlace #{interlace}" if $DEBUG
				
				raise 'Unhandled PNG format' if bitdepth != 8 or colourtype != 3 or compression != 0 or filter != 0 or interlace != 0
				
			when 'IEND'
				break
			
			when 'PLTE'
				@palette = Array.new
				(len/3).times { |i|
					@palette << [chunk[3*i], chunk[3*i+1], chunk[3*i+2]]
				}
				
				puts "loaded #{@palette.length} colors palette" if $DEBUG

			when 'IDAT'
				compresseddata << chunk
			
			when 'tRNS'
				# alpha transparency palette, 0=transparent, defaults to opaque
				if len == 1 and chunk[0] == 0
					@trans = true
				else
					puts 'unhandled transparency chunk - ignored'
				end
			
			else
				puts "ignoring unhandled chunk type #{chunktype}"
			end
		end
		
		puts 'Inflating image data' if $DEBUG
		img = Zlib::Inflate.inflate(compresseddata)

		@lines = Array.new
		@height.times { |i|
			off = i * (@width+1)
			raise 'Unhandled line encoding' if img[off] != 0
			@lines << img[(off+1)..(off+@width)]
		}

		puts 'png load successful' if $DEBUG
	end
	
	# returns [r, g, b] for the pixel at (x, y), or nil if it is transparent
	def px(x, y)
		raise 'bad y' if y >= @height
		l = @lines[y]
		raise 'bad x' if x >= l.length
		i = l[x]
		raise 'bad color index' if i >= @palette.length
		@palette[i] if i > 0 or not @trans
	end

	def encodeint(i)
		((i >> 24) & 255).chr << ((i>>16) & 255) << ((i>>8) & 255) << (i & 255)
	end
	
	def write_chunk(fd, id, chk)
		fd.write encodeint(chk.length)
		fd.write id
		fd.write chk
		fd.write encodeint(Zlib.crc32(id+chk))
	end
	private :encodeint, :write_chunk
	
	def write(filename, overwrite = false)
		raise 'File exists' if not overwrite and File.exists? filename

		File.open(filename, 'w') { |fd|
		
			fd.write 137.chr << 80 << 78 << 71 << 13 << 10 << 26 << 10
			
			chk = encodeint(@width) << encodeint(@height) << 8 << 3 << 0 << 0 << 0
			write_chunk(fd, 'IHDR', chk)
			
			chk = @palette.map{ |e| e[0].chr << e[1] << e[2] }.join
			write_chunk(fd, 'PLTE', chk)
			
			write_chunk(fd, 'tRNS', 0.chr) if @trans
			
			chk = @lines.map { |l| 0.chr + l }.join
			chk = Zlib::Deflate.deflate(chk)
			write_chunk(fd, 'IDAT', chk)

			write_chunk(fd, 'IEND', '')
		}
	end

	def inspect
		"<Png: #{@width}x#{@height}, #{@palette.length-1} colors>"
	end
end