use std::{cmp, fmt};

/// 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,
}

/// Position in the input string
#[derive(Debug, PartialEq, Clone, Default)]
pub struct SourcePosition {
    /// The line number on which the error occurred.
    pub line: usize,
    /// The column number on which the error occurred.
    pub column: usize,
    /// The index in the given string which caused the error.
    pub index: usize,
}

/// 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.line, self.pos.column, 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 {}

pub(crate) fn get_line_and_column(s: &str, pos: usize) -> SourcePosition {
    let mut line: usize = 1;
    let mut col: isize = -1;
    for c in s.chars().take(pos + 1) {
        if c == '\n' {
            line += 1;
            col = -1;
        } else {
            col += 1;
        }
    }
    SourcePosition {
        line,
        column: cmp::max(col, 0) as usize,
        index: pos,
    }
}

#[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))
}

/// Build a span
pub(crate) fn spos(s: &str, pos: &usize) -> SourcePosition {
    if *pos >= s.len() {
        Default::default()
    } else {
        get_line_and_column(s, *pos)
    }
}