/**
 * The system device
 */
#[derive(Debug)]
struct SystemDevice {
    buffer: [u8; 0xF],
}

impl SystemDevice {
    fn new() -> SystemDevice {
        SystemDevice { buffer: [0; 0xF] }
    }
}

impl Device for SystemDevice {
    fn dei1(&mut self, vm: &mut Uxn, port: u8) -> u8 {
        let slot = port & 0xF;
        match slot {
            0x2 => vm.wst.idx,
            0x3 => vm.rst.idx,
            x => self.buffer[x as usize],
        }
    }
    fn dei2(&mut self, vm: &mut Uxn, port: u8) -> u16 {
        return ((self.dei1(vm, port) as u16) << 8) + self.dei1(vm, port + 1) as u16;
    }
    fn deo1(&mut self, vm: &mut Uxn, port: u8, val: u8) {
        let slot = port & 0xF;
        match slot {
            // Note: This VM does not support mutating the stack pointers.
            // This operation is not well defined upstream
            0x2 => panic!("Invoked unsupported mutation of the data stack pointer"),
            0x3 => panic!("Invoked unsupported mutation of the return stack pointer"),
            x => self.buffer[x as usize] = val,
        }
    }
    fn deo2(&mut self, vm: &mut Uxn, port: u8, val: u16) {
        let slot = port & 0xF;
        assert!(slot < 0xF, "Double-write beyond the end of the device");
        match slot {
            0x2 => panic!("The data stack is single-width, panic on spurious double-write"),
            0x3 => panic!("The return stack is single-width, panic on spurious double-write"),
            x => {
                let [high, low] = val.to_be_bytes();
                self.deo1(vm, x, high);
                self.deo1(vm, x + 1, low)
            }
        }
    }
}