web-api-cat 0.7.4

//! `Element`-side native methods: `getAttribute`, `setAttribute`,
//! `hasAttribute`, `querySelector`, plus the v0.6.1 / v0.6.2 / v0.6.3 /
//! v0.6.7 / v0.6.8 structural, class-list, and parent-tracking
//! mutators (`appendChild`, `removeChild`, `insertBefore`,
//! `replaceChild`, `classList.add`, `classList.remove`,
//! `classList.contains`, `classList.toggle`, `remove`).
//!
//! v0.6.8 introduces a hidden `__parent__` slot on every element
//! (`Value::Object(parent_id)` when attached, `Value::Null` when
//! detached) and routes every structural mutator through
//! `set_parent_backref` / `clear_parent_backref` so the slot
//! stays in sync with the actual children-array membership.
//!
//! Element objects are boa-cat [`Object`]s carrying these properties:
//!
//! - `tagName`: lowercased tag name (string).
//! - `id`: id attribute or empty string.
//! - `className`: class attribute or empty string.
//! - `textContent`: concatenated text of all descendant text nodes.
//! - `children`: array of child element objects.
//! - `__attributes`: object mapping attribute name -> value.
//! - `getAttribute`, `setAttribute`, `hasAttribute`,
//!   `querySelector`: native callables.

use std::collections::BTreeMap;

use boa_cat::Value;
use boa_cat::fuel::Fuel;
use boa_cat::heap::Heap;
use boa_cat::outcome::{EvalResult, Outcome};
use boa_cat::value::{Object, ObjectId};

/// `Element.getAttribute(name)` native implementation.
///
/// # Errors
///
/// Never returns `Err`; bad input yields `Value::Null`.
#[allow(clippy::needless_pass_by_value)]
#[allow(clippy::unnecessary_wraps)]
pub fn get_attribute_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let name = string_arg(&args, 0);
    let outcome = read_attribute(&this, &name, &heap).map_or(Outcome::Normal(Value::Null), |v| {
        Outcome::Normal(Value::String(v))
    });
    Ok((outcome, heap, fuel))
}

/// `Element.hasAttribute(name)` native implementation.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value)]
#[allow(clippy::unnecessary_wraps)]
pub fn has_attribute_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let name = string_arg(&args, 0);
    let present = read_attribute(&this, &name, &heap).is_some();
    Ok((Outcome::Normal(Value::Boolean(present)), heap, fuel))
}

/// `Element.setAttribute(name, value)` native implementation.
///
/// # Errors
///
/// Never returns `Err`; missing element / attributes object yields the
/// unchanged heap.
#[allow(clippy::needless_pass_by_value)]
#[allow(clippy::unnecessary_wraps)]
pub fn set_attribute_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let name = string_arg(&args, 0);
    let value = string_arg(&args, 1);
    let new_heap = write_attribute(&this, &name, &value, heap);
    Ok((Outcome::Normal(Value::Undefined), new_heap, fuel))
}

fn object_id_of(value: &Value) -> Option<ObjectId> {
    match value {
        Value::Object(id) => Some(*id),
        Value::Undefined
        | Value::Null
        | Value::Boolean(_)
        | Value::Number(_)
        | Value::String(_)
        | Value::Function(_)
        | Value::Native(_)
        | Value::Promise(_) => None,
    }
}

fn attributes_id_of(element: &Object) -> Option<ObjectId> {
    match element.get("__attributes") {
        Some(value) => object_id_of(value),
        None => None,
    }
}

fn read_attribute(this: &Value, name: &str, heap: &Heap) -> Option<String> {
    let element_id = object_id_of(this)?;
    let element = heap.object(element_id)?;
    let attrs_id = attributes_id_of(element)?;
    let attrs = heap.object(attrs_id)?;
    match attrs.get(name) {
        Some(Value::String(s)) => Some(s.clone()),
        Some(_) | None => None,
    }
}

