chip8/src/emu.rs

use super::periph::{Display, Keypad, Timer};
use std::fs::File;
use std::io::{self, Read};
use std::path::Path;

#[derive(Copy, Clone)]
pub struct Chip8 {
    opcode: u16,
    i: u16,
    pc: u16,
    sp: u8,
    key: Keypad,
    v: [u8; 16],
    stack: [u16; 16],
    memory: [u8; 4096],
    delay: Timer,
    sound: Timer,
    pub display: Display,
}

impl Default for Chip8 {
    fn default() -> Self {
        let mut chip8 = Chip8 {
            opcode: 0,
            i: 0,
            pc: 0x200, // Progrm counter starts at 0x200
            sp: 0,
            key: Default::default(),
            v: [0; 16],
            stack: [0; 16],
            memory: [0; 4096],
            delay: Default::default(),
            sound: Default::default(),
            display: Display::default(),
        };

        chip8.load_font_set();
        chip8
    }
}

impl Chip8 {
    const FONT_SET: [u8; 80] = [
        0xF0, 0x90, 0x90, 0x90, 0xF0, // 0
        0x20, 0x60, 0x20, 0x20, 0x70, // 1
        0xF0, 0x10, 0xF0, 0x80, 0xF0, // 2
        0xF0, 0x10, 0xF0, 0x10, 0xF0, // 3
        0x90, 0x90, 0xF0, 0x10, 0x10, // 4
        0xF0, 0x80, 0xF0, 0x10, 0xF0, // 5
        0xF0, 0x80, 0xF0, 0x90, 0xF0, // 6
        0xF0, 0x10, 0x20, 0x40, 0x40, // 7
        0xF0, 0x90, 0xF0, 0x90, 0xF0, // 8
        0xF0, 0x90, 0xF0, 0x10, 0xF0, // 9
        0xF0, 0x90, 0xF0, 0x90, 0x90, // A
        0xE0, 0x90, 0xE0, 0x90, 0xE0, // B
        0xF0, 0x80, 0x80, 0x80, 0xF0, // C
        0xE0, 0x90, 0x90, 0x90, 0xE0, // D
        0xF0, 0x80, 0xF0, 0x80, 0xF0, // E
        0xF0, 0x80, 0xF0, 0x80, 0x80, // F
    ];

    pub fn execute_cycle(&mut self) {
        self.opcode = self.get_opcode();
        println!("{:#x}", self.opcode);
        self.handle_opcode();

        self.delay.tick();
        self.sound.tick();
    }

    pub fn load_font_set(&mut self) {
        self.memory[0..0x50].copy_from_slice(&Self::FONT_SET);
    }

    pub fn load_rom<P: AsRef<Path>>(&mut self, path: P) -> Result<(), io::Error> {
        let mut file = File::open(path.as_ref())?;
        let mut rom_buf: Vec<u8> = vec![];
        file.read_to_end(&mut rom_buf)?;

        self.memory[0x200..(0x200 + rom_buf.len())].copy_from_slice(&rom_buf);

        Ok(())
    }

    fn get_opcode(&self) -> u16 {
        let pc = self.pc as usize;

        ((self.memory[pc] as u16) << 8) | self.memory[pc + 1] as u16
    }

    fn get_nibs(opcode: u16) -> (u8, u8, u8, u8) {
        // Given 0xA2F0
        (
            ((opcode & 0xF000) >> 12) as u8, // 0xA
            ((opcode & 0x0F00) >> 8) as u8,  // 0x2
            ((opcode & 0x00F0) >> 4) as u8,  // 0xF
            (opcode & 0x000F) as u8,         // 0x0
        )
    }

    fn handle_opcode(&mut self) {
        let (nib_1, nib_2, nib_3, nib_4) = Self::get_nibs(self.opcode);

        let nnn: u16 = self.opcode & 0x0FFF;
        let kk: u8 = (self.opcode & 0x00FF) as u8;
        let x = nib_2;
        let y = nib_3;
        let n = nib_4;

