use std::any::TypeId;
use std::fmt::{self, Debug, Formatter};
use std::hash::{Hash, Hasher};
use std::iter::{self, Sum};
use std::marker::PhantomData;
use std::ops::{Add, AddAssign, Deref, DerefMut};
use std::sync::Arc;

use comemo::Tracked;
use ecow::{eco_format, EcoString};
use serde::{Serialize, Serializer};
use smallvec::smallvec;

use crate::diag::{SourceResult, StrResult};
use crate::engine::Engine;
use crate::foundations::{
    elem, func, scope, ty, Context, Dict, Element, Fields, IntoValue, Label,
    NativeElement, Recipe, RecipeIndex, Repr, Selector, Str, Style, StyleChain, Styles,
    Value,
};
use crate::introspection::Location;
use crate::layout::{AlignElem, Alignment, Axes, Length, MoveElem, PadElem, Rel, Sides};
use crate::model::{Destination, EmphElem, LinkElem, StrongElem};
use crate::syntax::Span;
use crate::text::UnderlineElem;
use crate::utils::{fat, singleton, LazyHash, SmallBitSet};

/// A piece of document content.
///
/// This type is at the heart of Typst. All markup you write and most
/// [functions]($function) you call produce content values. You can create a
/// content value by enclosing markup in square brackets. This is also how you
/// pass content to functions.
///
/// # Example
/// ```example
/// Type of *Hello!* is
/// #type([*Hello!*])
/// ```
///
/// Content can be added with the `+` operator,
/// [joined together]($scripting/#blocks) and multiplied with integers. Wherever
/// content is expected, you can also pass a [string]($str) or `{none}`.
///
/// # Representation
/// Content consists of elements with fields. When constructing an element with
/// its _element function,_ you provide these fields as arguments and when you
/// have a content value, you can access its fields with [field access
/// syntax]($scripting/#field-access).
///
/// Some fields are required: These must be provided when constructing an
/// element and as a consequence, they are always available through field access
/// on content of that type. Required fields are marked as such in the
/// documentation.
///
/// Most fields are optional: Like required fields, they can be passed to the
/// element function to configure them for a single element. However, these can
/// also be configured with [set rules]($styling/#set-rules) to apply them to
/// all elements within a scope. Optional fields are only available with field
/// access syntax when they were explicitly passed to the element function, not
/// when they result from a set rule.
///
/// Each element has a default appearance. However, you can also completely
/// customize its appearance with a [show rule]($styling/#show-rules). The show
/// rule is passed the element. It can access the element's field and produce
/// arbitrary content from it.
///
/// In the web app, you can hover over a content variable to see exactly which
/// elements the content is composed of and what fields they have.
/// Alternatively, you can inspect the output of the [`repr`] function.
#[ty(scope, cast)]
#[derive(Clone, Hash)]
#[allow(clippy::derived_hash_with_manual_eq)]
pub struct Content {
    /// The partially element-dependant inner data.
    inner: Arc<Inner<dyn Bounds>>,
    /// The element's source code location.
    span: Span,
}

/// The inner representation behind the `Arc`.
#[derive(Hash)]
struct Inner<T: ?Sized + 'static> {
    /// An optional label attached to the element.
    label: Option<Label>,
    /// The element's location which identifies it in the layouted output.
    location: Option<Location>,
    /// Manages the element during realization.
    /// - If bit 0 is set, the element is prepared.
    /// - If bit n is set, the element is guarded against the n-th show rule
    ///   recipe from the top of the style chain (counting from 1).
    lifecycle: SmallBitSet,
    /// The element's raw data.
    elem: LazyHash<T>,
}

impl Content {
    /// Creates a new content from an element.
    pub fn new<T: NativeElement>(elem: T) -> Self {
        Self {
            inner: Arc::new(Inner {
                label: None,
                location: None,
                lifecycle: SmallBitSet::new(),
                elem: elem.into(),
            }),
            span: Span::detached(),
        }
    }

    /// Creates a empty sequence content.
    pub fn empty() -> Self {
        singleton!(Content, SequenceElem::default().pack()).clone()
    }

    /// Get the element of this content.
    pub fn elem(&self) -> Element {
        self.inner.elem.dyn_elem()
    }

