use std::error::Error;
use std::fmt;
use rustc::mir::repr as mir;

#[derive(Clone, Debug)]
pub enum EvalError {
    DanglingPointerDeref,
    InvalidBool,
    InvalidDiscriminant,
    PointerOutOfBounds {
        offset: usize,
        size: usize,
        len: usize,
    },
    ReadPointerAsBytes,
    ReadBytesAsPointer,
    InvalidPointerMath,
    ReadUndefBytes,
    InvalidBoolOp(mir::BinOp),
    Unimplemented(String),
}

pub type EvalResult<T> = Result<T, EvalError>;

impl Error for EvalError {
    fn description(&self) -> &str {
        match *self {
            EvalError::DanglingPointerDeref =>
                "dangling pointer was dereferenced",
            EvalError::InvalidBool =>
                "invalid boolean value read",
            EvalError::InvalidDiscriminant =>
                "invalid enum discriminant value read",
            EvalError::PointerOutOfBounds { .. } =>
                "pointer offset outside bounds of allocation",
            EvalError::ReadPointerAsBytes =>
                "a raw memory access tried to access part of a pointer value as raw bytes",
            EvalError::ReadBytesAsPointer =>
                "attempted to interpret some raw bytes as a pointer address",
            EvalError::InvalidPointerMath =>
                "attempted to do math or a comparison on pointers into different allocations",
            EvalError::ReadUndefBytes =>
                "attempted to read undefined bytes",
            EvalError::InvalidBoolOp(_) =>
                "invalid boolean operation",
            EvalError::Unimplemented(ref msg) => msg,
        }
    }

    fn cause(&self) -> Option<&Error> { None }
}

impl fmt::Display for EvalError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            EvalError::PointerOutOfBounds { offset, size, len } => write!(f, "pointer offset ({} + {}) outside bounds ({}) of allocation", offset, size, len),
            _ => write!(f, "{}", self.description()),
        }
    }
}