        self.pc += 2;

        match (nib_1, nib_2, nib_3, nib_4) {
            // CLS
            (0x0, 0x0, 0xE, 0x0) => self.cls(),
            // 00EE
            (0x0, 0x0, 0xE, 0xE) => self.ret(),
            // 1NNN
            (0x1, _, _, _) => self.jmp_addr(nnn),
            // 2NNN
            (0x2, _, _, _) => self.call_addr(nnn),
            // 3XKK
            (0x3, _, _, _) => self.se_vx_byte(x, kk),
            // 4xkk
            (0x4, _, _, _) => self.sne_vx_byte(x, kk),
            // 5xy0
            (0x5, _, _, 0x0) => self.se_vx_vy(x, y),
            // 6xkk
            (0x6, _, _, _) => self.ld_vx_byte(x, kk),
            // 7xkk
            (0x7, _, _, _) => self.add_vx_byte(x, kk),
            // 8xy0
            (0x8, _, _, 0x0) => self.ld_vx_vy(x, y),
            // 8xy1
            (0x8, _, _, 0x1) => self.or(x, y),
            // 8xy2
            (0x8, _, _, 0x2) => self.and_vx_vy(x, y),
            // 8xy3
            (0x8, _, _, 0x3) => self.xor(x, y),
            // 8xy4
            (0x8, _, _, 0x4) => self.add_vx_vy(x, y),
            // 8xy5
            (0x8, _, _, 0x5) => self.sub_vx_vy(x, y),
            // 8xy6
            (0x8, _, _, 0x6) => self.shr(x),
            // 8xy7
            (0x8, _, _, 0x7) => self.subn_vx_vy(x, y),
            // 8xyE
            (0x8, _, _, 0xE) => self.shl(x),
            // 9xy0
            (0x9, _, _, 0x0) => self.sne_vx_vy(x, y),
            // Annn
            (0xA, _, _, _) => self.load_addr_to_i(nnn),
            // Bnnn
            (0xB, _, _, _) => self.jmp_addr_with_offset(nnn),
            // Cxkk
            (0xC, _, _, _) => self.rand(x, kk),
            //Dxyn
            (0xD, _, _, _) => self.draw(x, y, n),
            // Ex9E
            (0xE, _, 0x9, 0xE) => self.skip_on_press(x),
            // ExA1
            (0xE, _, 0xA, 0x1) => self.skip_not_pressed(x),
            // Fx07
            (0xF, _, 0x0, 0x7) => self.copy_delay_timer_val(x),
            // Fx0A
            (0xF, _, 0x0, 0xA) => self.loop_until_key_vx(x),
            // Fx15
            (0xF, _, 0x1, 0x15) => self.set_delay_to_vx(x),
            // Fx18
            (0xF, _, 0x1, 0x8) => self.set_sound_to_vx(x),
            // Fx1E
            (0xF, _, 0x1, 0xE) => self.add_vx_to_i(x),
            // Fx29
            (0xF, _, 0x2, 0x9) => self.set_i_to_hex_sprite_loc(x),
            // Fx33
            (0xF, _, 0x3, 0x3) => self.copy_digits_to_i(x),
            // Fx55
            (0xF, _, 0x5, 0x5) => self.copy_from_vx_to_memory(x),
            // Fx65
            (0xF, _, 0x6, 0x5) => self.load_into_vx_from_i(x),
            // Otherwise...
            _ => panic!("UNIMPLEMENTED OPCODE: {:#x}", self.opcode),
        }
    }

    fn cls(&mut self) {
        // Clear the display
        self.display.clear();
    }

    fn ret(&mut self) {
        // sets the program counter to the address at the top of the stack,
        // then subtracts one (1) from the stack pointer
        self.pc = self.stack[self.sp as usize];
        self.sp -= 1;
    }