    /// Get the span of the content.
    pub fn span(&self) -> Span {
        self.span
    }

    /// Set the span of the content.
    pub fn spanned(mut self, span: Span) -> Self {
        if self.span.is_detached() {
            self.span = span;
        }
        self
    }

    /// Get the label of the content.
    pub fn label(&self) -> Option<Label> {
        self.inner.label
    }

    /// Attach a label to the content.
    pub fn labelled(mut self, label: Label) -> Self {
        self.set_label(label);
        self
    }

    /// Set the label of the content.
    pub fn set_label(&mut self, label: Label) {
        self.make_mut().label = Some(label);
    }

    /// Assigns a location to the content.
    ///
    /// This identifies the content and e.g. makes it linkable by
    /// `.linked(Destination::Location(loc))`.
    ///
    /// Useful in combination with [`Location::variant`].
    pub fn located(mut self, loc: Location) -> Self {
        self.set_location(loc);
        self
    }

    /// Set the location of the content.
    pub fn set_location(&mut self, location: Location) {
        self.make_mut().location = Some(location);
    }

    /// Check whether a show rule recipe is disabled.
    pub fn is_guarded(&self, index: RecipeIndex) -> bool {
        self.inner.lifecycle.contains(index.0)
    }

    /// Disable a show rule recipe.
    pub fn guarded(mut self, index: RecipeIndex) -> Self {
        self.make_mut().lifecycle.insert(index.0);
        self
    }

    /// Whether this content has already been prepared.
    pub fn is_prepared(&self) -> bool {
        self.inner.lifecycle.contains(0)
    }

    /// Mark this content as prepared.
    pub fn mark_prepared(&mut self) {
        self.make_mut().lifecycle.insert(0);
    }

    /// Get a field by ID.
    ///
    /// This is the preferred way to access fields. However, you can only use it
    /// if you have set the field IDs yourself or are using the field IDs
    /// generated by the `#[elem]` macro.
    pub fn get(
        &self,
        id: u8,
        styles: Option<StyleChain>,
    ) -> Result<Value, FieldAccessError> {
        if id == 255 {
            if let Some(label) = self.label() {
                return Ok(label.into_value());
            }
        }
        match styles {
            Some(styles) => self.inner.elem.field_with_styles(id, styles),
            None => self.inner.elem.field(id),
        }
    }

    /// Get a field by name.
    ///
    /// If you have access to the field IDs of the element, use [`Self::get`]
    /// instead.
    pub fn get_by_name(&self, name: &str) -> Result<Value, FieldAccessError> {
        if name == "label" {
            if let Some(label) = self.label() {
                return Ok(label.into_value());
            }
        }
        let id = self.elem().field_id(name).ok_or(FieldAccessError::Unknown)?;
        self.get(id, None)
    }

    /// Get a field by ID, returning a missing field error if it does not exist.
    ///
    /// This is the preferred way to access fields. However, you can only use it
    /// if you have set the field IDs yourself or are using the field IDs
    /// generated by the `#[elem]` macro.
    pub fn field(&self, id: u8) -> StrResult<Value> {
        self.get(id, None)
            .map_err(|e| e.message(self, self.elem().field_name(id).unwrap()))
    }

    /// Get a field by name, returning a missing field error if it does not
    /// exist.
    ///
    /// If you have access to the field IDs of the element, use [`Self::field`]
    /// instead.
    pub fn field_by_name(&self, name: &str) -> StrResult<Value> {
        self.get_by_name(name).map_err(|e| e.message(self, name))
    }

    /// Resolve all fields with the styles and save them in-place.
    pub fn materialize(&mut self, styles: StyleChain) {
        self.make_mut().elem.materialize(styles);
    }

    /// Create a new sequence element from multiples elements.
    pub fn sequence(iter: impl IntoIterator<Item = Self>) -> Self {
        let mut iter = iter.into_iter();
        let Some(first) = iter.next() else { return Self::empty() };
        let Some(second) = iter.next() else { return first };
        SequenceElem::new(
            std::iter::once(first)
                .chain(std::iter::once(second))
                .chain(iter)
                .collect(),
        )
        .into()
    }

