///|
#cfg(target="js")
struct VDom {
  mut inode : INode
  target_element : @dom.Element?
  captured_link_listener : @dom.Listener
}

///|
#cfg(not(target="js"))
pub struct VDom {
  mut vnode : VNode
}

///|
#cfg(target="js")
priv struct IProps {
  props : Props
  slots : Map[String, Ref[(Event, &Scheduler) -> Unit]]
}

///|
#cfg(target="js")
priv enum INode {
  Elem(String, IProps, Children[INode], namespace_uri~ : String?, @dom.Element)
  Text(String, @dom.Element)
  Frag(Array[INode], @dom.Comment, @dom.Comment)
  Thunk(Int, INode)
}

///|
#cfg(target="js")
fn INode::start(self : Self) -> @dom.Node {
  match self {
    Elem(_, _, _, element, ..) | Text(_, element) => element.as_node()
    Thunk(_, node) => node.start()
    Frag(_, start, _) => start.as_node()
  }
}

///|
#cfg(target="js")
fn INode::end(self : Self) -> @dom.Node {
  match self {
    Elem(_, _, _, element, ..) | Text(_, element) => element.as_node()
    Thunk(_, node) => node.end()
    Frag(_, _, end) => end.as_node()
  }
}

///|
#cfg(target="js")
fn INode::needs_relocation(
  self : Self,
  anchor : @js.Nullable[@dom.Node],
) -> Bool {
  match (self.end().get_next_sibling().to_option(), anchor.to_option()) {
    (None, None) => false
    (Some(actual), Some(expected)) => !actual.is_same_node(expected)
    _ => true
  }
}

///|
#cfg(target="js")
fn INode::remove(self : Self, parent : @dom.Node) -> Unit {
  match self {
    Elem(_, _, _, element, ..) | Text(_, element) =>
      parent.remove_child(element.as_node())
    Thunk(_, node) => node.remove(parent)
    Frag(_, start, end) => {
      let start = start.as_node()
      let end = end.as_node()
      while start.get_next_sibling().to_option() is Some(node) &&
            !node.is_same_node(end) {
        parent.remove_child(node)
      }
      parent.remove_child(start)
      parent.remove_child(end)
    }
  }
}

///|
#cfg(target="js")
fn INode::relocate(
  self : Self,
  parent : @dom.Node,
  before : @js.Nullable[@dom.Node],
) -> Unit {
  guard self.needs_relocation(before) else { return }
  match self {
    Elem(_, _, _, element, ..) | Text(_, element) =>
      parent.insert_before(element.as_node(), before)
    Thunk(_, node) => node.relocate(parent, before)
    Frag(_, start, end) => {
      {
        guard! start.get_parent_node().to_option() is Some(a)
        guard! end.get_parent_node().to_option() is Some(b)
        guard! a.is_same_node(b)
      }
      let start = start.as_node()
      let end = end.as_node()
      let mut anchor = before
      while end.get_previous_sibling().to_option() is Some(node) &&
            !node.is_same_node(start) {
        parent.insert_before(node, anchor)
        anchor = nullable(node)
      }
      // For an empty fragment, anchor equals before: keep start before end.
      parent.insert_before(start, anchor)
      parent.insert_before(end, before)
    }
  }
}

///|
#cfg(target="js")
fn insert_props(
  element : @dom.Element,
  tag : String,
  props : Props,
  scheduler : &Scheduler,
  captured_link_listener : @dom.Listener,
) -> IProps {
  let { handlers, attrs, props: properties, styles, } = props
  let slots : Map[String, Ref[(Event, &Scheduler) -> Unit]] = Map([])
  for event, handler in handlers {
    let slot = @ref.new(handler)
    element.add_event_listener(event, event => (slot.val)(dom_event(event), scheduler))
    slots[event] = slot
  }
  if is_captured_link_tag(tag) {
    element.add_event_listener("click", captured_link_listener)
  }
  for name, value in attrs {
    element.set_attribute(name, value)
  }
  for name, value in properties {
    element.set_property(name, variant_to_js_value(value))
  }
  let sheet = if element.to_html_element() is Some(html) {
    html.get_style()
  } else if element.to_svg_element() is Some(svg) {
    svg.get_style()
  } else {
    panic()
  }
  for name, value in styles {
    // 类型化样式 → CSSOM。camelCase 键要转成 CSS 语法，长度要补 px。
    sheet.set_property(@style.css_property_name(name), value.to_css())
  }
  { props, slots, }
}

