use std::{fmt};
use super::SourcePosition;
use position::get_line_and_column;

/// The representation of an s-expression parse error.
pub struct Error {
    /// The error message.
    pub message: &'static str,
    /// Position in the source string where the error was detected
    pub pos: SourcePosition,
}

/// Since errors are the uncommon case, they're boxed. This keeps the size of
/// structs down, which helps performance in the common case.
///
/// For example, an `ERes<()>` becomes 8 bytes, instead of the 24 bytes it would
/// be if `Err` were unboxed.
type Err = Box<Error>;

/// Helps clean up type signatures, but shouldn't be exposed to the outside
/// world.
pub(crate) type ERes<T> = Result<T, Err>;

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(f, "{}: {}", self.pos, self.message)
    }
}

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

impl std::error::Error for Error {}

#[cold]
fn err_impl(message: &'static str, s: &str, pos: usize) -> Err {
    Box::new(Error {
        message,
        pos: get_line_and_column(s, pos),
    })
}

/// Build an error with span information
pub(crate) fn err<T>(message: &'static str, s: &str, pos: usize) -> ERes<T> {
    Err(err_impl(message, s, pos))
}