    /// Whether the contained element is of type `T`.
    pub fn is<T: NativeElement>(&self) -> bool {
        self.inner.elem.dyn_type_id() == TypeId::of::<T>()
    }

    /// Downcasts the element to a packed value.
    pub fn to_packed<T: NativeElement>(&self) -> Option<&Packed<T>> {
        Packed::from_ref(self)
    }

    /// Downcasts the element to a mutable packed value.
    pub fn to_packed_mut<T: NativeElement>(&mut self) -> Option<&mut Packed<T>> {
        Packed::from_mut(self)
    }

    /// Downcasts the element into an owned packed value.
    pub fn into_packed<T: NativeElement>(self) -> Result<Packed<T>, Self> {
        Packed::from_owned(self)
    }

    /// Extract the raw underlying element.
    pub fn unpack<T: NativeElement>(self) -> Result<T, Self> {
        self.into_packed::<T>().map(Packed::unpack)
    }

    /// Makes sure the content is not shared and returns a mutable reference to
    /// the inner data.
    fn make_mut(&mut self) -> &mut Inner<dyn Bounds> {
        let arc = &mut self.inner;
        if Arc::strong_count(arc) > 1 || Arc::weak_count(arc) > 0 {
            *self = arc.elem.dyn_clone(arc, self.span);
        }
        Arc::get_mut(&mut self.inner).unwrap()
    }

    /// Whether the contained element has the given capability.
    pub fn can<C>(&self) -> bool
    where
        C: ?Sized + 'static,
    {
        self.elem().can::<C>()
    }

    /// Cast to a trait object if the contained element has the given
    /// capability.
    pub fn with<C>(&self) -> Option<&C>
    where
        C: ?Sized + 'static,
    {
        // Safety: The vtable comes from the `Capable` implementation which
        // guarantees to return a matching vtable for `Packed<T>` and `C`.
        // Since any `Packed<T>` is a repr(transparent) `Content`, we can also
        // use a `*const Content` pointer.
        let vtable = self.elem().vtable()(TypeId::of::<C>())?;
        let data = self as *const Content as *const ();
        Some(unsafe { &*fat::from_raw_parts(data, vtable.as_ptr()) })
    }

    /// Cast to a mutable trait object if the contained element has the given
    /// capability.
    pub fn with_mut<C>(&mut self) -> Option<&mut C>
    where
        C: ?Sized + 'static,
    {
        // Safety: The vtable comes from the `Capable` implementation which
        // guarantees to return a matching vtable for `Packed<T>` and `C`.
        // Since any `Packed<T>` is a repr(transparent) `Content`, we can also
        // use a `*const Content` pointer.
        //
        // The resulting trait object contains an `&mut Packed<T>`. We do _not_
        // need to ensure that we hold the only reference to the `Arc` here
        // because `Packed<T>`'s DerefMut impl will take care of that if
        // mutable access is required.
        let vtable = self.elem().vtable()(TypeId::of::<C>())?;
        let data = self as *mut Content as *mut ();
        Some(unsafe { &mut *fat::from_raw_parts_mut(data, vtable.as_ptr()) })
    }

    /// Whether the content is an empty sequence.
    pub fn is_empty(&self) -> bool {
        let Some(sequence) = self.to_packed::<SequenceElem>() else {
            return false;
        };

        sequence.children.is_empty()
    }

    /// Also auto expands sequence of sequences into flat sequence
    pub fn sequence_recursive_for_each<'a>(&'a self, f: &mut impl FnMut(&'a Self)) {
        if let Some(sequence) = self.to_packed::<SequenceElem>() {
            for child in &sequence.children {
                child.sequence_recursive_for_each(f);
            }
        } else {
            f(self);
        }
    }

    /// Style this content with a recipe, eagerly applying it if possible.
    pub fn styled_with_recipe(
        self,
        engine: &mut Engine,
        context: Tracked<Context>,
        recipe: Recipe,
    ) -> SourceResult<Self> {
        if recipe.selector().is_none() {
            recipe.apply(engine, context, self)
        } else {
            Ok(self.styled(recipe))
        }
    }

    /// Repeat this content `count` times.
    pub fn repeat(&self, count: usize) -> Self {
        Self::sequence(std::iter::repeat_with(|| self.clone()).take(count))
    }