    fn jmp_addr(&mut self, nnn: u16) {
        // sets the program counter to addr (nnn)
        self.pc = nnn;
    }

    fn call_addr(&mut self, nnn: u16) {
        // increments the stack pointer, then puts current pc on top of stack
        // pc is then set to addr
        self.sp += 1;
        self.stack[self.sp as usize] = self.pc;
        self.pc = nnn;
    }

    fn se_vx_byte(&mut self, x: u8, kk: u8) {
        // compares Vx to kk. If they are equal, pc is incremented by 2
        if self.v[x as usize] == kk {
            self.pc += 2;
        }
    }

    fn sne_vx_byte(&mut self, x: u8, kk: u8) {
        // compares Vx to kk. If they are **not** equal, pc is incremented by 2
        if self.v[x as usize] != kk {
            self.pc += 2;
        }
    }

    fn se_vx_vy(&mut self, x: u8, y: u8) {
        // compares Vx to Vy. If they are equal, pc is incremented by 2
        if self.v[x as usize] == self.v[y as usize] {
            self.pc += 2;
        }
    }

    fn ld_vx_byte(&mut self, x: u8, kk: u8) {
        // put value kk into Vx
        self.v[x as usize] = kk;
    }

    fn add_vx_byte(&mut self, x: u8, kk: u8) {
        // calculate Vx + kk, then store it in Vx
        self.v[x as usize] += kk;
    }

    fn ld_vx_vy(&mut self, x: u8, y: u8) {
        // store Vy in Vx
        self.v[x as usize] = self.v[y as usize];
    }

    fn or(&mut self, x: u8, y: u8) {
        // calc bitwise OR on Vx and Vy, then store in Vx
        self.v[x as usize] |= self.v[y as usize];
    }

    fn and_vx_vy(&mut self, x: u8, y: u8) {
        // calc bitwise AND on Vx and Vy, then store in Vx
        self.v[x as usize] &= self.v[y as usize];
    }

    fn xor(&mut self, x: u8, y: u8) {
        // calc bitwise XOR on Vx and Vy, then store in Vx
        self.v[x as usize] ^= self.v[y as usize];
    }

    fn add_vx_vy(&mut self, x: u8, y: u8) {
        // add Vx and Vy, if result is greater than 8 bits
        // set VF to 1, otherwise 0
        // only the lowest 8 bits of result are stored in Vx
        let x = x as usize;

        let (res, did_overflow) = self.v[x].overflowing_add(self.v[y as usize]);
        self.v[0xF] = if did_overflow { 1 } else { 0 };
        self.v[x] = res;
    }

    fn sub_vx_vy(&mut self, x: u8, y: u8) {
        // TODO: Confirm that the default behaviour
        // for this opcode includes a wrapping subtraction

        // subtract Vx and Vy, if Vx > Vy VF is set to 1, otherwise 0
        // then set Vx to Vx - Vy
        let vx = self.v[x as usize];
        let vy = self.v[y as usize];

        self.v[0xF] = if vx > vy { 1 } else { 0 };
        self.v[x as usize] = vx.wrapping_sub(vy);
    }

    fn shr(&mut self, x: u8) {
        // if LSB is set to 1, set VF to 1, otherwise set VF to 0
        // then shift Vx one to the right
        let x = x as usize;

        self.v[0xF] = if (self.v[x] & 1) == 1 { 1 } else { 0 };
        self.v[x] >>= 1;
    }

    fn subn_vx_vy(&mut self, x: u8, y: u8) {
        // subtract Vy and Vx, if Vy > Vx VF is set to 1, otherwise 0
        // then set Vx = Vy - Vx
        let vx = self.v[x as usize];
        let vy = self.v[y as usize];

        self.v[0xF] = if vy > vx { 1 } else { 0 };
        self.v[x as usize] = vy.wrapping_sub(vx);
    }