///|
#cfg(target="js")
fn reapply_child_sensitive_properties(
  element : @dom.Element,
  tag : String,
  props : Props,
) -> Unit {
  // Adding  children resets a 's value to its first selected
  // option. Reapply the controlled property after child reconciliation so the
  // public value contract wins on initial mount and when options change.
  if tag == "select" && props.props.get("value") is Some(value) {
    element.set_property("value", variant_to_js_value(value))
  }
}

///|
#cfg(target="js")
fn insert_children(
  node : @dom.Node,
  children : Children[VNode],
  scheduler : &Scheduler,
  captured_link_listener : @dom.Listener,
) -> Children[INode] {
  match children {
    RawHtml(html) => {
      node.to_element().unwrap().set_inner_html(html)
      RawHtml(html)
    }
    Array(children) =>
      Array(
        children.map(child => {
          child.insert(scheduler, captured_link_listener, node, null())
        }),
      )
    Map(children) => {
      let mounted : Map[String, INode] = Map([])
      for key, child in children {
        mounted[key] = child.insert(
          scheduler,
          captured_link_listener,
          node,
          null(),
        )
      }
      Map(mounted)
    }
  }
}

///|
#cfg(target="js")
fn VNode::insert(
  self : Self,
  scheduler : &Scheduler,
  captured_link_listener : @dom.Listener,
  parent : @dom.Node,
  before : @js.Nullable[@dom.Node],
) -> INode {
  match self {
    Text(text) => {
      let element = @dom.document().create_text_node(text)
      parent.insert_before(element.as_node(), before)
      Text(text, element)
    }
    Thunk(hash, render) => {
      let node = render().insert(
        scheduler, captured_link_listener, parent, before,
      )
      Thunk(hash, node)
    }
    Frag(children) => {
      let document = @dom.document()
      let fragment = document.create_document_fragment()
      let start = document.create_comment(FRAGMENT_START_MARKER)
      let end = document.create_comment(FRAGMENT_END_MARKER)
      fragment.append_child(start.as_node())
      let children = children.map(child => {
        child.insert(
          scheduler,
          captured_link_listener,
          fragment.as_node(),
          null(),
        )
      })
      fragment.append_child(end.as_node())
      parent.insert_before(fragment.as_node(), before)
      Frag(children, start, end)
    }
    Elem(tag, props, children, namespace_uri~) => {
      let rendered_tag = rendered_tag(tag)
      let document = @dom.document()
      let element = match namespace_uri {
        None => document.create_element(rendered_tag)
        Some(namespace_uri) =>
          document.create_element_ns(namespace_uri, rendered_tag)
      }
      let props = insert_props(
        element, tag, props, scheduler, captured_link_listener,
      )
      let node = element.as_node()
      let children : Children[INode] = if is_html_void_element(tag) {
        Array([])
      } else {
        insert_children(node, children, scheduler, captured_link_listener)
      }
      reapply_child_sensitive_properties(element, tag, props.props)
      parent.insert_before(node, before)
      Elem(tag, props, children, namespace_uri~, element)
    }
  }
}

///|
#cfg(target="js")
fn INode::to_vnode(self : Self) -> VNode {
  match self {
    Elem(tag, mounted_props, children, namespace_uri~, _) =>
      Elem(
        tag,
        mounted_props.props,
        children.map(child => child.to_vnode()),
        namespace_uri~,
      )
    Text(text, _) => Text(text)
    Frag(children, _, _) => Frag(children.map(child => child.to_vnode()))
    Thunk(_, node) => node.to_vnode()
  }
}

///|
#cfg(target="js")
fn VDom::to_vnode(self : Self) -> VNode {
  self.inode.to_vnode()
}

///|
#cfg(not(target="js"))
fn VDom::to_vnode(self : Self) -> VNode {
  self.vnode
}

///|
pub fn VDom::to_string(self : Self) -> String {
  server_side_render(() => self.to_vnode())
}

///|
#cfg(target="js")
fn[A] nullable(value : A) -> @js.Nullable[A] {
  @js.Nullable::from_option(Some(value))
}