fn write_attribute(this: &Value, name: &str, value: &str, heap: Heap) -> Heap {
    let Some(element_id) = object_id_of(this) else {
        return heap;
    };
    let Some(element) = heap.object(element_id).cloned() else {
        return heap;
    };
    let Some(attrs_id) = attributes_id_of(&element) else {
        return heap;
    };
    let Some(attrs) = heap.object(attrs_id).cloned() else {
        return heap;
    };
    let updated_attrs = attrs.with(name.to_owned(), Value::String(value.to_owned()));
    let heap = heap
        .store_object(attrs_id, updated_attrs)
        .unwrap_or_else(|h| h);
    let mirror_key = match name {
        "id" => Some("id"),
        "class" => Some("className"),
        _other => None,
    };
    if let Some(key) = mirror_key {
        let updated_element = element.with(key.to_owned(), Value::String(value.to_owned()));
        heap.store_object(element_id, updated_element)
            .unwrap_or_else(|h| h)
    } else {
        heap
    }
}

fn string_arg(args: &[Value], idx: usize) -> String {
    match args.get(idx) {
        Some(Value::String(s)) => s.clone(),
        Some(other) => format!("{other}"),
        None => String::new(),
    }
}

/// Build an attributes object (`__attributes`) from a list of pairs.
#[must_use]
pub fn build_attributes_object(pairs: &[(String, String)], heap: Heap) -> (Value, Heap) {
    let map: BTreeMap<String, Value> = pairs
        .iter()
        .map(|(k, v)| (k.clone(), Value::String(v.clone())))
        .collect();
    let (id, heap) = heap.alloc_object(Object::from_properties(map));
    (Value::Object(id), heap)
}

/// `Element.querySelector(selector)` -- limited v0 selector subset
/// (`tag`, `.class`, `#id`, `tag.class`, `tag#id`).
///
/// # Errors
///
/// Never returns `Err`; no match yields `Value::Null`.
#[allow(clippy::needless_pass_by_value)]
#[allow(clippy::unnecessary_wraps)]
pub fn query_selector_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let selector = string_arg(&args, 0);
    let pattern = parse_simple_selector(&selector);
    let outcome =
        find_matching(&this, &pattern, &heap).map_or(Outcome::Normal(Value::Null), Outcome::Normal);
    Ok((outcome, heap, fuel))
}

#[derive(Debug, Clone, Default)]
struct SelectorPattern {
    tag: Option<String>,
    id: Option<String>,
    class: Option<String>,
}

fn parse_simple_selector(source: &str) -> SelectorPattern {
    let trimmed = source.trim();
    parse_selector_recursive(trimmed, 0, SelectorPattern::default())
}

fn parse_selector_recursive(source: &str, start: usize, acc: SelectorPattern) -> SelectorPattern {
    let bytes = source.as_bytes();
    if start >= bytes.len() {
        acc
    } else {
        match bytes.get(start) {
            Some(b'#') => {
                let end = scan_ident(bytes, start + 1);
                let name = source.get(start + 1..end).unwrap_or("").to_owned();
                parse_selector_recursive(
                    source,
                    end,
                    SelectorPattern {
                        id: Some(name),
                        ..acc
                    },
                )
            }
            Some(b'.') => {
                let end = scan_ident(bytes, start + 1);
                let name = source.get(start + 1..end).unwrap_or("").to_owned();
                parse_selector_recursive(
                    source,
                    end,
                    SelectorPattern {
                        class: Some(name),
                        ..acc
                    },
                )
            }
            Some(_) => {
                let end = scan_ident(bytes, start);
                let name = source.get(start..end).unwrap_or("").to_ascii_lowercase();
                if name.is_empty() {
                    acc
                } else {
                    parse_selector_recursive(
                        source,
                        end,
                        SelectorPattern {
                            tag: Some(name),
                            ..acc
                        },
                    )
                }
            }
            None => acc,
        }
    }
}

fn scan_ident(bytes: &[u8], start: usize) -> usize {
    bytes
        .iter()
        .enumerate()
        .skip(start)
        .find(|(_, b)| !is_ident_byte(**b))
        .map_or(bytes.len(), |(i, _)| i)
}