    fn shl(&mut self, x: u8) {
        // if MSB is set to 1, set VF to 1, otherwise set VF to 0
        // then shift Vx one to the left;
        let x = x as usize;

        self.v[0xF] = if (self.v[x] >> 7) == 1 { 1 } else { 0 };
        self.v[x] <<= 1;
    }

    fn sne_vx_vy(&mut self, x: u8, y: u8) {
        // if Vx != vy program counter is increased by 2
        if self.v[x as usize] != self.v[y as usize] {
            self.pc += 2;
        }
    }

    fn load_addr_to_i(&mut self, nnn: u16) {
        // set i to addr
        self.i = nnn;
    }

    fn jmp_addr_with_offset(&mut self, nnn: u16) {
        // set program counter to addr + V0
        self.pc = nnn + self.v[0] as u16;
    }

    fn rand(&mut self, x: u8, kk: u8) {
        // generate a random number from 0 to 255
        // AND with the value of kk, then store in Vx
        self.v[x as usize] = rand::random::<u8>() & kk;
    }

    fn draw(&mut self, x: u8, y: u8, n: u8) {
        // read n bytes from memory starting from self.i
        // then display them starting at (vx, vy)
        let disp_x = self.v[x as usize];
        let disp_y = self.v[y as usize];

        let mut display_buf = self.display.buf;

        let mut pixel: u8;
        self.v[0xF] = 0;

        // TODO: Rewrite Your Solution to better fit
        // the rest of your code base.
        // http://www.multigesture.net/articles/how-to-write-an-emulator-chip-8-interpreter/
        for i in 0..(n as usize) {
            // height of sprite (y-axis)
            pixel = self.memory[self.i as usize + i];
            for j in 0..8 {
                // Loop over the 8 bits (x-axis)
                if (pixel & (0x80 >> j)) != 0 {
                    let index = (disp_x as usize) + j + (((disp_y as usize) + i) * 64);

                    if display_buf[index] == 1 {
                        self.v[0xF] = 1;
                    }

                    display_buf[index] ^= 1;
                }
            }
        }
    }

    fn skip_on_press(&mut self, x: u8) {
        // if current key is the same as the on in Vx
        // program counter is increased by 2

        if let Some(key) = self.key.get_pressed() {
            if self.v[x as usize] == key {
                self.pc += 2;
            }
        }
    }

    fn skip_not_pressed(&mut self, x: u8) {
        // if current key is not the sameas the one in Vx
        // increment the program counter by 2

        match self.key.get_pressed() {
            Some(key) => {
                if self.v[x as usize] != key {
                    self.pc += 2;
                }
            }
            None => self.pc += 2,
        }
    }

    fn copy_delay_timer_val(&mut self, x: u8) {
        // set Vx to be the value of the delay timer
        self.v[x as usize] = self.delay.get();
    }

    fn loop_until_key_vx(&mut self, x: u8) {
        // wait (blocking) until a key is pressed
        // once pressed, store in Vx

        match self.key.get_pressed() {
            Some(key) => self.v[x as usize] = key,
            None => self.pc -= 2,
        }
    }

    fn set_delay_to_vx(&mut self, x: u8) {
        // set delay timer to be value of Vx
        self.delay.set(self.v[x as usize]);
    }

    fn set_sound_to_vx(&mut self, x: u8) {
        // set sound timer to be value of Vx
        self.delay.set(self.v[x as usize]);
    }

    fn add_vx_to_i(&mut self, x: u8) {
        // set I to be I + Vx
        self.i += self.v[x as usize] as u16;
    }

    fn set_i_to_hex_sprite_loc(&mut self, x: u8) {
        // set I to location of hex sprite related to Vx
        self.i = self.v[x as usize] as u16 * 5;
    }

    fn copy_digits_to_i(&mut self, x: u8) {
        // take hundreds digit and place it at I
        // take tens digit and place it at I + 1
        // take ones digit and place it at I + 2
        let i = self.i as usize;
        let mut iter = Self::digits(self.v[x as usize] as usize);