    /// Style this content with a style entry.
    pub fn styled(mut self, style: impl Into<Style>) -> Self {
        if let Some(style_elem) = self.to_packed_mut::<StyledElem>() {
            style_elem.styles.apply_one(style.into());
            self
        } else {
            self.styled_with_map(style.into().into())
        }
    }

    /// Style this content with a full style map.
    pub fn styled_with_map(mut self, styles: Styles) -> Self {
        if styles.is_empty() {
            return self;
        }

        if let Some(style_elem) = self.to_packed_mut::<StyledElem>() {
            style_elem.styles.apply(styles);
            self
        } else {
            StyledElem::new(self, styles).into()
        }
    }

    /// Style this content with a full style map in-place.
    pub fn style_in_place(&mut self, styles: Styles) {
        if styles.is_empty() {
            return;
        }

        if let Some(style_elem) = self.to_packed_mut::<StyledElem>() {
            style_elem.styles.apply(styles);
        } else {
            *self = StyledElem::new(std::mem::take(self), styles).into();
        }
    }

    /// Queries the content tree for all elements that match the given selector.
    ///
    /// Elements produced in `show` rules will not be included in the results.
    pub fn query(&self, selector: Selector) -> Vec<Content> {
        let mut results = Vec::new();
        self.traverse(&mut |element| {
            if selector.matches(&element, None) {
                results.push(element);
            }
        });
        results
    }

    /// Queries the content tree for the first element that match the given
    /// selector.
    ///
    /// Elements produced in `show` rules will not be included in the results.
    pub fn query_first(&self, selector: Selector) -> Option<Content> {
        let mut result = None;
        self.traverse(&mut |element| {
            if result.is_none() && selector.matches(&element, None) {
                result = Some(element);
            }
        });
        result
    }

    /// Extracts the plain text of this content.
    pub fn plain_text(&self) -> EcoString {
        let mut text = EcoString::new();
        self.traverse(&mut |element| {
            if let Some(textable) = element.with::<dyn PlainText>() {
                textable.plain_text(&mut text);
            }
        });
        text
    }

    /// Traverse this content.
    fn traverse<F>(&self, f: &mut F)
    where
        F: FnMut(Content),
    {
        f(self.clone());

        self.inner
            .elem
            .fields()
            .into_iter()
            .for_each(|(_, value)| walk_value(value, f));

        /// Walks a given value to find any content that matches the selector.
        fn walk_value<F>(value: Value, f: &mut F)
        where
            F: FnMut(Content),
        {
            match value {
                Value::Content(content) => content.traverse(f),
                Value::Array(array) => {
                    for value in array {
                        walk_value(value, f);
                    }
                }
                _ => {}
            }
        }
    }
}

impl Content {
    /// Strongly emphasize this content.
    pub fn strong(self) -> Self {
        StrongElem::new(self).pack()
    }

    /// Emphasize this content.
    pub fn emph(self) -> Self {
        EmphElem::new(self).pack()
    }

    /// Underline this content.
    pub fn underlined(self) -> Self {
        UnderlineElem::new(self).pack()
    }

    /// Link the content somewhere.
    pub fn linked(self, dest: Destination) -> Self {
        self.styled(LinkElem::set_dests(smallvec![dest]))
    }

    /// Set alignments for this content.
    pub fn aligned(self, align: Alignment) -> Self {
        self.styled(AlignElem::set_alignment(align))
    }

    /// Pad this content at the sides.
    pub fn padded(self, padding: Sides<Rel<Length>>) -> Self {
        PadElem::new(self)
            .with_left(padding.left)
            .with_top(padding.top)
            .with_right(padding.right)
            .with_bottom(padding.bottom)
            .pack()
    }

    /// Transform this content's contents without affecting layout.
    pub fn moved(self, delta: Axes<Rel<Length>>) -> Self {
        MoveElem::new(self).with_dx(delta.x).with_dy(delta.y).pack()
    }
}

#[scope]
impl Content {
    /// The content's element function. This function can be used to create the element
    /// contained in this content. It can be used in set and show rules for the
    /// element. Can be compared with global functions to check whether you have
    /// a specific
    /// kind of element.
    #[func]
    pub fn func(&self) -> Element {
        self.elem()
    }

