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use std::error::Error as StdError;
use std::ffi::CStr;
use std::fmt;
use std::result::Result as StdResult;

use libc::c_char;

// https://github.com/neovim/neovim/blob/master/src/nvim/api/private/defs.h#L62
#[derive(thiserror::Error, Eq, PartialEq)]
#[repr(C)]
pub struct Error {
    r#type: ErrorType,
    msg: *mut c_char,
}

// https://github.com/neovim/neovim/blob/master/src/nvim/api/private/defs.h#L26
#[allow(dead_code, non_camel_case_types)]
#[derive(Eq, PartialEq)]
#[repr(C)]
enum ErrorType {
    None = -1,
    Exception,
    Validation,
}

impl Error {
    pub const fn new() -> Self {
        Self { r#type: ErrorType::None, msg: std::ptr::null_mut() }
    }
}

impl Default for Error {
    #[inline]
    fn default() -> Self {
        Self::new()
    }
}

impl fmt::Debug for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        if !self.msg.is_null() {
            fmt::Debug::fmt(unsafe { CStr::from_ptr(self.msg) }, f)
        } else {
            use ErrorType::*;
            let msg = match self.r#type {
                None => return Ok(()),
                Exception => "exception",
                Validation => "validation",
            };
            write!(f, "{}", msg)
        }
    }
}

impl Error {
    /// Returns `Ok(f())` if it's not actually an error, or moves into a
    /// generic `std::error::Error` if it is.
    #[inline]
    pub fn into_err_or_else<Ok, Err, F>(self, f: F) -> StdResult<Ok, Err>
    where
        Err: StdError + From<self::Error>,
        F: FnOnce() -> Ok,
    {
        (!self.is_err()).then(f).ok_or_else(|| self.into())
    }

    #[inline]
    pub fn into_err_or_flatten<Ok, Err, F>(self, f: F) -> StdResult<Ok, Err>
    where
        Err: StdError + From<self::Error>,
        F: FnOnce() -> StdResult<Ok, Err>,
    {
        self.into_err_or_else(f)?
    }

    #[inline]
    pub const fn is_err(&self) -> bool {
        !matches!(self.r#type, ErrorType::None)
    }
}