//! Diagnostics.

use std::fmt::{self, Display, Formatter, Write as _};
use std::io;
use std::path::{Path, PathBuf};
use std::str::Utf8Error;
use std::string::FromUtf8Error;

use az::SaturatingAs;
use comemo::Tracked;
use ecow::{EcoVec, eco_vec};
use typst_syntax::package::{PackageSpec, PackageVersion};
use typst_syntax::{Lines, Span, Spanned, SyntaxError};
use utf8_iter::ErrorReportingUtf8Chars;

use crate::engine::Engine;
use crate::loading::{LoadSource, Loaded};
use crate::{World, WorldExt};

/// Early-return with a [`StrResult`] or [`SourceResult`].
///
/// If called with just a string and format args, returns with a
/// `StrResult`. If called with a span, a string and format args, returns
/// a `SourceResult`.
///
/// You can also emit hints with the `; hint: "..."` syntax.
///
/// ```ignore
/// bail!("bailing with a {}", "string result");
/// bail!(span, "bailing with a {}", "source result");
/// bail!(
///     span, "bailing with a {}", "source result";
///     hint: "hint 1"
/// );
/// bail!(
///     span, "bailing with a {}", "source result";
///     hint: "hint 1";
///     hint: "hint 2";
/// );
/// ```
#[macro_export]
#[doc(hidden)]
macro_rules! __bail {
    // For bail!("just a {}", "string")
    (
        $fmt:literal $(, $arg:expr)*
        $(; hint: $hint:literal $(, $hint_arg:expr)*)*
        $(,)?
    ) => {
        return Err($crate::diag::error!(
            $fmt $(, $arg)*
            $(; hint: $hint $(, $hint_arg)*)*
        ))
    };

    // For bail!(error!(..))
    ($error:expr) => {
        return Err(::ecow::eco_vec![$error])
    };

    // For bail(span, ...)
    ($($tts:tt)*) => {
        return Err(::ecow::eco_vec![$crate::diag::error!($($tts)*)])
    };
}

/// Construct an [`EcoString`], [`HintedString`] or [`SourceDiagnostic`] with
/// severity `Error`.
#[macro_export]
#[doc(hidden)]
macro_rules! __error {
    // For bail!("just a {}", "string").
    ($fmt:literal $(, $arg:expr)* $(,)?) => {
        $crate::diag::eco_format!($fmt, $($arg),*).into()
    };

    // For bail!("a hinted {}", "string"; hint: "some hint"; hint: "...")
    (
        $fmt:literal $(, $arg:expr)*
        $(; hint: $hint:literal $(, $hint_arg:expr)*)*
        $(,)?
    ) => {
        $crate::diag::HintedString::new(
            $crate::diag::eco_format!($fmt, $($arg),*)
        ) $(.with_hint($crate::diag::eco_format!($hint, $($hint_arg),*)))*
    };

    // For bail!(span, ...)
    (
        $span:expr, $fmt:literal $(, $arg:expr)*
        $(; hint: $hint:literal $(, $hint_arg:expr)*)*
        $(,)?
    ) => {
        $crate::diag::SourceDiagnostic::error(
            $span,
            $crate::diag::eco_format!($fmt, $($arg),*),
        )  $(.with_hint($crate::diag::eco_format!($hint, $($hint_arg),*)))*
    };
}

/// Construct a [`SourceDiagnostic`] with severity `Warning`.
///
/// You can also emit hints with the `; hint: "..."` syntax.
///
/// ```ignore
/// warning!(span, "warning with a {}", "source result");
/// warning!(
///     span, "warning with a {}", "source result";
///     hint: "hint 1"
/// );
/// warning!(
///     span, "warning with a {}", "source result";
///     hint: "hint 1";
///     hint: "hint 2";
/// );
/// ```
#[macro_export]
#[doc(hidden)]
macro_rules! __warning {
    (
        $span:expr,
        $fmt:literal $(, $arg:expr)*
        $(; hint: $hint:literal $(, $hint_arg:expr)*)*
        $(,)?
    ) => {
        $crate::diag::SourceDiagnostic::warning(
            $span,
            $crate::diag::eco_format!($fmt, $($arg),*),
        ) $(.with_hint($crate::diag::eco_format!($hint, $($hint_arg),*)))*
    };
}