        let ones = iter.next().unwrap(); // Ther has to at least be a ones lol
        let tens = iter.next().unwrap_or(0);
        let hundreds = iter.next().unwrap_or(0);

        self.memory[i] = hundreds as u8;
        self.memory[i + 1] = tens as u8;
        self.memory[i + 2] = ones as u8;
    }

    fn copy_from_vx_to_memory(&mut self, x: u8) {
        // copy values v0 -> Vx to memory starting at i

        for n in 0..=(x as usize) {
            self.memory[self.i as usize + n] = self.v[n];
        }
    }

    fn load_into_vx_from_i(&mut self, x: u8) {
        // read what will be values of v0 -> vx from memory starting at i
        let x = x as usize;

        for n in 0..=x {
            self.v[n] = self.memory[self.i as usize + n];
        }
    }

    // https://stackoverflow.com/questions/41536479/how-do-i-split-an-integer-into-individual-digits
    fn digits(mut num: usize) -> impl Iterator<Item = usize> {
        let mut divisor = 1;
        while num >= divisor * 10 {
            divisor *= 10;
        }

        std::iter::from_fn(move || {
            if divisor == 0 {
                None
            } else {
                let v = num / divisor;
                num %= divisor;
                divisor /= 10;
                Some(v)
            }
        })
    }
}

#[cfg(test)]
mod test {
    use super::Chip8;

    #[test]
    fn chip8_initializes_properly() {
        let chip8: Chip8 = Default::default();
        assert_eq!(chip8.pc, 0x200);

        let slice: &[u8] = &chip8.memory[0..0x50];
        let font_set: &[u8] = &Chip8::FONT_SET;

        for (i, byte) in slice.iter().enumerate() {
            assert_eq!(*byte, font_set[i]);
        }
    }

    #[test]
    fn get_opcode_works() {
        let mut chip8: Chip8 = Default::default();
        let expected = 0x00E0; // Clear Screen

        chip8.memory[0x202] = 0x00;
        chip8.memory[0x203] = 0xE0;
        chip8.pc = 0x202;

        assert_eq!(chip8.get_opcode(), expected);
    }

    #[test]
    fn get_nibs_works() {
        let opcode = 0xA2F0;

        let (n1, n2, n3, n4) = Chip8::get_nibs(opcode);

        assert_eq!(n1, 0xA);
        assert_eq!(n2, 0x2);
        assert_eq!(n3, 0xF);
        assert_eq!(n4, 0x0);
    }

    #[test]
    fn opcode_cls_works() {
        let mut chip8: Chip8 = Default::default();

        // Display is now noise
        for byte in chip8.display.buf.iter_mut() {
            *byte = rand::random();
        }
        chip8.cls();
        // now check that everything in the display buffer is zeroed out
        for byte in chip8.display.buf.iter() {
            assert_eq!(*byte, 0);
        }
    }

    #[test]
    fn opcode_ret_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.pc = 0x202;
        chip8.stack[0] = 0xABF0;
        chip8.stack[1] = 0x50D5; // 80
        chip8.sp = 1;

        chip8.ret();

        // Ret should set the programcouner to the address at the top of the stack
        // and then decrement the stack pointer by 1
        assert_eq!(chip8.sp, 0);
        assert_eq!(chip8.pc, 0x50D5);
    }

    #[test]
    fn opcode_jmp_addr_works() {
        let mut chip8: Chip8 = Default::default();

        let opcode = 0x1ABC;
        let nnn = opcode & 0x0FFF;

        chip8.jmp_addr(nnn);

        assert_eq!(chip8.pc, nnn);
    }

    #[test]
    fn opcode_call_addr_works() {
        let mut chip8: Chip8 = Default::default();

        let nnn = 0x0ABC;
        chip8.call_addr(nnn);

        assert_eq!(chip8.sp, 1);
        assert_eq!(chip8.stack[chip8.sp as usize], 0x200);
        assert_eq!(chip8.pc, nnn);
    }