///|
#cfg(target="js")
fn[A] null() -> @js.Nullable[A] {
  @js.Nullable::null()
}

///|
#cfg(target="js")
pub fn VDom::update(self : Self, root : VNode, scheduler : &Scheduler) -> Unit {
  self.inode = diff_document(
    self.inode,
    root,
    scheduler,
    container?=self.target_element,
    captured_link_listener=self.captured_link_listener,
  )
}

///|
#cfg(not(target="js"))
pub fn VDom::update(self : Self, vnode : VNode, _ : &Scheduler) -> Unit {
  self.vnode = vnode
}

///|
#cfg(target="js")
pub fn VDom::initialize_with_hydration(
  vnode : VNode,
  scheduler : &Scheduler,
) -> VDom {
  let captured_link_listener = new_captured_link_listener(scheduler)
  let inode = hydrate_root(vnode, scheduler, captured_link_listener)
  { inode, captured_link_listener, target_element: None, }
}

///|
#cfg(target="js")
fn new_captured_link_listener(scheduler : &Scheduler) -> @dom.Listener {
  event => {
    if event.to_mouse_event() is Some(mouse_event) &&
      !(mouse_event.get_meta_key() || mouse_event.get_ctrl_key()) {
      event.prevent_default()
      let href = event
        .current_target()
        .unwrap()
        .to_element()
        .unwrap()
        .get_property("href")
        .unwrap()
      scheduler.add(scheduler.inject_url_request(href))
    }
  }
}

///|
#cfg(target="js")
pub fn VDom::initialize(
  vnode : VNode,
  scheduler : &Scheduler,
  target_element_id? : String,
) -> VDom {
  let captured_link_listener = new_captured_link_listener(scheduler)
  match (target_element_id, vnode) {
    (Some(id), vnode) => {
      let container = @dom.document().get_element_by_id(id).unwrap()
      // Clear any existing DOM so mounting does not append beside stale content.
      container.set_inner_html("")
      let inode = vnode.insert(
        scheduler,
        captured_link_listener,
        container.as_node(),
        null(),
      )
      VDom::{ target_element: Some(container), inode, captured_link_listener, }
    }
    (
      None,
      Elem(
        "html",
        props,
        Array(
          [
            Elem("head", head_props, head_children, ..),
            Elem("body", body_props, body_children, ..),
          ]
        ),
        namespace_uri=None
      ),
    ) => {
      let head = {
        let element = @dom.document().get_head().unwrap().as_element()
        let props = insert_props(
          element, "head", head_props, scheduler, captured_link_listener,
        )
        let children = insert_children(
          element.as_node(),
          head_children,
          scheduler,
          captured_link_listener,
        )
        INode::Elem("head", props, children, namespace_uri=None, element)
      }
      let body = {
        let element = @dom.document().get_body().unwrap().as_element()
        let props = insert_props(
          element, "body", body_props, scheduler, captured_link_listener,
        )
        let children = insert_children(
          element.as_node(),
          body_children,
          scheduler,
          captured_link_listener,
        )
        INode::Elem("body", props, children, namespace_uri=None, element)
      }

      let html = @dom.document().get_document_element().unwrap()
      let props = insert_props(
        html, "html", props, scheduler, captured_link_listener,
      )
      let inode = INode::Elem(
        "html",
        props,
        Array([head, body]),
        html,
        namespace_uri=None,
      )
      VDom::{ target_element: None, inode, captured_link_listener, }
    }
    // VDom root invariant: the root must be either a `(Some(id), _)` or a 
    // `(None, Elem("html", _, Array([Elem("head", ..), Elem("body", ..)]), ..)`
    (_, Elem(_) | Text(_) | Frag(_) | Thunk(_)) => panic()
  }
}

///|
#cfg(not(target="js"))
pub fn VDom::initialize(
  vnode : VNode,
  _ : &Scheduler,
  target_element_id? : String,
) -> VDom {
  ignore(target_element_id)
  { vnode, }
}