fn is_ident_byte(b: u8) -> bool {
    b.is_ascii_alphanumeric() || b == b'-' || b == b'_'
}

fn find_matching(this: &Value, pattern: &SelectorPattern, heap: &Heap) -> Option<Value> {
    let root_id = object_id_of(this)?;
    walk_descendants(root_id, pattern, heap)
}

fn walk_descendants(node_id: ObjectId, pattern: &SelectorPattern, heap: &Heap) -> Option<Value> {
    let children = element_children(node_id, heap);
    children.iter().find_map(|child_id| {
        if matches_pattern(*child_id, pattern, heap) {
            Some(Value::Object(*child_id))
        } else {
            walk_descendants(*child_id, pattern, heap)
        }
    })
}

fn element_children(node_id: ObjectId, heap: &Heap) -> Vec<ObjectId> {
    let Some(object) = heap.object(node_id) else {
        return Vec::new();
    };
    let Some(children_id) = object.get("children").and_then(object_id_of) else {
        return Vec::new();
    };
    let Some(children) = heap.object(children_id) else {
        return Vec::new();
    };
    let length = array_length(children);
    (0..length)
        .filter_map(|i| children.get(&format!("{i}")).and_then(object_id_of))
        .collect()
}

fn array_length(array: &Object) -> u32 {
    match array.get("length") {
        Some(Value::Number(n)) if n.is_finite() && *n >= 0.0 => {
            #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
            let len = *n as u32;
            len
        }
        Some(_) | None => 0,
    }
}

fn matches_pattern(node_id: ObjectId, pattern: &SelectorPattern, heap: &Heap) -> bool {
    let Some(object) = heap.object(node_id) else {
        return false;
    };
    let tag_ok = pattern
        .tag
        .as_ref()
        .is_none_or(|want| string_property(object, "tagName").eq_ignore_ascii_case(want));
    let id_ok = pattern
        .id
        .as_ref()
        .is_none_or(|want| string_property(object, "id") == *want);
    let class_ok = pattern.class.as_ref().is_none_or(|want| {
        string_property(object, "className")
            .split_ascii_whitespace()
            .any(|c| c == want)
    });
    tag_ok && id_ok && class_ok
}

fn string_property(object: &Object, key: &str) -> String {
    match object.get(key) {
        Some(Value::String(s)) => s.clone(),
        Some(_) | None => String::new(),
    }
}

/// Public helper used by `document.querySelector` to perform the search
/// from the document root.
#[must_use]
pub fn find_first_descendant(root: ObjectId, pattern_source: &str, heap: &Heap) -> Option<Value> {
    let pattern = parse_simple_selector(pattern_source);
    walk_descendants(root, &pattern, heap)
}

/// Public helper for `document.getElementById`.
#[must_use]
pub fn find_by_id(root: ObjectId, id: &str, heap: &Heap) -> Option<Value> {
    let pattern = SelectorPattern {
        id: Some(id.to_owned()),
        ..SelectorPattern::default()
    };
    walk_descendants(root, &pattern, heap)
}

/// `Element.appendChild(child)` (v0.6.1): append `child` to
/// `this.children`, updating `length`.  Returns `child` per spec
/// so the caller can chain.  No-ops when `this` or `child` aren't
/// Object values, or when the children-array slot is missing /
/// non-object.
///
/// Mutation flow: clone the children array Object, write the new
/// numeric-key entry, write `length`, then `store_object` both the
/// children Object and the parent (the parent's `children` slot
/// already points at the same children-array `ObjectId`, so the
/// parent update is implicit -- only the children Object's
/// in-place mutation matters).  `extract_document` walks
/// `children` numerically in the next back-prop pass, so the new
/// child reaches layout / paint automatically.
///
/// # Errors
///
/// Never returns `Err`; bad inputs yield `Value::Undefined`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn append_child_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let child = args.first().cloned().unwrap_or(Value::Undefined);
    let outcome_heap = append_child_to_parent(&this, &child, heap);
    Ok((Outcome::Normal(child), outcome_heap, fuel))
}

