pub struct Metadata<'a> {
    program: &'a [u8],
}

impl<'a> Metadata<'a> {
    pub fn from(program: &'a [u8]) -> Option<Self> {
        if program.len() < 10 {
            return None;
        }
        let identifier = [0xE8,0x00,0x18,0x42,0x45,0x44,0x52,0x4F,0x43,0x4B];
        match program[0..10] == identifier {
            true => Some(Self { program }),
            false => None,
        }
    }

    pub fn name(&self) -> Option<String> {
        let pointer = self.get_pointer(0x000A)?;
        self.get_string(pointer)
    }

    pub fn authors(&self) -> Option<Vec<String>> {
        let pointer = self.get_pointer(0x000C)?;
        let string = self.get_string(pointer)?;
        Some(string.lines().map(String::from).collect())
    }

    pub fn description(&self) -> Option<String> {
        let pointer = self.get_pointer(0x000E)?;
        self.get_string(pointer)
    }

    pub fn bg_colour(&self) -> Option<MetadataColour> {
        let pointer = self.get_pointer(0x0010)?;
        let colour = self.get_pointer(pointer)?;
        Some(MetadataColour::from_double(colour))
    }

    pub fn fg_colour(&self) -> Option<MetadataColour> {
        let pointer = self.get_pointer(0x0012)?;
        let colour = self.get_pointer(pointer)?;
        Some(MetadataColour::from_double(colour))
    }

    pub fn small_icon(&self) -> Option<Vec<u8>> {
        let pointer = self.get_pointer(0x0014)?;
        self.get_icon(pointer, 72)
    }

    pub fn large_icon(&self) -> Option<Vec<u8>> {
        let pointer = self.get_pointer(0x0016)?;
        self.get_icon(pointer, 512)
    }

    fn get_byte(&self, address: u16) -> Option<u8> {
        self.program.get(address as usize).copied()
    }

    fn get_pointer(&self, address: u16) -> Option<u16> {
        let high = self.get_byte(address)?;
        let low = self.get_byte(address.checked_add(1)?)?;
        let pointer = u16::from_be_bytes([high, low]);
        if pointer != 0 { Some(pointer) } else { None }
    }

    fn get_string(&self, address: u16) -> Option<String> {
        let start = address as usize;
        let mut end = address as usize;
        loop {
            match self.program.get(end) {
                Some(0) => break,
                Some(_) => end += 1,
                None    => return None,
            }
        }
        let vec = self.program[start..end].to_vec();
        String::from_utf8(vec).ok()
    }

    fn get_icon(&self, address: u16, length: usize) -> Option<Vec<u8>> {
        let address = address as usize;
        let data = &self.program[address..address+length];
        match data.len() == length {
            true => Some(data.to_vec()),
            false => None,
        }
    }
}


pub struct MetadataColour {
    pub red: u8,
    pub green: u8,
    pub blue: u8,
}

impl MetadataColour {
    pub fn from_double(colour: u16) -> Self {
        let red   = (colour & 0x0F00 >> 8) as u8 * 17;
        let green = (colour & 0x00F0 >> 4) as u8 * 17;
        let blue  = (colour & 0x000F >> 0) as u8 * 17;
        Self { red, green, blue }
    }
}