///|
#cfg(target="js")
fn diff_document(
  old : INode,
  new : VNode,
  scheduler : &Scheduler,
  container? : @dom.Element,
  captured_link_listener~ : @dom.Listener,
) -> INode {
  match (container, old, new) {
    (
      None,
      Elem(
        "html",
        old_props,
        Array(
          [
            Elem("head", old_head_props, old_head_children, head_element, ..),
            Elem("body", old_body_props, old_body_children, body_element, ..),
          ]
        ),
        html_element,
        namespace_uri=None
      ),
      Elem(
        "html",
        new_props,
        Array(
          [
            Elem("head", new_head_props, new_head_children, ..),
            Elem("body", new_body_props, new_body_children, ..),
          ]
        ),
        namespace_uri=None
      ),
    ) => {
      let props = diff_props(old_props, new_props, scheduler, html_element)
      let head = {
        let props = diff_props(
          old_head_props, new_head_props, scheduler, head_element,
        )
        let children = diff_children(
          old_head_children,
          new_head_children,
          scheduler,
          captured_link_listener,
          head_element.as_node(),
          null(),
        )
        INode::Elem("head", props, children, namespace_uri=None, head_element)
      }
      let body = {
        let props = diff_props(
          old_body_props, new_body_props, scheduler, body_element,
        )
        let children = diff_children(
          old_body_children,
          new_body_children,
          scheduler,
          captured_link_listener,
          body_element.as_node(),
          null(),
        )
        INode::Elem("body", props, children, namespace_uri=None, body_element)
      }
      Elem("html", props, Array([head, body]), html_element, namespace_uri=None)
    }
    (Some(container), old, new) =>
      diff_node(
        old,
        new,
        scheduler,
        captured_link_listener,
        container.as_node(),
        null(),
      )
    // See VDom root invariant in `VDom::initialize`
    _ => panic()
  }
}

///|
#cfg(target="js")
fn diff_node(
  old : INode,
  new : VNode,
  scheduler : &Scheduler,
  captured_link_listener : @dom.Listener,
  parent : @dom.Node,
  anchor : @js.Nullable[@dom.Node],
) -> INode {
  match (old, new) {
    (
      Elem(tag1, props1, children1, namespace_uri=ns1, element),
      Elem(tag2, props2, children2, namespace_uri=ns2),
    ) =>
      if tag1 != tag2 || ns1 != ns2 {
        old.remove(parent)
        new.insert(scheduler, captured_link_listener, parent, anchor)
      } else {
        let props = diff_props(props1, props2, scheduler, element)
        let children = diff_children(
          children1,
          children2,
          scheduler,
          captured_link_listener,
          element.as_node(),
          null(),
        )
        reapply_child_sensitive_properties(element, tag2, props.props)
        Elem(tag2, props, children, namespace_uri=ns2, element)
      }
    (Text(text1, element), Text(text2)) => {
      if text1 != text2 {
        element.set_node_value(@js.Nullable::from_option(Some(text2)))
      }
      Text(text2, element)
    }
    (Frag(children1, start, end), Frag(children2)) => {
      let children = diff_children(
        Array(children1),
        Array(children2),
        scheduler,
        captured_link_listener,
        parent,
        nullable(end.as_node()),
      )
      guard! children is Array(children)
      Frag(children, start, end)
    }
    (Thunk(hash1, node) as old, Thunk(hash2, render)) =>
      if hash1 == hash2 {
        old
      } else {
        let vnode = render()
        let node = diff_node(
          node, vnode, scheduler, captured_link_listener, parent, anchor,
        )
        Thunk(hash2, node)
      }
    (_, _) => {
      old.remove(parent)
      new.insert(scheduler, captured_link_listener, parent, anchor)
    }
  }
}

///|
#cfg(target="js")
fn diff_props(
  old : IProps,
  new : Props,
  scheduler : &Scheduler,
  parent : @dom.Element,
) -> IProps {
  let slots = old.slots
  for event, slot in slots {
    if !new.handlers.contains(event) {
      slot.val = fn(_, _) {  }
    }
  }
  for event, handler in new.handlers {
    if slots.get(event) is Some(slot) {
      slot.val = handler
    } else {
      let slot = @ref.new(handler)
      parent.add_event_listener(event, event => (slot.val)(dom_event(event), scheduler))
      slots[event] = slot
    }
  }