fn append_child_to_parent(this: &Value, child: &Value, heap: Heap) -> Heap {
    let Some(parent_id) = object_id_of(this) else {
        return heap;
    };
    if let Some((children_id, children_obj)) = children_object_of(this, &heap) {
        let length = array_length(&children_obj);
        let updated_children = children_obj
            .with(format!("{length}"), child.clone())
            .with("length".to_owned(), Value::Number(f64::from(length + 1)));
        let heap = heap
            .store_object(children_id, updated_children)
            .unwrap_or_else(|h| h);
        set_parent_backref(child, parent_id, heap)
    } else {
        heap
    }
}

/// v0.6.8 helper: set `child_value.__parent__` to
/// `Value::Object(parent_id)`.  No-op when `child_value` isn't an
/// Object or its heap slot is missing.  Called by every structural
/// mutator and by `document::build_element` / `document::clone_element`
/// to wire up parent backrefs.
#[must_use]
pub fn set_parent_backref(child_value: &Value, parent_id: ObjectId, heap: Heap) -> Heap {
    let Some(child_id) = object_id_of(child_value) else {
        return heap;
    };
    let Some(child_obj) = heap.object(child_id).cloned() else {
        return heap;
    };
    let updated = child_obj.with("__parent__".to_owned(), Value::Object(parent_id));
    heap.store_object(child_id, updated).unwrap_or_else(|h| h)
}

/// v0.6.8 helper: clear `child_value.__parent__` to `Value::Null`.
/// No-op when `child_value` isn't an Object or its heap slot is
/// missing.  Called when detaching a child from its parent
/// (`removeChild`, `replaceChild`'s old-child slot, `remove`,
/// `innerHTML` setter's discarded old children).
#[must_use]
pub fn clear_parent_backref(child_value: &Value, heap: Heap) -> Heap {
    let Some(child_id) = object_id_of(child_value) else {
        return heap;
    };
    let Some(child_obj) = heap.object(child_id).cloned() else {
        return heap;
    };
    let updated = child_obj.with("__parent__".to_owned(), Value::Null);
    heap.store_object(child_id, updated).unwrap_or_else(|h| h)
}

fn children_object_of(this: &Value, heap: &Heap) -> Option<(ObjectId, Object)> {
    let parent_id = object_id_of(this)?;
    let parent = heap.object(parent_id)?;
    let children_id = parent.get("children").and_then(|v| match v {
        Value::Object(id) => Some(*id),
        Value::Undefined
        | Value::Null
        | Value::Boolean(_)
        | Value::Number(_)
        | Value::String(_)
        | Value::Function(_)
        | Value::Native(_)
        | Value::Promise(_) => None,
    })?;
    let children_obj = heap.object(children_id)?.clone();
    Some((children_id, children_obj))
}

/// `Element.removeChild(child)` (v0.6.2): remove `child` from
/// `this.children` and return it.  Remaining children shift down
/// to fill the freed slot; `length` decrements.  No-ops (and still
/// returns the requested `child`) when `this` or `child` aren't
/// Object values, the children-array slot is missing / non-object,
/// or `child` isn't currently a child of `this` (the DOM spec
/// throws `NotFoundError` here; this scoped impl no-ops to stay
/// inside the always-`Ok` `NativeFn` contract).
///
/// # Errors
///
/// Never returns `Err`; bad inputs no-op.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn remove_child_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let child = args.first().cloned().unwrap_or(Value::Undefined);
    let outcome_heap = remove_child_from_parent(&this, &child, heap);
    Ok((Outcome::Normal(child), outcome_heap, fuel))
}