#[rustfmt::skip]
#[doc(inline)]
pub use {
    crate::__bail as bail,
    crate::__error as error,
    crate::__warning as warning,
    ecow::{eco_format, EcoString},
};

/// A result that can carry multiple source errors.
pub type SourceResult<T> = Result<T, EcoVec<SourceDiagnostic>>;

/// An output alongside warnings generated while producing it.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub struct Warned<T> {
    /// The produced output.
    pub output: T,
    /// Warnings generated while producing the output.
    pub warnings: EcoVec<SourceDiagnostic>,
}

/// An error or warning in a source or text file.
///
/// The contained spans will only be detached if any of the input source files
/// were detached.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub struct SourceDiagnostic {
    /// Whether the diagnostic is an error or a warning.
    pub severity: Severity,
    /// The span of the relevant node in the source code.
    pub span: Span,
    /// A diagnostic message describing the problem.
    pub message: EcoString,
    /// The trace of function calls leading to the problem.
    pub trace: EcoVec<Spanned<Tracepoint>>,
    /// Additional hints to the user, indicating how this problem could be avoided
    /// or worked around.
    pub hints: EcoVec<EcoString>,
}

/// The severity of a [`SourceDiagnostic`].
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
pub enum Severity {
    /// A fatal error.
    Error,
    /// A non-fatal warning.
    Warning,
}

impl SourceDiagnostic {
    /// Create a new, bare error.
    pub fn error(span: Span, message: impl Into<EcoString>) -> Self {
        Self {
            severity: Severity::Error,
            span,
            trace: eco_vec![],
            message: message.into(),
            hints: eco_vec![],
        }
    }

    /// Create a new, bare warning.
    pub fn warning(span: Span, message: impl Into<EcoString>) -> Self {
        Self {
            severity: Severity::Warning,
            span,
            trace: eco_vec![],
            message: message.into(),
            hints: eco_vec![],
        }
    }

    /// Adds a single hint to the diagnostic.
    pub fn hint(&mut self, hint: impl Into<EcoString>) {
        self.hints.push(hint.into());
    }

    /// Adds a single hint to the diagnostic.
    pub fn with_hint(mut self, hint: impl Into<EcoString>) -> Self {
        self.hint(hint);
        self
    }

    /// Adds user-facing hints to the diagnostic.
    pub fn with_hints(mut self, hints: impl IntoIterator<Item = EcoString>) -> Self {
        self.hints.extend(hints);
        self
    }
}

impl From<SyntaxError> for SourceDiagnostic {
    fn from(error: SyntaxError) -> Self {
        Self {
            severity: Severity::Error,
            span: error.span,
            message: error.message,
            trace: eco_vec![],
            hints: error.hints,
        }
    }
}

/// Destination for a deprecation message when accessing a deprecated value.
pub trait DeprecationSink {
    /// Emits the given deprecation message into this sink.
    fn emit(self, message: &str);
}

impl DeprecationSink for () {
    fn emit(self, _: &str) {}
}

impl DeprecationSink for (&mut Engine<'_>, Span) {
    /// Emits the deprecation message as a warning.
    fn emit(self, message: &str) {
        self.0.sink.warn(SourceDiagnostic::warning(self.1, message));
    }
}

/// A part of a diagnostic's [trace](SourceDiagnostic::trace).
#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub enum Tracepoint {
    /// A function call.
    Call(Option<EcoString>),
    /// A show rule application.
    Show(EcoString),
    /// A module import.
    Import,
}

impl Display for Tracepoint {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Tracepoint::Call(Some(name)) => {
                write!(f, "error occurred in this call of function `{name}`")
            }
            Tracepoint::Call(None) => {
                write!(f, "error occurred in this function call")
            }
            Tracepoint::Show(name) => {
                write!(f, "error occurred while applying show rule to this {name}")
            }
            Tracepoint::Import => {
                write!(f, "error occurred while importing this module")
            }
        }
    }
}

