gb-emu/src/processor/mod.rs

use std::{
    mem::transmute,
    ops::{BitAnd, BitOr, BitXor},
};

use crate::{verbose_println, Inner, Memory, Register, State};

mod opcodes;

#[allow(dead_code)]
#[derive(PartialEq)]
enum FLAGS {
    Zero = 7,
    NSubtract = 6,
    HalfCarry = 5,
    Carry = 4,
}

enum Direction {
    Left,
    Right,
}

pub struct CPU {
    pub memory: Memory,
    pub state: State,
    pub last_instruction: u8,
    pub last_instruction_addr: u16,
}

impl CPU {
    pub fn exec_next(&mut self) {
        unsafe { self.last_instruction_addr = self.state.pc.as_u16 };
        let opcode = self.next_opcode();
        self.last_instruction = opcode;
        verbose_println!(
            "exec {:#4X} from pc: {:#X}",
            opcode,
            self.last_instruction_addr
        );
        self.run_opcode(opcode);
    }

    fn next_opcode(&mut self) -> u8 {
        unsafe {
            let opcode = self.memory.get(self.state.pc.as_u16);
            self.state.pc.as_u16 = self.state.pc.as_u16.wrapping_add(0x1);
            return opcode;
        };
    }

    fn rlc(&mut self, byte: u8) -> u8 {
        self.rotate_c(byte, Direction::Left)
    }

    fn rrc(&mut self, byte: u8) -> u8 {
        self.rotate_c(byte, Direction::Right)
    }

    fn rl(&mut self, byte: u8) -> u8 {
        self.rotate(byte, Direction::Left)
    }

    fn rr(&mut self, byte: u8) -> u8 {
        self.rotate(byte, Direction::Right)
    }

    fn rotate_c(&mut self, byte: u8, direction: Direction) -> u8 {
        let (mut rotated, carry) = rotate(byte, &direction);
        if carry {
            rotated += get_rotation_carry(&direction);
            self.set_flag(FLAGS::Carry);
        }
        return rotated;
    }

    fn rotate(&mut self, byte: u8, direction: Direction) -> u8 {
        let old_carry = self.get_flag(FLAGS::Carry);
        let (mut rotated, carry) = rotate(byte, &direction);
        if old_carry > 0 {
            rotated += get_rotation_carry(&direction);
        }
        if carry {
            self.set_flag(FLAGS::Carry);
        }
        return rotated;
    }

    fn sla(&mut self, byte: u8) -> u8 {
        self.shift(byte, Direction::Left)
    }

    fn sra(&mut self, byte: u8) -> u8 {
        let b = get_bit(byte, 7);
        let val = self.shift(byte, Direction::Right);
        if b {
            val + 0b10000000
        } else {
            val
        }
    }

    fn srl(&mut self, byte: u8) -> u8 {
        self.shift(byte, Direction::Right)
    }

    fn shift(&mut self, byte: u8, direction: Direction) -> u8 {
        let (rotated, carry) = rotate(byte, &direction);
        if carry {
            self.set_flag(FLAGS::Carry);
        }
        return rotated;
    }

    fn bit(&mut self, byte: u8, bit: u8) {
        self.set_or_clear_flag(FLAGS::Zero, !get_bit(byte, bit));
    }

    fn rst(&mut self, address: u16) {
        self.push(self.state.pc);
        self.state.pc.as_u8s.left = 0x0;
        self.state.pc.as_u8s.right = self.memory.get(address);
    }

    fn push(&mut self, register: Register) {
        unsafe {
            let address = self.state.sp.as_u16;
            self.memory
                .set(address.wrapping_sub(1), register.as_u8s.right);
            self.memory
                .set(address.wrapping_sub(2), register.as_u8s.left);
            self.state.sp.as_u16 = address.wrapping_sub(2);
        }
    }

    fn pop_word(&mut self) -> Register {
        unsafe {
            let address = self.state.sp.as_u16;
            self.state.sp.as_u16 = self.state.sp.as_u16.wrapping_add(0x2);
            Register {
                as_u8s: Inner {
                    left: self.memory.get(address),
                    right: self.memory.get(address.wrapping_add(1)),
                },
            }
        }
    }

    fn store_word(&mut self, address: u16, word: Register) {
        unsafe {
            self.memory.set(address, word.as_u8s.left);
            self.memory.set(address + 1, word.as_u8s.right);
        };
    }

    fn ld_immediate_word(&mut self) -> Register {
        Register {
            as_u8s: Inner {
                left: self.next_opcode(),
                right: self.next_opcode(),
            },
        }
    }

