//const ADD_XY: u8 = 0x8;
//
//struct CPU {
//    current_operation: u16,
//    registers: [u8; 2],
//}
//
//impl CPU {
//    fn run(&mut self) {
//        let encoded_op = self.current_operation;
//        let op = ((encoded_op & 0xf000) >> 12) as u8;
//        let x = ((encoded_op & 0x0f00) >> 8) as u8;
//        let y = ((encoded_op & 0x00f0) >> 4) as u8;
//
//        match op {
//            ADD_XY => {
//                self.add_xy(x, y);
//            },
//            _ => unimplemented!(),
//        }
//    }
//
//    fn add_xy(&mut self, x: u8, y: u8) {
//        self.registers[x as usize] += self.registers[y as usize];
//    }
//}
//
//
//fn main() {
//    let mut cpu = CPU {
//        current_operation: 0x8014,
//        registers: [0; 2],
//    };
//
//    cpu.registers[0] = 5;
//    cpu.registers[1] = 10;
//    cpu.run();
//
//    assert_eq!(cpu.registers[0], 15);
//    println!("5 + 10 = {}", cpu.registers[0]);
//}

//const ARITHMETIC_AND_LOGIC: u8 = 0x8;
//const HALT: u8 = 0x0;
//const ADD_XY: u8 = 0x4;
//
//struct CPU {
//    // current_operation: u16
//    registers: [u8; 16],
//    position_in_memory: usize,
//    memory: [u8; 4096],
//}
//
//impl CPU {
//    fn run(&mut self) {
//        loop {
//            let op_byte1 = self.memory[self.position_in_memory] as u16;
//            let op_byte2 = self.memory[self.position_in_memory + 1] as u16;
//            let raw_op = op_byte1 << 8 | op_byte2;
//
//            let op_major = ((raw_op & 0xf000) >> 12) as u8;
//            let x = ((raw_op & 0x0f00) >> 8) as u8;
//            let y = ((raw_op & 0x00f0) >> 4) as u8;
//            let op_minor = (raw_op & 0x000f) as u8;
//
//            self.position_in_memory += 2;
//
//            match (op_major, op_minor) {
//                (HALT, HALT) => { return; },
//                (ARITHMETIC_AND_LOGIC, ADD_XY) => self.add_xy(x, y),
//                _ => unimplemented!("opcode: {:04x}", raw_op),
//            }
//        }
//    }
//
//    fn add_xy(&mut self, x: u8, y: u8) {
//        self.registers[x as usize] += self.registers[y as usize];
//    }
//}
//
//fn main() {
//    let mut cpu = CPU {
//        registers: [0; 16],
//        memory: [0; 4096],
//        position_in_memory: 0,
//    };
//
//    cpu.registers[0] = 5;
//    cpu.registers[1] = 10;
//    cpu.registers[2] = 10;
//    cpu.registers[3] = 10;
//
//    cpu.memory[0] = 0x80;
//    cpu.memory[1] = 0x14;
//    cpu.memory[2] = 0x80;
//    cpu.memory[3] = 0x24;
//    cpu.memory[4] = 0x80;
//    cpu.memory[5] = 0x34;
//
//    cpu.run();
//
//    assert_eq!(cpu.registers[0], 35);
//    println!("5 + 10 + 10 + 10 = {}", cpu.registers[0]);
//
//}

struct CPU {
    registers: [u8; 16],
    position_in_memory: usize,
    memory: [u8; 4096],
    stack: [u16; 16],
    stack_pointer: usize,
}

impl CPU {
    fn run(&mut self) {
        loop {
            let op_byte1 = self.memory[self.position_in_memory] as u16;
            let op_byte2 = self.memory[self.position_in_memory + 1] as u16;
            let opcode = op_byte1 << 8 | op_byte2;

            let x = ((opcode & 0x0f00) >> 8) as u8;
            let y = ((opcode & 0x00f0) >> 4) as u8;
            let op_minor = (opcode & 0x000f) as u8;
            let addr = opcode & 0x0fff;

            self.position_in_memory += 2;

            match opcode {
                0x0000 => { return; },
                0x00ee => { self.ret(); },
                0x2000...0x2fff => { self.call(addr); },
                0x8000...0x8fff => {
                    match op_minor {
                        4 => { self.add_xy(x, y); }
                        _ => { unimplemented!("opcode: {:04x}", opcode); },
                    }
                },
                _ => unimplemented!("opcode {:04x}", opcode),
            }
        }
    }

    fn call(&mut self, addr: u16) {
        let sp = self.stack_pointer;
        let stack = &mut self.stack;

        if sp > stack.len() {
            panic!("stack overflow!");
        }

        stack[sp] = self.position_in_memory as u16;
        self.stack_pointer += 1;
        self.position_in_memory = addr as usize;

    }

    fn ret(&mut self) {
        if self.stack_pointer == 0 {
            panic!("stack underflow!");
        }

        self.stack_pointer -= 1;
        self.position_in_memory = self.stack[self.stack_pointer] as usize;
    }

    fn add_xy(&mut self, x: u8, y: u8) {
        self.registers[x as usize] += self.registers[y as usize];
    }
}

fn main() {
    let mut cpu = CPU {
        registers: [0; 16],
        memory: [0; 4096],
        position_in_memory: 0,
        stack: [0; 16],
        stack_pointer: 0,
    };

    cpu.registers[0] = 5;
    cpu.registers[1] = 10;

    cpu.memory[0x000] = 0x21;
    cpu.memory[0x001] = 0x00;
    cpu.memory[0x002] = 0x21;
    cpu.memory[0x003] = 0x00;
    cpu.memory[0x100] = 0x80;
    cpu.memory[0x101] = 0x14;
    cpu.memory[0x102] = 0x80;
    cpu.memory[0x103] = 0x14;
    cpu.memory[0x104] = 0x00;
    cpu.memory[0x105] = 0xee;

    cpu.run();

    assert_eq!(cpu.registers[0], 45);
    dbg!(cpu.registers[0]);
}