//! HoleyBytes Virtual Machine
//!
//! # Alloc feature
//! - Enabled by default
//! - Provides mapping / unmapping, as well as [`Default`] and [`Drop`]
//!   implementations for soft-paged memory implementation

// # General safety notice:
// - Validation has to assure there is 256 registers (r0 - r255)
// - Instructions have to be valid as specified (values and sizes)
// - Mapped pages should be at least 4 KiB

#![no_std]
#![cfg_attr(feature = "nightly", feature(fn_align))]
#![warn(missing_docs)]

use mem::{Memory, Address};

#[cfg(feature = "alloc")]
extern crate alloc;

pub mod mem;
pub mod value;

mod bmc;
mod vmrun;
mod utils;

use {bmc::BlockCopier, value::Value};

/// HoleyBytes Virtual Machine
pub struct Vm<Mem, const TIMER_QUOTIENT: usize> {
    /// Holds 256 registers
    ///
    /// Writing to register 0 is considered undefined behaviour
    /// in terms of HoleyBytes program execution
    pub registers: [Value; 256],

    /// Memory implementation
    pub memory: Mem,

    /// Program counter
    pub pc: Address,

    /// Program timer
    timer: usize,

    /// Saved block copier
    copier: Option<BlockCopier>,
}

impl<Mem, const TIMER_QUOTIENT: usize> Vm<Mem, TIMER_QUOTIENT>
where
    Mem: Memory,
{
    /// Create a new VM with program and trap handler
    ///
    /// # Safety
    /// Program code has to be validated
    pub unsafe fn new(memory: Mem, entry: Address) -> Self {
        Self {
            registers: [Value::from(0_u64); 256],
            memory,
            pc: entry,
            timer: 0,
            copier: None,
        }
    }
}

/// Virtual machine halt error
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[repr(u8)]
pub enum VmRunError {
    /// Tried to execute invalid instruction
    InvalidOpcode(u8),

    /// Unhandled load access exception
    LoadAccessEx(Address),

    /// Unhandled instruction load access exception
    ProgramFetchLoadEx(Address),

    /// Unhandled store access exception
    StoreAccessEx(Address),

    /// Register out-of-bounds access
    RegOutOfBounds,

    /// Address out-of-bounds
    AddrOutOfBounds,

    /// Reached unreachable code
    Unreachable,
}

/// Virtual machine halt ok
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum VmRunOk {
    /// Program has eached its end
    End,

    /// Program was interrupted by a timer
    Timer,

    /// Environment call
    Ecall,
}