pub struct Stack {
    pub mem: [u8; 256],
    pub sp: u8,
}

impl Stack {
    pub fn new() -> Self {
        Self {
            mem: [0; 256],
            sp: 0,
        }
    }

    pub fn push_u8(&mut self, val: u8) {
        self.mem[self.sp as usize] = val;
        self.sp = self.sp.wrapping_add(1);
    }

    pub fn pop_u8(&mut self) -> u8 {
        self.sp = self.sp.wrapping_sub(1);
        self.mem[self.sp as usize]
    }

    pub fn push_u16(&mut self, val: u16) {
        let [high, low] = u16::to_be_bytes(val);
        self.mem[self.sp as usize] = high;
        self.sp  = self.sp.wrapping_add(1);
        self.mem[self.sp as usize] = low;
        self.sp  = self.sp.wrapping_add(1);
    }

    pub fn pop_u16(&mut self) -> u16 {
        self.sp  = self.sp.wrapping_sub(1);
        let low  = self.mem[self.sp as usize];
        self.sp  = self.sp.wrapping_sub(1);
        let high = self.mem[self.sp as usize];
        u16::from_be_bytes([high, low])
    }
}