    fn ld_immediate_byte(&mut self) -> u8 {
        self.next_opcode()
    }

    fn get_flag(&mut self, flag: FLAGS) -> u8 {
        unsafe {
            if get_bit(self.state.af.as_u8s.right, flag as u8) {
                0x1
            } else {
                0x0
            }
        }
    }

    fn set_flag(&mut self, flag: FLAGS) {
        if flag == FLAGS::Zero {
            verbose_println!("setting z flag");
        }
        unsafe {
            verbose_println!(
                "setting flag: currently {0:#b} / {0:#X}",
                self.state.af.as_u8s.right
            );
            self.state.af.as_u8s.right = self.state.af.as_u8s.right.bitor(1 << flag as u8);
            verbose_println!(
                "                    now {0:#b} / {0:#X}",
                self.state.af.as_u8s.right
            );
        };
    }

    fn clear_flag(&mut self, flag: FLAGS) {
        unsafe {
            self.state.af.as_u8s.right = self.state.af.as_u8s.right.bitand(!(1 << flag as u8));
        };
    }

    fn toggle_flag(&mut self, flag: FLAGS) {
        unsafe {
            self.state.af.as_u8s.right = self.state.af.as_u8s.right.bitxor(1 << flag as u8);
        };
    }

    fn set_or_clear_flag(&mut self, flag: FLAGS, state: bool) {
        if state {
            self.set_flag(flag)
        } else {
            self.clear_flag(flag)
        }
    }

    fn add_u8s(&mut self, first: u8, second: u8) -> u8 {
        let (result, carry) = first.overflowing_add(second);
        self.clear_flag(FLAGS::NSubtract);
        self.set_or_clear_flag(FLAGS::Carry, carry);
        self.set_or_clear_flag(FLAGS::Zero, result == 0x0);
        self.set_or_clear_flag(
            FLAGS::HalfCarry,
            (((first & 0xF).wrapping_add(second & 0xF)) & 0x10) == 0x10,
        );
        return result;
    }

    fn add_u16s(&mut self, first: u16, second: u16) -> u16 {
        let (result, carry) = first.overflowing_add(second);
        self.clear_flag(FLAGS::NSubtract);
        self.set_or_clear_flag(FLAGS::Carry, carry);
        self.set_or_clear_flag(FLAGS::Zero, result == 0x0);
        self.set_or_clear_flag(
            FLAGS::HalfCarry,
            (((first & 0xFFF).wrapping_add(second & 0xFFF)) & 0x1000) == 0x1000,
        );
        return result;
    }

    fn sub_u8s(&mut self, first: u8, second: u8) -> u8 {
        let (result, carry) = first.overflowing_sub(second);
        self.set_flag(FLAGS::NSubtract);
        self.set_or_clear_flag(FLAGS::Carry, carry);
        self.set_or_clear_flag(FLAGS::Zero, result == 0x0);
        self.set_or_clear_flag(
            FLAGS::HalfCarry,
            (((first & 0xF).wrapping_sub(second & 0xF)) & 0x10) == 0x10,
        );
        return result;
    }

    fn sub_u16s(&mut self, first: u16, second: u16) -> u16 {
        let (result, carry) = first.overflowing_sub(second);
        self.set_flag(FLAGS::NSubtract);
        self.set_or_clear_flag(FLAGS::Carry, carry);
        self.set_or_clear_flag(FLAGS::Zero, result == 0x0);
        self.set_or_clear_flag(
            FLAGS::HalfCarry,
            (((first & 0xFFF).wrapping_sub(second & 0xFFF)) & 0x1000) == 0x1000,
        );
        return result;
    }
}

fn as_signed(unsigned: u8) -> i8 {
    unsafe {
        return transmute(unsigned);
    }
}

fn get_bit(byte: u8, flag: u8) -> bool {
    let mask = 1 << flag;
    let got = byte & mask;
    return got > 0x0;
}

fn rotate(byte: u8, direction: &Direction) -> (u8, bool) {
    match direction {
        Direction::Left => {
            let carry = get_bit(byte, 7);
            let r = byte << 1;
            return (r, carry);
        }
        Direction::Right => {
            let carry = get_bit(byte, 0);
            let r = byte >> 1;
            return (r, carry);
        }
    }
}

fn get_rotation_carry(direction: &Direction) -> u8 {
    match direction {
        Direction::Left => 0b1,
        Direction::Right => 0b10000000,
    }
}

fn swap_nibbles(byte: u8) -> u8 {
    (byte & 0x0F) << 4 | (byte & 0xF0) >> 4
}

fn res(byte: u8, bit: u8) -> u8 {
    byte & !(1 << bit)
}

fn set(byte: u8, bit: u8) -> u8 {
    byte | (1 << bit)
}