//! HoleyBytes Virtual Machine
//! 
//! All unsafe code here should be sound, if input bytecode passes validation.

// # General safety notice:
// - Validation has to assure there is 60 registers (r0 - r59)
// - Instructions have to be valid as specified (values and sizes)
// - Mapped pages should be at least 8 KiB
// - Yes, I am aware of the UB when jumping in-mid of instruction where
//   the read byte corresponds to an instruction whose lenght exceets the
//   program size. If you are (rightfully) worried about the UB, for now just
//   append your program with 11 zeroes.

mod mem;
mod value;

use {
    crate::validate,
    core::ops,
    hbbytecode::{OpParam, ParamRI, ParamRR, ParamRRI, ParamRRR},
    mem::{ma_size, Memory},
    static_assertions::assert_impl_one,
    value::Value,
};

macro_rules! param {
    ($self:expr, $ty:ty) => {{
        assert_impl_one!($ty: OpParam);
        let data = $self
            .program
            .as_ptr()
            .add($self.pc + 1)
            .cast::<$ty>()
            .read();
        $self.pc += 1 + core::mem::size_of::<$ty>();
        data
    }};
}

macro_rules! binary_op {
    ($self:expr, $ty:ident, $handler:expr) => {{
        let ParamRRR(tg, a0, a1) = param!($self, ParamRRR);
        $self.write_reg(
            tg,
            $handler(
                Value::$ty(&$self.read_reg(a0)),
                Value::$ty(&$self.read_reg(a1)),
            )
            .into(),
        );
    }};
}

macro_rules! binary_op_imm {
    ($self:expr, $ty:ident, $handler:expr) => {{
        let ParamRRI(tg, a0, imm) = param!($self, ParamRRI);
        $self.write_reg(
            tg,
            $handler(Value::$ty(&$self.read_reg(a0)), Value::$ty(&imm.into())).into(),
        );
    }};
}

macro_rules! load {
    ($self:expr, $size:ty) => {{
        let ParamRRI(tg, a0, offset) = param!($self, ParamRRI);
        $self.write_reg(
            tg,
            match $self
                .memory
                .load::<$size>($self.read_reg(a0).int() + offset)
            {
                Some(x) => x,
                None => return HaltReason::LoadAccessEx,
            },
        );
    }};
}

macro_rules! store {
    ($self:expr, $size:ty) => {{
        let ParamRRI(src, a0, offset) = param!($self, ParamRRI);
        if let Err(()) = $self
            .memory
            .store::<$size>($self.read_reg(a0).int() + offset, $self.read_reg(src))
        {
            return HaltReason::StoreAccessEx;
        }
    }};
}

macro_rules! cond_jump {
    ($self:expr, $ty:ident, $expected:ident) => {{
        let ParamRRI(a0, a1, jt) = param!($self, ParamRRI);
        if core::cmp::Ord::cmp(&$self.read_reg(a0), &$self.read_reg(a1))
            == core::cmp::Ordering::$expected
        {
            $self.pc = jt as usize;
        }
    }};
}

pub struct Vm<'a> {
    pub registers: [Value; 60],
    pub memory: Memory,
    pc: usize,
    program: &'a [u8],
}

impl<'a> Vm<'a> {
    /// # Safety
    /// Program code has to be validated
    pub unsafe fn new_unchecked(program: &'a [u8]) -> Self {
        Self {
            registers: [Value::from(0_u64); 60],
            memory: Default::default(),
            pc: 0,
            program,
        }
    }

    pub fn new_validated(program: &'a [u8]) -> Result<Self, validate::Error> {
        validate::validate(program)?;
        Ok(unsafe { Self::new_unchecked(program) })
    }

    pub fn run(&mut self) -> HaltReason {
        use hbbytecode::opcode::*;
        loop {
            let Some(&opcode) = self.program.get(self.pc)
                else { return HaltReason::ProgramEnd };

