#!/usr/bin/env ruby

def find_in_io(io, s, bufsz = 4096)
  buf1 = ''
  until io.eof?
    buf2 = io.read(bufsz)
    buf = buf1 + buf2
    pos = buf.index(s)
    if pos
      ret = io.pos - buf.size + pos
      io.seek(ret + s.size)
      return ret
    end
    buf1 = buf2
  end
  nil
end

def carve(io_in, io_out, from, to, bufsz = 4096)
  count = to - from
  io_in.seek(from)
  while count > 0
    chunksz = [bufsz, count].min
    io_out.write(io_in.read(chunksz))
    count -= chunksz
  end
end

PNG_START = "\x89PNG\r\n\x1A\n".b
PNG_END = 'IEND'.b

def carve_pngs(io)
  1.step do |i|
    break unless (start_pos = find_in_io(io, PNG_START))
    break unless (end_pos = find_in_io(io, PNG_END))
    end_pos += 8 # 4 bytes chunk name, 4 bytes CRC32 checksum
    File.open(format('%i.png', i), 'wb') { |f| carve(io, f, start_pos, end_pos) }
  end
end

def die(msg)
  STDERR.puts(msg)
  exit(1)
end

die("usage: #{$PROGRAM_NAME} <infile>") unless ARGV.count == 1
File.open(ARGV[0], 'rb') { |f| carve_pngs(f) }