fn remove_child_from_parent(this: &Value, child: &Value, heap: Heap) -> Heap {
    let Some((children_id, children_obj)) = children_object_of(this, &heap) else {
        return heap;
    };
    let Some(child_id) = object_id_of(child) else {
        return heap;
    };
    let length = array_length(&children_obj);
    let Some(removal_index) = (0..length)
        .find(|i| children_obj.get(&format!("{i}")).and_then(object_id_of) == Some(child_id))
    else {
        return heap;
    };
    let new_length = length.saturating_sub(1);
    let pairs: BTreeMap<String, Value> = (0..length)
        .filter(|i| *i != removal_index)
        .enumerate()
        .map(|(new_idx, old_idx)| {
            let value = children_obj
                .get(&format!("{old_idx}"))
                .cloned()
                .unwrap_or(Value::Null);
            (format!("{new_idx}"), value)
        })
        .chain(std::iter::once((
            "length".to_owned(),
            Value::Number(f64::from(new_length)),
        )))
        .collect();
    let new_children = Object::from_properties(pairs);
    let heap = heap
        .store_object(children_id, new_children)
        .unwrap_or_else(|h| h);
    clear_parent_backref(child, heap)
}

/// `Element.insertBefore(newNode, referenceNode)` (v0.6.2): insert
/// `newNode` into `this.children` immediately before
/// `referenceNode`, returning `newNode`.  Existing children at and
/// after `referenceNode`'s slot shift up; `length` increments.  If
/// `referenceNode` is `null` / `undefined` / any non-Object value
/// the call falls through to `appendChild`-like end-of-list
/// behaviour (matching the DOM spec's `null` case).  No-ops if
/// `this` isn't an Object, the children-array slot is missing /
/// non-object, or `referenceNode` is a real Object but isn't
/// actually a child of `this` (the DOM spec throws
/// `NotFoundError`; this scoped impl no-ops).
///
/// # Errors
///
/// Never returns `Err`; bad inputs yield `Value::Undefined`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn insert_before_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let new_node = args.first().cloned().unwrap_or(Value::Undefined);
    let ref_node = args.get(1).cloned().unwrap_or(Value::Null);
    let outcome_heap = insert_before_into_parent(&this, &new_node, &ref_node, heap);
    Ok((Outcome::Normal(new_node), outcome_heap, fuel))
}

fn insert_before_into_parent(this: &Value, new_node: &Value, ref_node: &Value, heap: Heap) -> Heap {
    let ref_id_opt = object_id_of(ref_node);
    if ref_id_opt.is_none() {
        append_child_to_parent(this, new_node, heap)
    } else {
        let Some(parent_id) = object_id_of(this) else {
            return heap;
        };
        let Some((children_id, children_obj)) = children_object_of(this, &heap) else {
            return heap;
        };
        let Some(ref_id) = ref_id_opt else {
            return heap;
        };
        let length = array_length(&children_obj);
        let Some(insert_index) = (0..length)
            .find(|i| children_obj.get(&format!("{i}")).and_then(object_id_of) == Some(ref_id))
        else {
            return heap;
        };
        let new_length = length + 1;
        let pairs: BTreeMap<String, Value> = (0..new_length)
            .map(|new_idx| {
                let value = match new_idx.cmp(&insert_index) {
                    std::cmp::Ordering::Less => children_obj
                        .get(&format!("{new_idx}"))
                        .cloned()
                        .unwrap_or(Value::Null),
                    std::cmp::Ordering::Equal => new_node.clone(),
                    std::cmp::Ordering::Greater => {
                        let old_idx = new_idx - 1;
                        children_obj
                            .get(&format!("{old_idx}"))
                            .cloned()
                            .unwrap_or(Value::Null)
                    }
                };
                (format!("{new_idx}"), value)
            })
            .chain(std::iter::once((
                "length".to_owned(),
                Value::Number(f64::from(new_length)),
            )))
            .collect();
        let new_children = Object::from_properties(pairs);
        let heap = heap
            .store_object(children_id, new_children)
            .unwrap_or_else(|h| h);
        set_parent_backref(new_node, parent_id, heap)
    }
}

