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    use std::cmp;
    use std::error;
    use std::fmt;
    use std::result;

    use ast;
    use hir;

    /// A type alias for dealing with errors returned by this crate.
    pub type Result<T> = result::Result<T, Error>;

    /// This error type encompasses any error that can be returned by this crate.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum Error {
        /// An error that occurred while translating concrete syntax into abstract
        /// syntax (AST).
        Parse(ast::Error),
        /// An error that occurred while translating abstract syntax into a high
        /// level intermediate representation (HIR).
        Translate(hir::Error),
        /// Hints that destructuring should not be exhaustive.
        ///
        /// This enum may grow additional variants, so this makes sure clients
        /// don't count on exhaustive matching. (Otherwise, adding a new variant
        /// could break existing code.)
        #[doc(hidden)]
        __Nonexhaustive,
    }

    impl From<ast::Error> for Error {
        fn from(err: ast::Error) -> Error {
            Error::Parse(err)
        }
    }

    impl From<hir::Error> for Error {
        fn from(err: hir::Error) -> Error {
            Error::Translate(err)
        }
    }

    impl error::Error for Error {
        fn description(&self) -> &str {
            match *self {
                Error::Parse(ref x) => x.description(),
                Error::Translate(ref x) => x.description(),
                _ => unreachable!(),
            }
        }
    }

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                Error::Parse(ref x) => x.fmt(f),
                Error::Translate(ref x) => x.fmt(f),
                _ => unreachable!(),
            }
        }
    }

    /// A helper type for formatting nice error messages.
    ///
    /// This type is responsible for reporting regex parse errors in a nice human
    /// readable format. Most of its complexity is from interspersing notational
    /// markers pointing out the position where an error occurred.
    #[derive(Debug)]
    pub struct Formatter<'e, E: 'e> {
        /// The original regex pattern in which the error occurred.
        pattern: &'e str,
        /// The error kind. It must impl fmt::Display.
        err: &'e E,
        /// The primary span of the error.
        span: &'e ast::Span,
        /// An auxiliary and optional span, in case the error needs to point to
        /// two locations (e.g., when reporting a duplicate capture group name).
        aux_span: Option<&'e ast::Span>,
    }

    impl<'e> From<&'e ast::Error> for Formatter<'e, ast::ErrorKind> {
        fn from(err: &'e ast::Error) -> Self {
            Formatter {
                pattern: err.pattern(),
                err: err.kind(),
                span: err.span(),
                aux_span: err.auxiliary_span(),
            }
        }
    }

    impl<'e> From<&'e hir::Error> for Formatter<'e, hir::ErrorKind> {
        fn from(err: &'e hir::Error) -> Self {
            Formatter {
                pattern: err.pattern(),
                err: err.kind(),
                span: err.span(),
                aux_span: None,
            }
        }
    }

    impl<'e, E: fmt::Display> fmt::Display for Formatter<'e, E> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let spans = Spans::from_formatter(self);
            if self.pattern.contains('\n') {
                let divider = repeat_char('~', 79);

                writeln!(f, "regex parse error:")?;
                writeln!(f, "{}", divider)?;
                let notated = spans.notate();
                write!(f, "{}", notated)?;
                writeln!(f, "{}", divider)?;
                // If we have error spans that cover multiple lines, then we just
                // note the line numbers.
                if !spans.multi_line.is_empty() {
                    let mut notes = vec![];
                    for span in &spans.multi_line {
                        notes.push(format!(
                            "on line {} (column {}) through line {} (column {})",
                            span.start.line, span.start.column,
                            span.end.line, span.end.column - 1));
                    }
                    writeln!(f, "{}", notes.join("\n"))?;
                }
                write!(f, "error: {}", self.err)?;
            } else {
                writeln!(f, "regex parse error:")?;
                let notated = Spans::from_formatter(self).notate();
                write!(f, "{}", notated)?;
                write!(f, "error: {}", self.err)?;
            }
            Ok(())
        }
    }

    /// This type represents an arbitrary number of error spans in a way that makes
    /// it convenient to notate the regex pattern. ("Notate" means "point out
    /// exactly where the error occurred in the regex pattern.")
    ///
    /// Technically, we can only ever have two spans given our current error
    /// structure. However, after toiling with a specific algorithm for handling
    /// two spans, it became obvious that an algorithm to handle an arbitrary
    /// number of spans was actually much simpler.
    struct Spans<'p> {
        /// The original regex pattern string.
        pattern: &'p str,
        /// The total width that should be used for line numbers. The width is
        /// used for left padding the line numbers for alignment.
        ///
        /// A value of `0` means line numbers should not be displayed. That is,
        /// the pattern is itself only one line.
        line_number_width: usize,
        /// All error spans that occur on a single line. This sequence always has
        /// length equivalent to the number of lines in `pattern`, where the index
        /// of the sequence represents a line number, starting at `0`. The spans
        /// in each line are sorted in ascending order.
        by_line: Vec<Vec<ast::Span>>,
        /// All error spans that occur over one or more lines. That is, the start
        /// and end position of the span have different line numbers. The spans are
        /// sorted in ascending order.
        multi_line: Vec<ast::Span>,
    }