    /// Whether the content has the specified field.
    #[func]
    pub fn has(
        &self,
        /// The field to look for.
        field: Str,
    ) -> bool {
        if field.as_str() == "label" {
            return self.label().is_some();
        }

        let Some(id) = self.elem().field_id(&field) else {
            return false;
        };

        self.inner.elem.has(id)
    }

    /// Access the specified field on the content. Returns the default value if
    /// the field does not exist or fails with an error if no default value was
    /// specified.
    #[func]
    pub fn at(
        &self,
        /// The field to access.
        field: Str,
        /// A default value to return if the field does not exist.
        #[named]
        default: Option<Value>,
    ) -> StrResult<Value> {
        self.get_by_name(&field)
            .or_else(|e| default.ok_or(e))
            .map_err(|e| e.message_no_default(self, &field))
    }

    /// Returns the fields of this content.
    ///
    /// ```example
    /// #rect(
    ///   width: 10cm,
    ///   height: 10cm,
    /// ).fields()
    /// ```
    #[func]
    pub fn fields(&self) -> Dict {
        let mut dict = self.inner.elem.fields();
        if let Some(label) = self.label() {
            dict.insert("label".into(), label.into_value());
        }
        dict
    }

    /// The location of the content. This is only available on content returned
    /// by [query] or provided by a [show rule]($reference/styling/#show-rules),
    /// for other content it will be `{none}`. The resulting location can be
    /// used with [counters]($counter), [state] and [queries]($query).
    #[func]
    pub fn location(&self) -> Option<Location> {
        self.inner.location
    }
}

impl Default for Content {
    fn default() -> Self {
        Self::empty()
    }
}

impl Debug for Content {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        self.inner.elem.fmt(f)
    }
}

impl<T: NativeElement> From<T> for Content {
    fn from(value: T) -> Self {
        Self::new(value)
    }
}

impl PartialEq for Content {
    fn eq(&self, other: &Self) -> bool {
        // Additional short circuit for different elements.
        self.elem() == other.elem() && self.inner.elem.dyn_eq(other)
    }
}

impl Repr for Content {
    fn repr(&self) -> EcoString {
        self.inner.elem.repr()
    }
}

impl Add for Content {
    type Output = Self;

    fn add(self, mut rhs: Self) -> Self::Output {
        let mut lhs = self;
        match (lhs.to_packed_mut::<SequenceElem>(), rhs.to_packed_mut::<SequenceElem>()) {
            (Some(seq_lhs), Some(rhs)) => {
                seq_lhs.children.extend(rhs.children.iter().cloned());
                lhs
            }
            (Some(seq_lhs), None) => {
                seq_lhs.children.push(rhs);
                lhs
            }
            (None, Some(rhs_seq)) => {
                rhs_seq.children.insert(0, lhs);
                rhs
            }
            (None, None) => Self::sequence([lhs, rhs]),
        }
    }
}

impl<'a> Add<&'a Self> for Content {
    type Output = Self;

    fn add(self, rhs: &'a Self) -> Self::Output {
        let mut lhs = self;
        match (lhs.to_packed_mut::<SequenceElem>(), rhs.to_packed::<SequenceElem>()) {
            (Some(seq_lhs), Some(rhs)) => {
                seq_lhs.children.extend(rhs.children.iter().cloned());
                lhs
            }
            (Some(seq_lhs), None) => {
                seq_lhs.children.push(rhs.clone());
                lhs
            }
            (None, Some(_)) => {
                let mut rhs = rhs.clone();
                rhs.to_packed_mut::<SequenceElem>().unwrap().children.insert(0, lhs);
                rhs
            }
            (None, None) => Self::sequence([lhs, rhs.clone()]),
        }
    }
}

impl AddAssign for Content {
    fn add_assign(&mut self, rhs: Self) {
        *self = std::mem::take(self) + rhs;
    }
}

impl AddAssign<&Self> for Content {
    fn add_assign(&mut self, rhs: &Self) {
        *self = std::mem::take(self) + rhs;
    }
}

impl Sum for Content {
    fn sum<I: Iterator<Item = Self>>(iter: I) -> Self {
        Self::sequence(iter)
    }
}

impl Serialize for Content {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.collect_map(
            iter::once(("func".into(), self.func().name().into_value()))
                .chain(self.fields()),
        )
    }
}