/// `Element.replaceChild(newChild, oldChild)` (v0.6.7): swap
/// `oldChild` for `newChild` in `this.children`.  Length is
/// preserved (one slot in, one slot out); the surrounding
/// children stay at their indexes.  Returns `oldChild` per spec.
/// No-ops (and still returns the requested `oldChild`) when
/// `this` isn't an Object, the children-array slot is missing /
/// non-object, `oldChild` isn't an Object, or `oldChild` isn't
/// currently a child of `this` (the DOM spec throws
/// `NotFoundError`; this scoped impl no-ops).
///
/// # Errors
///
/// Never returns `Err`; bad inputs no-op.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn replace_child_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let new_child = args.first().cloned().unwrap_or(Value::Undefined);
    let old_child = args.get(1).cloned().unwrap_or(Value::Undefined);
    let outcome_heap = replace_child_in_parent(&this, &new_child, &old_child, heap);
    Ok((Outcome::Normal(old_child), outcome_heap, fuel))
}

fn replace_child_in_parent(this: &Value, new_child: &Value, old_child: &Value, heap: Heap) -> Heap {
    let Some(parent_id) = object_id_of(this) else {
        return heap;
    };
    let Some((children_id, children_obj)) = children_object_of(this, &heap) else {
        return heap;
    };
    let Some(old_id) = object_id_of(old_child) else {
        return heap;
    };
    let length = array_length(&children_obj);
    let Some(replace_index) = (0..length)
        .find(|i| children_obj.get(&format!("{i}")).and_then(object_id_of) == Some(old_id))
    else {
        return heap;
    };
    let updated_children = children_obj.with(format!("{replace_index}"), new_child.clone());
    let heap = heap
        .store_object(children_id, updated_children)
        .unwrap_or_else(|h| h);
    let heap = set_parent_backref(new_child, parent_id, heap);
    clear_parent_backref(old_child, heap)
}

/// `Element.remove()` (v0.6.8): detach `this` from its parent.
/// Reads `this.__parent__`; if it's an Object, calls the same
/// `remove_child_from_parent` helper that backs `removeChild`,
/// which both excises `this` from the parent's children-array
/// Object and clears `this.__parent__`.  No-op when `this` has no
/// parent (already detached, or it's the document root).
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn remove_impl(_args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let parent_value = parent_value_of(&this, &heap);
    let outcome_heap = match parent_value {
        Value::Object(_) => remove_child_from_parent(&parent_value, &this, heap),
        Value::Undefined
        | Value::Null
        | Value::Boolean(_)
        | Value::Number(_)
        | Value::String(_)
        | Value::Function(_)
        | Value::Native(_)
        | Value::Promise(_) => heap,
    };
    Ok((Outcome::Normal(Value::Undefined), outcome_heap, fuel))
}

fn parent_value_of(this: &Value, heap: &Heap) -> Value {
    object_id_of(this)
        .and_then(|id| heap.object(id))
        .and_then(|obj| obj.get("__parent__").cloned())
        .unwrap_or(Value::Null)
}

/// v0.6.8 getter for `parentElement` / `parentNode` accessors:
/// returns `this.__parent__` (an `Object` value when attached,
/// `Null` when detached or root).  Same implementation for both
/// accessors -- we don't yet distinguish elements from other node
/// types, so `parentElement` and `parentNode` produce identical
/// results.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn parent_getter_impl(_args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let parent = parent_value_of(&this, &heap);
    Ok((Outcome::Normal(parent), heap, fuel))
}

/// v0.6.9 getter for `firstElementChild`: `this.children[0]` or
/// `Value::Null` if `this` has no children.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn first_element_child_getter_impl(
    _args: Vec<Value>,
    this: Value,
    heap: Heap,
    fuel: Fuel,
) -> EvalResult {
    let value = own_children_view(&this, &heap)
        .filter(|(_, length)| *length > 0)
        .and_then(|(children, _)| children.get("0").cloned())
        .unwrap_or(Value::Null);
    Ok((Outcome::Normal(value), heap, fuel))
}