/// Enrich a [`SourceResult`] with a tracepoint.
pub trait Trace<T> {
    /// Add the tracepoint to all errors that lie outside the `span`.
    fn trace<F>(self, world: Tracked<dyn World + '_>, make_point: F, span: Span) -> Self
    where
        F: Fn() -> Tracepoint;
}

impl<T> Trace<T> for SourceResult<T> {
    fn trace<F>(self, world: Tracked<dyn World + '_>, make_point: F, span: Span) -> Self
    where
        F: Fn() -> Tracepoint,
    {
        self.map_err(|mut errors| {
            let Some(trace_range) = world.range(span) else { return errors };
            for error in errors.make_mut().iter_mut() {
                // Skip traces that surround the error.
                if let Some(error_range) = world.range(error.span)
                    && error.span.id() == span.id()
                    && trace_range.start <= error_range.start
                    && trace_range.end >= error_range.end
                {
                    continue;
                }

                error.trace.push(Spanned::new(make_point(), span));
            }
            errors
        })
    }
}

/// A result type with a string error message.
pub type StrResult<T> = Result<T, EcoString>;

/// A common trait to add a span to any error
pub trait ErrAt {
    /// Attach a span to a more specialized error and turn it into
    /// a generic error, optionally with hints.
    fn err_at(self, span: Span) -> SourceDiagnostic;
}

/// Blanket implementation for anything that doesn't implement its own
/// convertion to a SourceDiagnostic.
impl<S> ErrAt for S
where
    S: Into<EcoString>,
{
    fn err_at(self, span: Span) -> SourceDiagnostic {
        SourceDiagnostic::error(span, self)
    }
}

/// Convert a [`StrResult`] or [`HintedStrResult`] to a [`SourceResult`] by
/// adding span information.
pub trait At<T> {
    /// Add the span information.
    fn at(self, span: Span) -> SourceResult<T>;
}

impl<T, S> At<T> for Result<T, S>
where
    S: ErrAt,
{
    fn at(self, span: Span) -> SourceResult<T> {
        self.map_err(|e| eco_vec![e.err_at(span)])
    }
}

/// A result type with a string error message and hints.
pub type HintedStrResult<T> = Result<T, HintedString>;

/// A string message with hints.
///
/// This is internally represented by a vector of strings.
/// The first element of the vector contains the message.
/// The remaining elements are the hints.
/// This is done to reduce the size of a HintedString.
/// The vector is guaranteed to not be empty.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub struct HintedString(EcoVec<EcoString>);

impl HintedString {
    /// Creates a new hinted string with the given message.
    pub fn new(message: EcoString) -> Self {
        Self(eco_vec![message])
    }

    /// A diagnostic message describing the problem.
    pub fn message(&self) -> &EcoString {
        self.0.first().unwrap()
    }

    /// Additional hints to the user, indicating how this error could be avoided
    /// or worked around.
    pub fn hints(&self) -> &[EcoString] {
        self.0.get(1..).unwrap_or(&[])
    }

    /// Adds a single hint to the hinted string.
    pub fn hint(&mut self, hint: impl Into<EcoString>) {
        self.0.push(hint.into());
    }

    /// Adds a single hint to the hinted string.
    pub fn with_hint(mut self, hint: impl Into<EcoString>) -> Self {
        self.hint(hint);
        self
    }

    /// Adds user-facing hints to the hinted string.
    pub fn with_hints(mut self, hints: impl IntoIterator<Item = EcoString>) -> Self {
        self.0.extend(hints);
        self
    }
}

impl<S> From<S> for HintedString
where
    S: Into<EcoString>,
{
    fn from(value: S) -> Self {
        Self::new(value.into())
    }
}

impl<T> At<T> for HintedStrResult<T> {
    fn at(self, span: Span) -> SourceResult<T> {
        self.map_err(|err| {
            let mut components = err.0.into_iter();
            let message = components.next().unwrap();
            let diag = SourceDiagnostic::error(span, message).with_hints(components);
            eco_vec![diag]
        })
    }
}

