//! This crate defines the centralized error types and `Result` alias used
//! throughout the entire `nixjit` evaluation pipeline. By consolidating error
//! handling here, we ensure a consistent approach to reporting failures across
//! different stages of processing, from parsing to final evaluation.

use std::rc::Rc;
use thiserror::Error;

/// A specialized `Result` type used for all fallible operations within the
/// `nixjit` crates. It defaults to the crate's `Error` type.
pub type Result<T> = core::result::Result<T, Error>;

/// The primary error enum, encompassing all potential failures that can occur
/// during the lifecycle of a Nix expression's evaluation.
#[derive(Error, Debug)]
pub enum ErrorKind {
    /// An error occurred during the initial parsing phase. This typically
    /// indicates a syntax error in the Nix source code, as detected by the
    /// `rnix` parser.
    #[error("error occurred during parse stage: {0}")]
    ParseError(String),

    /// An error occurred while "downgrading" the `rnix` AST to the
    /// High-Level IR (HIR). This can happen if the AST has a structure that is
    /// syntactically valid but semantically incorrect for our IR.
    #[error("error occurred during downgrade stage: {0}")]
    DowngradeError(String),

    /// An error occurred during the variable resolution phase, where the HIR is
    /// converted to the Low-Level IR (LIR). This is most commonly caused by
    /// an unbound or undefined variable.
    #[error("error occurred during variable resolve stage: {0}")]
    ResolutionError(String),

    /// An error occurred during the final evaluation of the LIR. This covers
    /// all unrecoverable runtime errors, such as type mismatches (e.g., adding
    /// a string to an integer), division by zero, or primitive operation
    /// argument arity mismatch
    #[error("error occurred during evaluation stage: {0}")]
    EvalError(String),

    /// Represents an error that can be generated by `throw` or `assert`, and
    /// can be caught by `builtins.tryEval`.
    #[error("{0}")]
    Catchable(String),

    /// A catch-all for any error that does not fit into the other categories.
    #[error("an unknown or unexpected error occurred")]
    Unknown,
}

#[derive(Debug)]
pub struct Error {
    pub kind: ErrorKind,
    pub span: Option<rnix::TextRange>,
    pub source: Option<Rc<str>>,
}

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        // Basic display
        write!(f, "{}", self.kind)?;

        // If we have source and span, print context
        if let (Some(source), Some(span)) = (&self.source, self.span) {
            let start_byte = usize::from(span.start());
            let end_byte = usize::from(span.end());

            if start_byte > source.len() || end_byte > source.len() {
                return Ok(()); // Span is out of bounds
            }

            let mut start_line = 1;
            let mut start_col = 1usize;
            let mut line_start_byte = 0;
            for (i, c) in source.char_indices() {
                if i >= start_byte {
                    break;
                }
                if c == '\n' {
                    start_line += 1;
                    start_col = 1;
                    line_start_byte = i + 1;
                } else {
                    start_col += 1;
                }
            }

            let line_end_byte = source[line_start_byte..]
                .find('\n')
                .map(|i| line_start_byte + i)
                .unwrap_or(source.len());

            let line_str = &source[line_start_byte..line_end_byte];

            let underline_len = if end_byte > start_byte {
                end_byte - start_byte
            } else {
                1
            };

            write!(f, "\n --> {}:{}", start_line, start_col)?;
            write!(f, "\n  |\n")?;
            writeln!(f, "{:4} | {}", start_line, line_str)?;
            write!(
                f,
                "  | {}{}",
                " ".repeat(start_col.saturating_sub(1)),
                "^".repeat(underline_len)
            )?;
        }
        Ok(())
    }
}

impl std::error::Error for Error {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        Some(&self.kind)
    }
}

impl Error {
    pub fn new(kind: ErrorKind) -> Self {
        Self {
            kind,
            span: None,
            source: None,
        }
    }

    pub fn with_span(mut self, span: rnix::TextRange) -> Self {
        self.span = Some(span);
        self
    }

    pub fn with_source(mut self, source: Rc<str>) -> Self {
        self.source = Some(source);
        self
    }

    pub fn parse_error(msg: String) -> Self {
        Self::new(ErrorKind::ParseError(msg))
    }
    pub fn downgrade_error(msg: String) -> Self {
        Self::new(ErrorKind::DowngradeError(msg))
    }
    pub fn resolution_error(msg: String) -> Self {
        Self::new(ErrorKind::ResolutionError(msg))
    }
    pub fn eval_error(msg: String) -> Self {
        Self::new(ErrorKind::EvalError(msg))
    }
    pub fn catchable(msg: String) -> Self {
        Self::new(ErrorKind::Catchable(msg))
    }
    pub fn unknown() -> Self {
        Self::new(ErrorKind::Unknown)
    }
}