/// v0.6.9 getter for `lastElementChild`: `this.children[length-1]`
/// or `Value::Null` if `this` has no children.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn last_element_child_getter_impl(
    _args: Vec<Value>,
    this: Value,
    heap: Heap,
    fuel: Fuel,
) -> EvalResult {
    let value = own_children_view(&this, &heap)
        .filter(|(_, length)| *length > 0)
        .and_then(|(children, length)| children.get(&format!("{}", length - 1)).cloned())
        .unwrap_or(Value::Null);
    Ok((Outcome::Normal(value), heap, fuel))
}

/// v0.6.9 getter for `previousElementSibling`: looks `this` up by
/// id in its parent's children-array Object, returns the entry at
/// `this_index - 1` if it exists, else `Value::Null`.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn previous_element_sibling_getter_impl(
    _args: Vec<Value>,
    this: Value,
    heap: Heap,
    fuel: Fuel,
) -> EvalResult {
    let value = parent_children_view(&this, &heap)
        .filter(|(_, _, this_index)| *this_index > 0)
        .and_then(|(children, _, this_index)| children.get(&format!("{}", this_index - 1)).cloned())
        .unwrap_or(Value::Null);
    Ok((Outcome::Normal(value), heap, fuel))
}

/// v0.6.9 getter for `nextElementSibling`: looks `this` up by id
/// in its parent's children-array Object, returns the entry at
/// `this_index + 1` if it exists, else `Value::Null`.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn next_element_sibling_getter_impl(
    _args: Vec<Value>,
    this: Value,
    heap: Heap,
    fuel: Fuel,
) -> EvalResult {
    let value = parent_children_view(&this, &heap)
        .filter(|(_, length, this_index)| this_index + 1 < *length)
        .and_then(|(children, _, this_index)| children.get(&format!("{}", this_index + 1)).cloned())
        .unwrap_or(Value::Null);
    Ok((Outcome::Normal(value), heap, fuel))
}

fn own_children_view<'h>(this: &Value, heap: &'h Heap) -> Option<(&'h Object, u32)> {
    let this_id = object_id_of(this)?;
    let this_obj = heap.object(this_id)?;
    let children_id = this_obj.get("children").and_then(object_id_of)?;
    let children = heap.object(children_id)?;
    let length = array_length(children);
    Some((children, length))
}

fn parent_children_view<'h>(this: &Value, heap: &'h Heap) -> Option<(&'h Object, u32, u32)> {
    let this_id = object_id_of(this)?;
    let this_obj = heap.object(this_id)?;
    let parent_id = this_obj.get("__parent__").and_then(object_id_of)?;
    let parent = heap.object(parent_id)?;
    let children_id = parent.get("children").and_then(object_id_of)?;
    let children = heap.object(children_id)?;
    let length = array_length(children);
    let this_index = (0..length)
        .find(|i| children.get(&format!("{i}")).and_then(object_id_of) == Some(this_id))?;
    Some((children, length, this_index))
}

/// `Element.classList.add(token)` (v0.6.3): if `token` isn't
/// already among the parent element's class tokens, append it and
/// write the rejoined token list back through `setAttribute`-style
/// mirroring (so both `className` and `__attributes.class` stay in
/// sync; `getAttribute('class')` keeps working).  No-ops if `this`
/// isn't a `classList` Object or its `__element__` backref is
/// missing / non-Object.
///
/// # Errors
///
/// Never returns `Err`; bad inputs no-op.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn class_list_add_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let token = string_arg(&args, 0);
    let new_heap = add_class_token(&this, &token, heap);
    Ok((Outcome::Normal(Value::Undefined), new_heap, fuel))
}

/// `Element.classList.remove(token)` (v0.6.3): drop every
/// occurrence of `token` from the parent element's class list and
/// write the rejoined token list back through `setAttribute`-style
/// mirroring.  No-ops if `this` isn't a `classList` Object.
///
/// # Errors
///
/// Never returns `Err`; bad inputs no-op.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn class_list_remove_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let token = string_arg(&args, 0);
    let new_heap = remove_class_token(&this, &token, heap);
    Ok((Outcome::Normal(Value::Undefined), new_heap, fuel))
}