/// Enrich a [`StrResult`] or [`HintedStrResult`] with a hint.
pub trait Hint<T> {
    /// Add the hint.
    fn hint(self, hint: impl Into<EcoString>) -> HintedStrResult<T>;
}

impl<T, S> Hint<T> for Result<T, S>
where
    S: Into<EcoString>,
{
    fn hint(self, hint: impl Into<EcoString>) -> HintedStrResult<T> {
        self.map_err(|message| HintedString::new(message.into()).with_hint(hint))
    }
}

impl<T> Hint<T> for HintedStrResult<T> {
    fn hint(self, hint: impl Into<EcoString>) -> HintedStrResult<T> {
        self.map_err(|mut error| {
            error.hint(hint.into());
            error
        })
    }
}

/// A result type with a file-related error.
pub type FileResult<T> = Result<T, FileError>;

/// An error that occurred while trying to load of a file.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub enum FileError {
    /// A file was not found at this path.
    NotFound(PathBuf),
    /// A file could not be accessed.
    AccessDenied(PathBuf),
    /// A directory was found, but a file was expected.
    IsDirectory(PathBuf),
    /// The file is not a Typst source file, but should have been.
    NotSource(PathBuf),
    /// The file was not valid UTF-8, but should have been.
    InvalidUtf8(Option<PathBuf>),
    /// The package the file is part of could not be loaded.
    Package(PackageError),
    /// Another error.
    ///
    /// The optional string can give more details, if available.
    Other(Option<EcoString>),
}

impl FileError {
    /// Create a file error from an I/O error.
    pub fn from_io(err: io::Error, path: &Path) -> Self {
        match err.kind() {
            io::ErrorKind::NotFound => Self::NotFound(path.into()),
            io::ErrorKind::PermissionDenied => Self::AccessDenied(path.into()),
            io::ErrorKind::InvalidData
                if err.to_string().contains("stream did not contain valid UTF-8") =>
            {
                Self::InvalidUtf8(Some(path.into()))
            }
            _ => Self::Other(Some(eco_format!("{err}"))),
        }
    }

    fn write_hints(&self) -> EcoVec<EcoString> {
        match self {
            Self::NotFound(_) => eco_vec![],
            Self::AccessDenied(_) => {
                eco_vec![
                    eco_format!("cannot read file outside of project root"),
                    eco_format!(
                        "you can adjust the project root with the --root argument"
                    ),
                ]
            }
            Self::IsDirectory(_) => eco_vec![],
            Self::NotSource(_) => eco_vec![],
            Self::InvalidUtf8(Some(path)) => {
                eco_vec![eco_format!("tried to read {}", path.display())]
            }
            Self::InvalidUtf8(None) => eco_vec![],
            Self::Package(error) => error.write_hints(),
            Self::Other(Some(err)) => eco_vec![eco_format!("{err}")],
            Self::Other(None) => eco_vec![],
        }
    }
}

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

impl Display for FileError {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::NotFound(path) => {
                write!(f, "file not found (searched at {})", path.display())
            }
            Self::NotSource(path) => {
                write!(f, "{} is not a typst source file", path.display())
            }
            Self::InvalidUtf8(_) => write!(f, "file is not valid utf-8"),
            Self::IsDirectory(path) => {
                write!(f, "failed to load file {} (is a directory)", path.display())
            }
            Self::AccessDenied(path) => {
                write!(f, "failed to load file {} (access denied)", path.display())
            }
            Self::Other(_) => {
                write!(f, "failed to load file")
            }
            Self::Package(error) => write!(f, "{error}"),
        }
    }
}

impl ErrAt for FileError {
    fn err_at(self, span: Span) -> SourceDiagnostic {
        let hints = self.write_hints();
        SourceDiagnostic::error(span, eco_format!("{}", self)).with_hints(hints)
    }
}

impl From<Utf8Error> for FileError {
    fn from(_: Utf8Error) -> Self {
        Self::InvalidUtf8(None)
    }
}

impl From<FromUtf8Error> for FileError {
    fn from(_: FromUtf8Error) -> Self {
        Self::InvalidUtf8(None)
    }
}

impl From<PackageError> for FileError {
    fn from(err: PackageError) -> Self {
        Self::Package(err)
    }
}