/// The trait that combines all the other traits into a trait object.
trait Bounds: Debug + Repr + Fields + Send + Sync + 'static {
    fn dyn_type_id(&self) -> TypeId;
    fn dyn_elem(&self) -> Element;
    fn dyn_clone(&self, inner: &Inner<dyn Bounds>, span: Span) -> Content;
    fn dyn_hash(&self, hasher: &mut dyn Hasher);
    fn dyn_eq(&self, other: &Content) -> bool;
}

impl<T: NativeElement> Bounds for T {
    fn dyn_type_id(&self) -> TypeId {
        TypeId::of::<Self>()
    }

    fn dyn_elem(&self) -> Element {
        Self::elem()
    }

    fn dyn_clone(&self, inner: &Inner<dyn Bounds>, span: Span) -> Content {
        Content {
            inner: Arc::new(Inner {
                label: inner.label,
                location: inner.location,
                lifecycle: inner.lifecycle.clone(),
                elem: LazyHash::reuse(self.clone(), &inner.elem),
            }),
            span,
        }
    }

    fn dyn_hash(&self, mut state: &mut dyn Hasher) {
        TypeId::of::<Self>().hash(&mut state);
        self.hash(&mut state);
    }

    fn dyn_eq(&self, other: &Content) -> bool {
        let Some(other) = other.to_packed::<Self>() else {
            return false;
        };
        *self == **other
    }
}

impl Hash for dyn Bounds {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.dyn_hash(state);
    }
}

/// A packed element of a static type.
#[derive(Clone, PartialEq, Hash)]
#[repr(transparent)]
pub struct Packed<T: NativeElement>(
    /// Invariant: Must be of type `T`.
    Content,
    PhantomData<T>,
);

impl<T: NativeElement> Packed<T> {
    /// Pack element while retaining its static type.
    pub fn new(element: T) -> Self {
        // Safety: The element is known to be of type `T`.
        Packed(element.pack(), PhantomData)
    }

    /// Try to cast type-erased content into a statically known packed element.
    pub fn from_ref(content: &Content) -> Option<&Self> {
        if content.is::<T>() {
            // Safety:
            // - We have checked the type.
            // - Packed<T> is repr(transparent).
            return Some(unsafe { std::mem::transmute::<&Content, &Packed<T>>(content) });
        }
        None
    }

    /// Try to cast type-erased content into a statically known packed element.
    pub fn from_mut(content: &mut Content) -> Option<&mut Self> {
        if content.is::<T>() {
            // Safety:
            // - We have checked the type.
            // - Packed<T> is repr(transparent).
            return Some(unsafe {
                std::mem::transmute::<&mut Content, &mut Packed<T>>(content)
            });
        }
        None
    }

    /// Try to cast type-erased content into a statically known packed element.
    pub fn from_owned(content: Content) -> Result<Self, Content> {
        if content.is::<T>() {
            // Safety:
            // - We have checked the type.
            // - Packed<T> is repr(transparent).
            return Ok(unsafe { std::mem::transmute::<Content, Packed<T>>(content) });
        }
        Err(content)
    }

    /// Pack back into content.
    pub fn pack(self) -> Content {
        self.0
    }

    /// Extract the raw underlying element.
    pub fn unpack(self) -> T {
        // This function doesn't yet need owned self, but might in the future.
        (*self).clone()
    }

    /// The element's span.
    pub fn span(&self) -> Span {
        self.0.span()
    }

    /// Set the span of the element.
    pub fn spanned(self, span: Span) -> Self {
        Self(self.0.spanned(span), PhantomData)
    }

    /// Accesses the label of the element.
    pub fn label(&self) -> Option<Label> {
        self.0.label()
    }

    /// Accesses the location of the element.
    pub fn location(&self) -> Option<Location> {
        self.0.location()
    }