  for key, _ in old.props.attrs {
    if !new.attrs.contains(key) {
      parent.remove_attribute(key)
    }
  }
  for key, value2 in new.attrs {
    if old.props.attrs.get(key) is Some(value1) {
      if value1 != value2 {
        parent.set_attribute(key, value2)
      }
    } else {
      parent.set_attribute(key, value2)
    }
  }
  for key, value in old.props.props {
    if !new.props.contains(key) {
      // remove_property only deletes own JS properties, leaving native DOM state
      // exposed by prototype accessors unchanged. Use Elm's reset convention:
      // assign "" for strings and null otherwise.
      let reset = if value is String(_) { nullable("") } else { null() }
      parent.set_property(key, @js.Value::cast_from(reset))
    }
  }
  for key, value2 in new.props {
    if old.props.props.get(key) is Some(value1) {
      if value1 != value2 {
        parent.set_property(key, variant_to_js_value(value2))
      }
    } else {
      parent.set_property(key, variant_to_js_value(value2))
    }
  }
  let stylesheet = if parent.to_html_element() is Some(html) {
    html.get_style()
  } else if parent.to_svg_element() is Some(svg) {
    svg.get_style()
  } else {
    panic()
  }
  for key, _ in old.props.styles {
    if !new.styles.contains(key) {
      stylesheet.remove_property(@style.css_property_name(key)) |> ignore
    }
  }
  for key, value2 in new.styles {
    if old.props.styles.get(key) is Some(value1) {
      if value1 != value2 {
        stylesheet.set_property(@style.css_property_name(key), value2.to_css())
      }
    } else {
      stylesheet.set_property(@style.css_property_name(key), value2.to_css())
    }
  }
  { props: new, slots, }
}

///|
#cfg(target="js")
fn diff_children(
  old : Children[INode],
  new : Children[VNode],
  scheduler : &Scheduler,
  captured_link_listener : @dom.Listener,
  parent : @dom.Node,
  anchor : @js.Nullable[@dom.Node],
) -> Children[INode] {
  match (old, new) {
    (Array(old), Array(new)) => {
      let old_length = old.length()
      let new_length = new.length()
      let mut before = anchor
      let acc = []
      let length = if old_length > new_length {
        for index in old_length>..new_length {
          old[index].remove(parent)
        }
        new_length
      } else {
        for index in new_length>..old_length {
          let node = new[index].insert(
            scheduler, captured_link_listener, parent, before,
          )
          acc.push(node)
          before = nullable(node.start())
        }
        old_length
      }
      for index in length>..0 {
        let node = diff_node(
          old[index],
          new[index],
          scheduler,
          captured_link_listener,
          parent,
          before,
        )
        acc.push(node)
        before = nullable(node.start())
      }
      Array(acc.rev())
    }
    (Map(old), Map(new)) => {
      for key, node in old {
        if !new.contains(key) {
          node.remove(parent)
        }
      }
      let order = new.to_array()
      let mut before = null()
      let reversed = []
      for index in order.length()>..0 {
        let (key, vnode) = order[index]
        let node = if old.get(key) is Some(node) {
          let node = diff_node(
            node, vnode, scheduler, captured_link_listener, parent, before,
          )
          node.relocate(parent, before)
          node
        } else {
          vnode.insert(scheduler, captured_link_listener, parent, before)
        }
        reversed.push((key, node))
        before = nullable(node.start())
      }
      Map(Map::from_array(reversed))
    }
    (RawHtml(old), RawHtml(new)) => {
      if old != new {
        parent.to_element().unwrap().set_inner_html(new)
      }
      RawHtml(new)
    }
    (_, RawHtml(new)) => {
      parent.to_element().unwrap().set_inner_html(new)
      RawHtml(new)
    }
    (_, _) => {
      parent.to_element().unwrap().set_inner_html("")
      new.map(child => {
        child.insert(scheduler, captured_link_listener, parent, null())
      })
    }
  }
}

///|
#cfg(target="js")
fn[A, B] Children::map(self : Children[A], f : (A) -> B) -> Children[B] {
  match self {
    Array(children) => Array(children.map(f))
    Map(children) => Map(children.map((_, child) => f(child)))
    RawHtml(html) => RawHtml(html)
  }
}

///|
#cfg(target="js")
fn variant_to_js_value(value : @variant.Variant) -> @js.Value {
  match value {
    String(value) => @js.Value::cast_from(value)
    Floating(value) => @js.Value::cast_from(value)
    Integer(value) => @js.Value::cast_from(value)
    Boolean(value) => @js.Value::cast_from(value)
  }
}