/// A result type with a package-related error.
pub type PackageResult<T> = Result<T, PackageError>;

/// An error that occurred while trying to load a package.
///
/// Some variants have an optional string that can give more details, if available.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub enum PackageError {
    /// The specified package does not exist.
    /// Additionally provides information on where we tried to find the package.
    NotFound(PackageSpec, EcoString),
    /// The specified package found, but the version does not exist.
    /// TODO: make the registry part of the error better typed
    VersionNotFound(PackageSpec, Option<PackageVersion>, EcoString),
    /// Failed to retrieve the package through the network.
    NetworkFailed(Option<EcoString>),
    /// The package archive was malformed.
    MalformedArchive(Option<EcoString>),
    /// Another error.
    Other(Option<EcoString>),
}

impl PackageError {
    fn write_hints(&self) -> EcoVec<EcoString> {
        match self {
            Self::NotFound(_, detail) => {
                eco_vec![eco_format!("{detail}")]
            }
            Self::VersionNotFound(spec, latest, registry) => {
                if let Some(version) = latest {
                    eco_vec![
                        eco_format!(
                            "the package {} exists, but not with version {}",
                            spec.name,
                            spec.version
                        ),
                        eco_format!(
                            "the registry at {registry} provides up to version {version}"
                        ),
                    ]
                } else {
                    eco_vec![eco_format!(
                        "the registry at {registry} contains no versions for this package"
                    )]
                }
            }
            Self::NetworkFailed(Some(err)) => eco_vec![eco_format!("{err}")],
            Self::NetworkFailed(None) => eco_vec![],
            Self::MalformedArchive(Some(err)) => eco_vec![eco_format!("{err}")],
            Self::MalformedArchive(None) => {
                eco_vec![eco_format!("the archive is malformed")]
            }
            Self::Other(Some(err)) => eco_vec![eco_format!("{err}")],
            Self::Other(None) => eco_vec![],
        }
    }
}

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

impl Display for PackageError {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::NotFound(spec, _) => write!(f, "package {spec} not found"),
            Self::VersionNotFound(spec, _, _) => {
                write!(f, "package version {spec} not found")
            }
            Self::NetworkFailed(_) => write!(f, "failed to download package"),
            Self::MalformedArchive(_) => write!(f, "failed to decompress package"),
            Self::Other(_) => f.pad("failed to load package"),
        }
    }
}

/// A result type with a data-loading-related error.
pub type LoadResult<T> = Result<T, LoadError>;

/// A call site independent error that occurred during data loading. This avoids
/// polluting the memoization with [`Span`]s and [`FileId`]s from source files.
/// Can be turned into a [`SourceDiagnostic`] using the [`LoadedWithin::within`]
/// method available on [`LoadResult`].
///
/// [`FileId`]: typst_syntax::FileId
#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct LoadError {
    /// The position in the file at which the error occured.
    pos: ReportPos,
    /// Must contain a message formatted like this: `"failed to do thing (cause)"`.
    message: EcoString,
}

impl LoadError {
    /// Creates a new error from a position in a file, a base message
    /// (e.g. `failed to parse JSON`) and a concrete error (e.g. `invalid
    /// number`)
    pub fn new(
        pos: impl Into<ReportPos>,
        message: impl std::fmt::Display,
        error: impl std::fmt::Display,
    ) -> Self {
        Self {
            pos: pos.into(),
            message: eco_format!("{message} ({error})"),
        }
    }
}

impl From<Utf8Error> for LoadError {
    fn from(err: Utf8Error) -> Self {
        let start = err.valid_up_to();
        let end = start + err.error_len().unwrap_or(0);
        LoadError::new(
            start..end,
            "failed to convert to string",
            "file is not valid utf-8",
        )
    }
}