    /// Sets the location of the element.
    pub fn set_location(&mut self, location: Location) {
        self.0.set_location(location);
    }
}

impl<T: NativeElement> AsRef<T> for Packed<T> {
    fn as_ref(&self) -> &T {
        self
    }
}

impl<T: NativeElement> AsMut<T> for Packed<T> {
    fn as_mut(&mut self) -> &mut T {
        self
    }
}

impl<T: NativeElement> Deref for Packed<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        // Safety:
        // - Packed<T> guarantees that the content trait object wraps
        //   an element of type `T`.
        // - This downcast works the same way as dyn Any's does. We can't reuse
        //   that one because we don't want to pay the cost for every deref.
        let elem = &*self.0.inner.elem;
        unsafe { &*(elem as *const dyn Bounds as *const T) }
    }
}

impl<T: NativeElement> DerefMut for Packed<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        // Safety:
        // - Packed<T> guarantees that the content trait object wraps
        //   an element of type `T`.
        // - We have guaranteed unique access thanks to `make_mut`.
        // - This downcast works the same way as dyn Any's does. We can't reuse
        //   that one because we don't want to pay the cost for every deref.
        let elem = &mut *self.0.make_mut().elem;
        unsafe { &mut *(elem as *mut dyn Bounds as *mut T) }
    }
}

impl<T: NativeElement + Debug> Debug for Packed<T> {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}

/// A sequence of content.
#[elem(Debug, Repr, PartialEq)]
pub struct SequenceElem {
    /// The elements.
    #[required]
    pub children: Vec<Content>,
}

impl Debug for SequenceElem {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        write!(f, "Sequence ")?;
        f.debug_list().entries(&self.children).finish()
    }
}

// Derive is currently incompatible with `elem` macro.
#[allow(clippy::derivable_impls)]
impl Default for SequenceElem {
    fn default() -> Self {
        Self { children: Default::default() }
    }
}

impl PartialEq for SequenceElem {
    fn eq(&self, other: &Self) -> bool {
        self.children.iter().eq(other.children.iter())
    }
}

impl Repr for SequenceElem {
    fn repr(&self) -> EcoString {
        if self.children.is_empty() {
            "[]".into()
        } else {
            let elements = crate::foundations::repr::pretty_array_like(
                &self.children.iter().map(|c| c.inner.elem.repr()).collect::<Vec<_>>(),
                false,
            );
            eco_format!("sequence{}", elements)
        }
    }
}

/// Content alongside styles.
#[elem(Debug, Repr, PartialEq)]
pub struct StyledElem {
    /// The content.
    #[required]
    pub child: Content,
    /// The styles.
    #[required]
    pub styles: Styles,
}

impl Debug for StyledElem {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        for style in self.styles.iter() {
            writeln!(f, "#{style:?}")?;
        }
        self.child.fmt(f)
    }
}

impl PartialEq for StyledElem {
    fn eq(&self, other: &Self) -> bool {
        self.child == other.child
    }
}

impl Repr for StyledElem {
    fn repr(&self) -> EcoString {
        eco_format!("styled(child: {}, ..)", self.child.repr())
    }
}

/// Tries to extract the plain-text representation of the element.
pub trait PlainText {
    /// Write this element's plain text into the given buffer.
    fn plain_text(&self, text: &mut EcoString);
}

/// An error arising when trying to access a field of content.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub enum FieldAccessError {
    Unknown,
    Unset,
    Internal,
}

impl FieldAccessError {
    /// Formats the error message given the content and the field name.
    #[cold]
    pub fn message(self, content: &Content, field: &str) -> EcoString {
        let elem_name = content.elem().name();
        match self {
            FieldAccessError::Unknown => {
                eco_format!("{elem_name} does not have field {}", field.repr())
            }
            FieldAccessError::Unset => {
                eco_format!(
                    "field {} in {elem_name} is not known at this point",
                    field.repr()
                )
            }
            FieldAccessError::Internal => {
                eco_format!(
                    "internal error when accessing field {} in {elem_name} – this is a bug",
                    field.repr()
                )
            }
        }
    }

    /// Formats the error message for an `at` calls without a default value.
    #[cold]
    pub fn message_no_default(self, content: &Content, field: &str) -> EcoString {
        let mut msg = self.message(content, field);
        msg.push_str(" and no default was specified");
        msg
    }
}
