//! Validate if program is sound to execute

/// Program validation error kind
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum ErrorKind {
    /// Unknown opcode
    InvalidInstruction,
    /// VM doesn't implement this valid opcode
    Unimplemented,
    /// Attempted to copy over register boundary
    RegisterArrayOverflow,
    /// Program is not validly terminated
    InvalidEnd,
}

/// Error
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Error {
    /// Kind
    pub kind:  ErrorKind,
    /// Location in bytecode
    pub index: usize,
}

/// Perform bytecode validation. If it passes, the program should be
/// sound to execute.
pub fn validate(mut program: &[u8]) -> Result<(), Error> {
    use hbbytecode::opcode::*;

    if program.len() < 12 {
        return Err(Error {
            kind:  ErrorKind::InvalidEnd,
            index: 0,
        });
    }

    for (index, item) in program.iter().enumerate().skip(program.len() - 12) {
        if *item != 0 {
            return Err(Error {
                kind: ErrorKind::InvalidEnd,
                index,
            });
        }
    }

    let start = program;
    loop {
        // Match on instruction types and perform necessary checks
        program = match program {
            [] => return Ok(()),
            [LD..=ST, reg, _, _, _, _, _, _, _, _, count_0, count_1, ..]
                if usize::from(*reg) * 8
                    + usize::from(u16::from_le_bytes([*count_1, *count_0]))
                    > 2048 =>
            {
                return Err(Error {
                    kind:  ErrorKind::RegisterArrayOverflow,
                    index: (program.as_ptr() as usize) - (start.as_ptr() as usize),
                })
            }
            [BRC, src, dst, count, ..]
                if src.checked_add(*count).is_none() || dst.checked_add(*count).is_none() =>
            {
                return Err(Error {
                    kind:  ErrorKind::RegisterArrayOverflow,
                    index: (program.as_ptr() as usize) - (start.as_ptr() as usize),
                })
            }
            [UN | NOP | ECALL, rest @ ..]
            | [DIR | DIRF, _, _, _, _, rest @ ..]
            | [ADD..=CMPU | BRC | ADDF..=MULF, _, _, _, rest @ ..]
            | [NEG..=NOT | CP..=SWA | NEGF..=FTI, _, _, rest @ ..]
            | [LI, _, _, _, _, _, _, _, _, _, rest @ ..]
            | [ADDI..=XORI | CMPI..=CMPUI | BMC | JAL..=JGTU | ADDFI..=MULFI, _, _, _, _, _, _, _, _, _, _, rest @ ..]
            | [SLI..=SRSI, _, _, _, _, rest @ ..]
            | [LD..=ST, _, _, _, _, _, _, _, _, _, _, _, _, rest @ ..] => rest,
            _ => {
                return Err(Error {
                    kind:  ErrorKind::InvalidInstruction,
                    index: (program.as_ptr() as usize) - (start.as_ptr() as usize),
                })
            }
        }
    }
}