/// Convert a [`LoadResult`] to a [`SourceResult`] by adding the [`Loaded`]
/// context.
pub trait LoadedWithin<T> {
    /// Report an error, possibly in an external file.
    fn within(self, loaded: &Loaded) -> SourceResult<T>;
}

impl<T, E> LoadedWithin<T> for Result<T, E>
where
    E: Into<LoadError>,
{
    fn within(self, loaded: &Loaded) -> SourceResult<T> {
        self.map_err(|err| {
            let LoadError { pos, message } = err.into();
            load_err_in_text(loaded, pos, message)
        })
    }
}

/// Report an error, possibly in an external file. This will delegate to
/// [`load_err_in_invalid_text`] if the data isn't valid utf-8.
fn load_err_in_text(
    loaded: &Loaded,
    pos: impl Into<ReportPos>,
    mut message: EcoString,
) -> EcoVec<SourceDiagnostic> {
    let pos = pos.into();
    // This also does utf-8 validation. Only report an error in an external
    // file if it is human readable (valid utf-8), otherwise fall back to
    // `load_err_in_invalid_text`.
    let lines = Lines::try_from(&loaded.data);
    match (loaded.source.v, lines) {
        (LoadSource::Path(file_id), Ok(lines)) => {
            if let Some(range) = pos.range(&lines) {
                let span = Span::from_range(file_id, range);
                return eco_vec![SourceDiagnostic::error(span, message)];
            }

            // Either `ReportPos::None` was provided, or resolving the range
            // from the line/column failed. If present report the possibly
            // wrong line/column in the error message anyway.
            let span = Span::from_range(file_id, 0..loaded.data.len());
            if let Some(pair) = pos.line_col(&lines) {
                message.pop();
                let (line, col) = pair.numbers();
                write!(&mut message, " at {line}:{col})").ok();
            }
            eco_vec![SourceDiagnostic::error(span, message)]
        }
        (LoadSource::Bytes, Ok(lines)) => {
            if let Some(pair) = pos.line_col(&lines) {
                message.pop();
                let (line, col) = pair.numbers();
                write!(&mut message, " at {line}:{col})").ok();
            }
            eco_vec![SourceDiagnostic::error(loaded.source.span, message)]
        }
        _ => load_err_in_invalid_text(loaded, pos, message),
    }
}

/// Report an error (possibly from an external file) that isn't valid utf-8.
fn load_err_in_invalid_text(
    loaded: &Loaded,
    pos: impl Into<ReportPos>,
    mut message: EcoString,
) -> EcoVec<SourceDiagnostic> {
    let line_col = pos.into().try_line_col(&loaded.data).map(|p| p.numbers());
    match (loaded.source.v, line_col) {
        (LoadSource::Path(file), _) => {
            message.pop();
            if let Some(package) = file.package() {
                write!(
                    &mut message,
                    " in {package}{}",
                    file.vpath().as_rooted_path().display()
                )
                .ok();
            } else {
                write!(&mut message, " in {}", file.vpath().as_rootless_path().display())
                    .ok();
            };
            if let Some((line, col)) = line_col {
                write!(&mut message, ":{line}:{col}").ok();
            }
            message.push(')');
        }
        (LoadSource::Bytes, Some((line, col))) => {
            message.pop();
            write!(&mut message, " at {line}:{col})").ok();
        }
        (LoadSource::Bytes, None) => (),
    }
    eco_vec![SourceDiagnostic::error(loaded.source.span, message)]
}

/// A position at which an error was reported.
#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
pub enum ReportPos {
    /// Contains a range, and a line/column pair.
    Full(std::ops::Range<u32>, LineCol),
    /// Contains a range.
    Range(std::ops::Range<u32>),
    /// Contains a line/column pair.
    LineCol(LineCol),
    #[default]
    None,
}

impl From<std::ops::Range<usize>> for ReportPos {
    fn from(value: std::ops::Range<usize>) -> Self {
        Self::Range(value.start.saturating_as()..value.end.saturating_as())
    }
}

impl From<LineCol> for ReportPos {
    fn from(value: LineCol) -> Self {
        Self::LineCol(value)
    }
}

impl ReportPos {
    /// Creates a position from a pre-existing range and line-column pair.
    pub fn full(range: std::ops::Range<usize>, pair: LineCol) -> Self {
        let range = range.start.saturating_as()..range.end.saturating_as();
        Self::Full(range, pair)
    }