    #[test]
    fn opcode_se_vx_byte_works() {
        let mut chip8: Chip8 = Default::default();

        // Opcode: 0x32D5

        // Test Vx == kk
        chip8.pc = 0x200;
        chip8.v[2] = 0xD5;
        chip8.se_vx_byte(0x2, 0xD5);

        assert_eq!(chip8.pc, 0x202);

        // Test Vx != kk
        chip8.pc = 0x200;
        chip8.v[2] = 0xD6;
        chip8.se_vx_byte(0x2, 0xD5);

        assert_ne!(chip8.pc, 0x202);
    }

    #[test]
    fn opcode_sne_vx_byte_works() {
        let mut chip8: Chip8 = Default::default();

        // Opcode: 0x32D6

        // Test Vx != kk
        chip8.pc = 0x200;
        chip8.v[2] = 0xD5;
        chip8.sne_vx_byte(0x2, 0xD6);

        assert_eq!(chip8.pc, 0x202);

        // Test Vx == kk
        chip8.pc = 0x200;
        chip8.v[2] = 0xD6;
        chip8.sne_vx_byte(0x2, 0xD6);

        assert_ne!(chip8.pc, 0x202);
    }

    #[test]
    fn opcode_se_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        // When Equal
        chip8.pc = 0x200;
        chip8.v[4] = 0xAB;
        chip8.v[7] = 0xAB;
        chip8.se_vx_vy(0x4, 0x7);

        assert_eq!(chip8.pc, 0x202);

        // When not Equal
        chip8.pc = 0x200;
        chip8.v[4] = 0xAC;
        chip8.v[7] = 0xAB;

        chip8.se_vx_vy(0x4, 0x7);
        assert_eq!(chip8.pc, 0x200);
    }

    #[test]
    fn opcode_ld_vx_byte_works() {
        let mut chip8: Chip8 = Default::default();

        assert_ne!(chip8.v[5], 0xAB);
        chip8.ld_vx_byte(0x5, 0xAB);
        assert_eq!(chip8.v[5], 0xAB);
    }

    #[test]
    fn opcode_add_vx_byte_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[3] = 0x0A;
        chip8.add_vx_byte(0x3, 0x36);
        assert_eq!(chip8.v[3], 0x40);
    }

    #[test]
    fn opcode_ld_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[1] = 0x64;
        chip8.v[4] = 0x96;

        chip8.ld_vx_vy(0x1, 0x4);
        assert_eq!(chip8.v[1], 0x96);
    }

    #[test]
    fn opcode_or_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xA] = 0xAB;
        chip8.v[0x9] = 0xC6;

        chip8.or(0xA, 0x9);
        assert_eq!(chip8.v[0xA], 0xAB | 0xC6);
    }

    #[test]
    fn opcode_and_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xC] = 0x58;
        chip8.v[0x2] = 0x9a;

        chip8.and_vx_vy(0xC, 0x2);
        assert_eq!(chip8.v[0xC], 0x58 & 0x9a);
    }

    #[test]
    fn opcode_xor_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0x5] = 0x38;
        chip8.v[0xB] = 0x4C;

        chip8.xor(0x5, 0xB);
        assert_eq!(chip8.v[0x5], 0x38 ^ 0x4C);
    }

    #[test]
    fn opcode_add_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xA] = 0x01;
        chip8.v[0x8] = 0xC8;

        chip8.add_vx_vy(0xA, 0x8);
        assert_eq!(chip8.v[0xA], 0x01 + 0xC8);
        assert_eq!(chip8.v[0xF], 0);
    }

    #[test]
    fn opcode_add_vx_vy_works_vf_set() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xE] = 0xFF;
        chip8.v[0x7] = 0x01;

        chip8.add_vx_vy(0xE, 0x7);

        assert_eq!(chip8.v[0xA], (0xFF as u8).wrapping_add(0x01));
        assert_eq!(chip8.v[0xF], 1);
    }