/// `Element.classList.contains(token)` (v0.6.3): return `true` iff
/// `token` is among the parent element's whitespace-separated class
/// tokens.  Returns `false` if `this` isn't a `classList` Object.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn class_list_contains_impl(
    args: Vec<Value>,
    this: Value,
    heap: Heap,
    fuel: Fuel,
) -> EvalResult {
    let token = string_arg(&args, 0);
    let present = element_of_class_list(&this, &heap)
        .is_some_and(|(_, element)| class_list_tokens(&element).iter().any(|t| t == &token));
    Ok((Outcome::Normal(Value::Boolean(present)), heap, fuel))
}

/// `Element.classList.toggle(token)` (v0.6.3): remove `token` if it
/// was already present, otherwise add it; return the new
/// presence-state (matches MDN: `true` after add, `false` after
/// remove).  No-ops (returning `false`) if `this` isn't a
/// `classList` Object.
///
/// # Errors
///
/// Never returns `Err`.
#[allow(clippy::needless_pass_by_value, clippy::unnecessary_wraps)]
pub fn class_list_toggle_impl(args: Vec<Value>, this: Value, heap: Heap, fuel: Fuel) -> EvalResult {
    let token = string_arg(&args, 0);
    let (present, new_heap) = toggle_class_token(&this, &token, heap);
    Ok((Outcome::Normal(Value::Boolean(present)), new_heap, fuel))
}

fn element_of_class_list(this: &Value, heap: &Heap) -> Option<(ObjectId, Object)> {
    let list_id = object_id_of(this)?;
    let list = heap.object(list_id)?;
    let element_id = list.get("__element__").and_then(|v| match v {
        Value::Object(id) => Some(*id),
        Value::Undefined
        | Value::Null
        | Value::Boolean(_)
        | Value::Number(_)
        | Value::String(_)
        | Value::Function(_)
        | Value::Native(_)
        | Value::Promise(_) => None,
    })?;
    let element = heap.object(element_id)?.clone();
    Some((element_id, element))
}

fn class_list_tokens(element: &Object) -> Vec<String> {
    match element.get("className") {
        Some(Value::String(s)) => s.split_ascii_whitespace().map(str::to_owned).collect(),
        Some(_) | None => Vec::new(),
    }
}

fn add_class_token(this: &Value, token: &str, heap: Heap) -> Heap {
    let Some((element_id, element)) = element_of_class_list(this, &heap) else {
        return heap;
    };
    let tokens = class_list_tokens(&element);
    let new_tokens: Vec<String> = if tokens.iter().any(|t| t == token) {
        tokens
    } else {
        tokens
            .into_iter()
            .chain(std::iter::once(token.to_owned()))
            .collect()
    };
    let new_class = new_tokens.join(" ");
    let element_value = Value::Object(element_id);
    write_attribute(&element_value, "class", &new_class, heap)
}

fn remove_class_token(this: &Value, token: &str, heap: Heap) -> Heap {
    let Some((element_id, element)) = element_of_class_list(this, &heap) else {
        return heap;
    };
    let tokens = class_list_tokens(&element);
    let new_tokens: Vec<String> = tokens.into_iter().filter(|t| t != token).collect();
    let new_class = new_tokens.join(" ");
    let element_value = Value::Object(element_id);
    write_attribute(&element_value, "class", &new_class, heap)
}

fn toggle_class_token(this: &Value, token: &str, heap: Heap) -> (bool, Heap) {
    let Some((element_id, element)) = element_of_class_list(this, &heap) else {
        return (false, heap);
    };
    let tokens = class_list_tokens(&element);
    let was_present = tokens.iter().any(|t| t == token);
    let new_tokens: Vec<String> = if was_present {
        tokens.into_iter().filter(|t| t != token).collect()
    } else {
        tokens
            .into_iter()
            .chain(std::iter::once(token.to_owned()))
            .collect()
    };
    let new_class = new_tokens.join(" ");
    let element_value = Value::Object(element_id);
    let new_heap = write_attribute(&element_value, "class", &new_class, heap);
    (!was_present, new_heap)
}