    /// Tries to determine the byte range for this position.
    fn range(&self, lines: &Lines<String>) -> Option<std::ops::Range<usize>> {
        match self {
            ReportPos::Full(range, _) => Some(range.start as usize..range.end as usize),
            ReportPos::Range(range) => Some(range.start as usize..range.end as usize),
            &ReportPos::LineCol(pair) => {
                let i =
                    lines.line_column_to_byte(pair.line as usize, pair.col as usize)?;
                Some(i..i)
            }
            ReportPos::None => None,
        }
    }

    /// Tries to determine the line/column for this position.
    fn line_col(&self, lines: &Lines<String>) -> Option<LineCol> {
        match self {
            &ReportPos::Full(_, pair) => Some(pair),
            ReportPos::Range(range) => {
                let (line, col) = lines.byte_to_line_column(range.start as usize)?;
                Some(LineCol::zero_based(line, col))
            }
            &ReportPos::LineCol(pair) => Some(pair),
            ReportPos::None => None,
        }
    }

    /// Either gets the line/column pair, or tries to compute it from possibly
    /// invalid utf-8 data.
    fn try_line_col(&self, bytes: &[u8]) -> Option<LineCol> {
        match self {
            &ReportPos::Full(_, pair) => Some(pair),
            ReportPos::Range(range) => {
                LineCol::try_from_byte_pos(range.start as usize, bytes)
            }
            &ReportPos::LineCol(pair) => Some(pair),
            ReportPos::None => None,
        }
    }
}

/// A line/column pair.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct LineCol {
    /// The 0-based line.
    line: u32,
    /// The 0-based column.
    col: u32,
}

impl LineCol {
    /// Constructs the line/column pair from 0-based indices.
    pub fn zero_based(line: usize, col: usize) -> Self {
        Self {
            line: line.saturating_as(),
            col: col.saturating_as(),
        }
    }

    /// Constructs the line/column pair from 1-based numbers.
    pub fn one_based(line: usize, col: usize) -> Self {
        Self::zero_based(line.saturating_sub(1), col.saturating_sub(1))
    }

    /// Try to compute a line/column pair from possibly invalid utf-8 data.
    pub fn try_from_byte_pos(pos: usize, bytes: &[u8]) -> Option<Self> {
        let bytes = &bytes[..pos];
        let mut line = 0;
        #[allow(clippy::double_ended_iterator_last)]
        let line_start = memchr::memchr_iter(b'\n', bytes)
            .inspect(|_| line += 1)
            .last()
            .map(|i| i + 1)
            .unwrap_or(bytes.len());

        let col = ErrorReportingUtf8Chars::new(&bytes[line_start..]).count();
        Some(LineCol::zero_based(line, col))
    }

    /// Returns the 0-based line/column indices.
    pub fn indices(&self) -> (usize, usize) {
        (self.line as usize, self.col as usize)
    }

    /// Returns the 1-based line/column numbers.
    pub fn numbers(&self) -> (usize, usize) {
        (self.line as usize + 1, self.col as usize + 1)
    }
}

/// Format a user-facing error message for an XML-like file format.
pub fn format_xml_like_error(format: &str, error: roxmltree::Error) -> LoadError {
    let pos = LineCol::one_based(error.pos().row as usize, error.pos().col as usize);
    let message = match error {
        roxmltree::Error::UnexpectedCloseTag(expected, actual, _) => {
            eco_format!(
                "failed to parse {format} (found closing tag '{actual}' instead of '{expected}')"
            )
        }
        roxmltree::Error::UnknownEntityReference(entity, _) => {
            eco_format!("failed to parse {format} (unknown entity '{entity}')")
        }
        roxmltree::Error::DuplicatedAttribute(attr, _) => {
            eco_format!("failed to parse {format} (duplicate attribute '{attr}')")
        }
        roxmltree::Error::NoRootNode => {
            eco_format!("failed to parse {format} (missing root node)")
        }
        err => eco_format!("failed to parse {format} ({err})"),
    };

    LoadError { pos: pos.into(), message }
}