    impl<'p> Spans<'p> {
        /// Build a sequence of spans from a formatter.
        fn from_formatter<'e, E: fmt::Display>(
            fmter: &'p Formatter<'e, E>,
        ) -> Spans<'p> {
            let mut line_count = fmter.pattern.lines().count();
            // If the pattern ends with a `\n` literal, then our line count is
            // off by one, since a span can occur immediately after the last `\n`,
            // which is consider to be an additional line.
            if fmter.pattern.ends_with('\n') {
                line_count += 1;
            }
            let line_number_width =
                if line_count <= 1 {
                    0
                } else {
                    line_count.to_string().len()
                };
            let mut spans = Spans {
                pattern: &fmter.pattern,
                line_number_width: line_number_width,
                by_line: vec![vec![]; line_count],
                multi_line: vec![],
            };
            spans.add(fmter.span.clone());
            if let Some(span) = fmter.aux_span {
                spans.add(span.clone());
            }
            spans
        }

        /// Add the given span to this sequence, putting it in the right place.
        fn add(&mut self, span: ast::Span) {
            // This is grossly inefficient since we sort after each add, but right
            // now, we only ever add two spans at most.
            if span.is_one_line() {
                let i = span.start.line - 1; // because lines are 1-indexed
                self.by_line[i].push(span);
                self.by_line[i].sort();
            } else {
                self.multi_line.push(span);
                self.multi_line.sort();
            }
        }

        /// Notate the pattern string with carents (`^`) pointing at each span
        /// location. This only applies to spans that occur within a single line.
        fn notate(&self) -> String {
            let mut notated = String::new();
            for (i, line) in self.pattern.lines().enumerate() {
                if self.line_number_width > 0 {
                    notated.push_str(&self.left_pad_line_number(i + 1));
                    notated.push_str(": ");
                } else {
                    notated.push_str("    ");
                }
                notated.push_str(line);
                notated.push('\n');
                if let Some(notes) = self.notate_line(i) {
                    notated.push_str(&notes);
                    notated.push('\n');
                }
            }
            notated
        }

        /// Return notes for the line indexed at `i` (zero-based). If there are no
        /// spans for the given line, then `None` is returned. Otherwise, an
        /// appropriately space padded string with correctly positioned `^` is
        /// returned, accounting for line numbers.
        fn notate_line(&self, i: usize) -> Option<String> {
            let spans = &self.by_line[i];
            if spans.is_empty() {
                return None;
            }
            let mut notes = String::new();
            for _ in 0..self.line_number_padding() {
                notes.push(' ');
            }
            let mut pos = 0;
            for span in spans {
                for _ in pos..(span.start.column - 1) {
                    notes.push(' ');
                    pos += 1;
                }
                let note_len = span.end.column.saturating_sub(span.start.column);
                for _ in 0..cmp::max(1, note_len) {
                    notes.push('^');
                    pos += 1;
                }
            }
            Some(notes)
        }

        /// Left pad the given line number with spaces such that it is aligned with
        /// other line numbers.
        fn left_pad_line_number(&self, n: usize) -> String {
            let n = n.to_string();
            let pad = self.line_number_width.checked_sub(n.len()).unwrap();
            let mut result = repeat_char(' ', pad);
            result.push_str(&n);
            result
        }

        /// Return the line number padding beginning at the start of each line of
        /// the pattern.
        ///
        /// If the pattern is only one line, then this returns a fixed padding
        /// for visual indentation.
        fn line_number_padding(&self) -> usize {
            if self.line_number_width == 0 {
                4
            } else {
                2 + self.line_number_width
            }
        }
    }

    fn repeat_char(c: char, count: usize) -> String {
        ::std::iter::repeat(c).take(count).collect()
    }

    #[cfg(test)]
    mod tests {
        use ast::parse::Parser;

        fn assert_panic_message(pattern: &str, expected_msg: &str) -> () {
            let result = Parser::new().parse(pattern);
            match result {
                Ok(_) => {
                    panic!("regex should not have parsed");
                }
                Err(err) => {
                    assert_eq!(err.to_string(), expected_msg.trim());
                }
            }
        }

        // See: https://github.com/rust-lang/regex/issues/464
        #[test]
        fn regression_464() {
            let err = Parser::new().parse("a{\n").unwrap_err();
            // This test checks that the error formatter doesn't panic.
            assert!(!err.to_string().is_empty());
        }

        // See: https://github.com/rust-lang/regex/issues/545
        #[test]
        fn repetition_quantifier_expects_a_valid_decimal() {
            assert_panic_message(r"\\u{[^}]*}", r#"
    regex parse error:
        \\u{[^}]*}
            ^
    error: repetition quantifier expects a valid decimal
    "#);
        }
    }

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