    #[test]
    fn opcode_sub_vx_vy_works_vf_set() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0x3] = 0xD5;
        chip8.v[0xD] = 0x71;

        chip8.sub_vx_vy(0x3, 0xD);

        assert_eq!(chip8.v[0xF], 1);
        assert_eq!(chip8.v[0x3], 0xD5 - 0x71);
    }

    #[test]
    fn opcode_sub_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xB] = 0x23;
        chip8.v[0x5] = 0xCA;

        chip8.sub_vx_vy(0xB, 0x5);

        assert_eq!(chip8.v[0xF], 0);
        assert_eq!(chip8.v[0xB], (0x23 as u8).wrapping_sub(0xCA))
    }

    #[test]
    fn opcode_shr_works() {
        let mut chip8: Chip8 = Default::default();

        // LSB == 1
        chip8.v[0x6] = 0xC8;
        chip8.shr(0x6);

        assert_eq!(chip8.v[0xF], 0);
        assert_eq!(chip8.v[0x6], 0xC8 >> 1);
    }

    #[test]
    fn opcode_shr_works_vf_set() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xD] = 0xB5;
        chip8.shr(0xD);

        assert_eq!(chip8.v[0xF], 1);
        assert_eq!(chip8.v[0xD], 0xB5 >> 1);
    }

    #[test]
    fn opcode_subn_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xC] = 0x87;
        chip8.v[0x9] = 0x04;

        chip8.subn_vx_vy(0xC, 0x9);
        assert_eq!(chip8.v[0xC], (0x04 as u8).wrapping_sub(0x87));
        assert_eq!(chip8.v[0xF], 0);
    }

    #[test]
    fn opcode_subn_vx_vy_works_vf_set() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0x6] = 0x4C;
        chip8.v[0x4] = 0xE8;

        chip8.subn_vx_vy(0x6, 0x4);

        assert_eq!(chip8.v[0xF], 1);
        assert_eq!(chip8.v[0x6], 0xE8 - 0x4C);
    }

    #[test]
    fn opcode_shl_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0xC] = 0x47;
        chip8.shl(0xC);

        assert_eq!(chip8.v[0xC], 0x47 << 1);
        assert_eq!(chip8.v[0xF], 0);
    }

    #[test]
    fn opcode_shl_works_vf_set() {
        let mut chip8: Chip8 = Default::default();

        chip8.v[0x7] = 0xC7;
        chip8.shl(0x7);

        assert_eq!(chip8.v[0x7], 0xC7 << 1);
        assert_eq!(chip8.v[0xF], 1);
    }

    #[test]
    fn opcode_sne_vx_vy_works() {
        let mut chip8: Chip8 = Default::default();

        // Vx != Vy
        chip8.pc = 0x200;
        chip8.v[0x4] = 0x5F;
        chip8.v[0x3] = 0xF8;

        chip8.sne_vx_vy(0x4, 0x3);

        assert_eq!(chip8.pc, 0x202);

        // Vx == Vy
        chip8.pc = 0x200;
        chip8.v[0x4] = 0x5F;
        chip8.v[0x3] = 0x5F;

        chip8.sne_vx_vy(0x4, 0x3);

        assert_ne!(chip8.pc, 0x202);
    }

    #[test]
    fn opcode_load_addr_to_i_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.i = 0;
        chip8.load_addr_to_i(0x1ABC & 0x0FFF);

        assert_eq!(chip8.i, 0x0ABC);
    }

    #[test]
    fn opcode_jmp_addr_with_offset_works() {
        let mut chip8: Chip8 = Default::default();

        chip8.pc = 0x200;
        chip8.v[0x0] = 0xA2;
        chip8.jmp_addr_with_offset(0x05D2);

        assert_eq!(chip8.pc, 0x05D2 + 0xA2);
    }
}