Code shared from the Rust Playground

playground.rs

pub struct Cpu {
    pub memory: Vec<u8>,
    pub v: Vec<u8>,
    pub i: u16,
    pub dt: u8,
    pub st: u8,
    pub pc: u16,
    pub sp: u8,
    pub stack: Vec<u16>,
}

// impl Default and a bunch of methods omitted...

enum Instruction {
    CLS,
    RET,
    JMP(u16),
    CALL(u16),
    SE(u16),
    SNE(u16),
    // etc, etc...
}


#[derive(Debug)]
enum DecodeError {
    OpcodeNotRecognized(u16),
}

// impl Error and fmt::Display omitted...

impl Instruction {
    
    pub fn decode(cpu: &Cpu) -> Result<Instruction, DecodeError> {
        let first_byte: u16 = cpu.memory[usize::from(cpu.pc)].into();
        let second_byte: u16 = cpu.memory[usize::from(cpu.pc + 0x01)].into();
        let opcode: u16 = first_byte << 8 | second_byte;
        
        match opcode & 0xF000 {
            0x0000 => match opcode & 0x000F {
                0x0000 => Ok(Instruction::CLS),
                0x000E => Ok(Instruction::RET),
                _ => Err(DecodeError::OpcodeNotRecognized(opcode)),
            },
            // bunch of other opcode/instruction matches here...
            _ => Err(Decode::Error::OpcodeNotRecognized(opcode)),
        }
    }
}