            unsafe {
                match opcode {
                    NOP => param!(self, ()),
                    ADD => binary_op!(self, int, u64::wrapping_add),
                    SUB => binary_op!(self, int, u64::wrapping_sub),
                    MUL => binary_op!(self, int, u64::wrapping_mul),
                    DIV => binary_op!(self, int, u64::wrapping_div),
                    REM => binary_op!(self, int, u64::wrapping_rem),
                    AND => binary_op!(self, int, ops::BitAnd::bitand),
                    OR => binary_op!(self, int, ops::BitOr::bitor),
                    XOR => binary_op!(self, int, ops::BitXor::bitxor),
                    SL => binary_op!(self, int, ops::Shl::shl),
                    SR => binary_op!(self, int, ops::Shr::shr),
                    SRS => binary_op!(self, sint, ops::Shr::shr),
                    CMP => {
                        let ParamRRR(tg, a0, a1) = param!(self, ParamRRR);
                        self.write_reg(
                            tg,
                            (self.read_reg(a0).sint().cmp(&self.read_reg(a1).sint()) as i64).into(),
                        );
                    }
                    CMPU => {
                        let ParamRRR(tg, a0, a1) = param!(self, ParamRRR);
                        self.write_reg(
                            tg,
                            (self.read_reg(a0).int().cmp(&self.read_reg(a1).int()) as i64).into(),
                        );
                    }
                    NOT => {
                        let param = param!(self, ParamRR);
                        self.write_reg(param.0, (!self.read_reg(param.1).int()).into());
                    }
                    ADDF => binary_op!(self, float, ops::Add::add),
                    SUBF => binary_op!(self, float, ops::Sub::sub),
                    MULF => binary_op!(self, float, ops::Mul::mul),
                    DIVF => binary_op!(self, float, ops::Div::div),
                    ADDI => binary_op_imm!(self, int, ops::Add::add),
                    MULI => binary_op_imm!(self, int, ops::Mul::mul),
                    REMI => binary_op_imm!(self, int, ops::Rem::rem),
                    ANDI => binary_op_imm!(self, int, ops::BitAnd::bitand),
                    ORI => binary_op_imm!(self, int, ops::BitOr::bitor),
                    XORI => binary_op_imm!(self, int, ops::BitXor::bitxor),
                    SLI => binary_op_imm!(self, int, ops::Shl::shl),
                    SRI => binary_op_imm!(self, int, ops::Shr::shr),
                    SRSI => binary_op_imm!(self, sint, ops::Shr::shr),
                    ADDFI => binary_op_imm!(self, float, ops::Add::add),
                    MULFI => binary_op_imm!(self, float, ops::Mul::mul),
                    CMPI => {
                        let ParamRRI(tg, a0, imm) = param!(self, ParamRRI);
                        self.write_reg(
                            tg,
                            (self.read_reg(a0).sint().cmp(&Value::from(imm).sint()) as i64).into(),
                        );
                    }
                    CMPUI => {
                        let ParamRRI(tg, a0, imm) = param!(self, ParamRRI);
                        self.write_reg(tg, (self.read_reg(a0).int().cmp(&imm) as i64).into());
                    }
                    CP => {
                        let param = param!(self, ParamRR);
                        self.write_reg(param.0, self.read_reg(param.1));
                    }
                    LI => {
                        let param = param!(self, ParamRI);
                        self.write_reg(param.0, param.1.into());
                    }
                    LB => load!(self, ma_size::Byte),
                    LD => load!(self, ma_size::Doublet),
                    LQ => load!(self, ma_size::Quadlet),
                    LO => load!(self, ma_size::Octlet),
                    SB => store!(self, ma_size::Byte),
                    SD => store!(self, ma_size::Doublet),
                    SQ => store!(self, ma_size::Quadlet),
                    SO => store!(self, ma_size::Octlet),
                    JMP => {
                        let ParamRI(reg, offset) = param!(self, ParamRI);
                        self.pc = (self.read_reg(reg).int() + offset) as usize;
                    }
                    JEQ => cond_jump!(self, int, Equal),
                    JNE => {
                        let ParamRRI(a0, a1, jt) = param!(self, ParamRRI);
                        if self.read_reg(a0) != self.read_reg(a1) {
                            self.pc = jt as usize;
                        }
                    }
                    JLT => cond_jump!(self, int, Less),
                    JGT => cond_jump!(self, int, Greater),
                    JLTU => cond_jump!(self, sint, Less),
                    JGTU => cond_jump!(self, sint, Greater),
                    ECALL => {
                        param!(self, ());
                        return HaltReason::Ecall;
                    }
                    _ => core::hint::unreachable_unchecked(),
                }
            }
        }
    }

    #[inline]
    unsafe fn read_reg(&self, n: u8) -> Value {
        if n == 0 {
            0_u64.into()
        } else {
            *self.registers.get_unchecked(n as usize)
        }
    }

    #[inline]
    unsafe fn write_reg(&mut self, n: u8, value: Value) {
        if n != 0 {
            *self.registers.get_unchecked_mut(n as usize) = value;
        }
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[repr(u8)]
pub enum HaltReason {
    ProgramEnd,
    Ecall,
    LoadAccessEx,
    StoreAccessEx,
}