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use std::{error::Error, fmt};

/// An error related to parsing of an SPDX license expression
/// or identifier
#[derive(Debug, PartialEq)]
pub struct ParseError<'a> {
    /// The string that was parsed
    pub original: &'a str,
    /// The range of characters in the original string that result
    /// in this error
    pub span: std::ops::Range<usize>,
    /// The specific reason for the error
    pub reason: Reason,
}

/// The particular reason for a `ParseError`
#[derive(Debug, PartialEq)]
pub enum Reason {
    /// The specified license short-identifier was not
    /// found the SPDX list
    UnknownLicense,
    /// The specified exception short-identifier was not
    /// found the SPDX list
    UnknownException,
    /// The characters are not valid in an SDPX license expression
    InvalidCharacters,
    /// An opening parens was unmatched with a closing parens
    UnclosedParens,
    /// A closing parens was unmatched with an opening parens
    UnopenedParens,
    /// The expression does not contain any valid terms
    Empty,
    /// Found an unexpected term, which wasn't one of the
    /// expected terms that is listed
    Unexpected(&'static [&'static str]),
    /// A + was found after whitespace, which is not allowed
    /// by the SPDX spec
    SeparatedPlus,
    /// When lexing, a term was found that was
    /// 1. Not a license short-id
    /// 2. Not an exception short-id
    /// 3. Not a document/license ref
    /// 4. Not an AND, OR, or WITH
    UnknownTerm,
    /// GNU suffix licenses don't allow `+` because they already have
    /// the `-or-later` suffix to denote that
    GnuNoPlus,
}

impl<'a> fmt::Display for ParseError<'a> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.original)?;
        f.write_str("\n")?;

        for _ in 0..self.span.start {
            f.write_str(" ")?;
        }

        // Mismatched parens have a slightly different output
        // than the other errors
        match &self.reason {
            Reason::UnclosedParens => f.write_fmt(format_args!("- {}", Reason::UnclosedParens)),
            Reason::UnopenedParens => f.write_fmt(format_args!("^ {}", Reason::UnopenedParens)),
            other => {
                for _ in self.span.start..self.span.end {
                    f.write_str("^")?;
                }

                f.write_fmt(format_args!(" {}", other))
            }
        }
    }
}

impl fmt::Display for Reason {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::UnknownLicense => f.write_str("unknown license id"),
            Self::UnknownException => f.write_str("unknown exception id"),
            Self::InvalidCharacters => f.write_str("invalid character(s)"),
            Self::UnclosedParens => f.write_str("unclosed parens"),
            Self::UnopenedParens => f.write_str("unopened parens"),
            Self::Empty => f.write_str("empty expression"),
            Self::Unexpected(expected) => {
                if expected.len() > 1 {
                    f.write_str("expected one of ")?;

                    for (i, exp) in expected.iter().enumerate() {
                        f.write_fmt(format_args!("{}`{}`", if i > 0 { ", " } else { "" }, exp))?;
                    }
                    f.write_str(" here")
                } else if !expected.is_empty() {
                    f.write_fmt(format_args!("expected a `{}` here", expected[0]))
                } else {
                    f.write_str("the term was not expected here")
                }
            }
            Self::SeparatedPlus => f.write_str("`+` must not follow whitespace"),
            Self::UnknownTerm => f.write_str("unknown term"),
            Self::GnuNoPlus => f.write_str("a GNU license was followed by a `+`"),
        }
    }
}

impl<'a> Error for ParseError<'a> {
    fn description(&self) -> &str {
        match self.reason {
            Reason::UnknownLicense => "unknown license id",
            Reason::UnknownException => "unknown exception id",
            Reason::InvalidCharacters => "invalid character(s)",
            Reason::UnclosedParens => "unclosed parens",
            Reason::UnopenedParens => "unopened parens",
            Reason::Empty => "empty expression",
            Reason::Unexpected(_) => "unexpected term",
            Reason::SeparatedPlus => "`+` must not follow whitespace",
            Reason::UnknownTerm => "unknown term",
            Reason::GnuNoPlus => "a GNU license was followed by a `+`",
        }
    }
}