use std::str::FromStr;

use ecow::eco_format;
use smallvec::SmallVec;

use crate::diag::{bail, SourceResult};
use crate::engine::Engine;
use crate::foundations::{
    cast, elem, scope, Array, Content, NativeElement, Packed, Show, Smart, StyleChain,
    Styles, TargetElem,
};
use crate::html::{attr, tag, HtmlAttr, HtmlElem};
use crate::layout::{Alignment, BlockElem, Em, HAlignment, Length, VAlignment, VElem};
use crate::model::{ListItemLike, ListLike, Numbering, NumberingPattern, ParElem};

/// A numbered list.
///
/// Displays a sequence of items vertically and numbers them consecutively.
///
/// # Example
/// ```example
/// Automatically numbered:
/// + Preparations
/// + Analysis
/// + Conclusions
///
/// Manually numbered:
/// 2. What is the first step?
/// 5. I am confused.
/// +  Moving on ...
///
/// Multiple lines:
/// + This enum item has multiple
///   lines because the next line
///   is indented.
///
/// Function call.
/// #enum[First][Second]
/// ```
///
/// You can easily switch all your enumerations to a different numbering style
/// with a set rule.
/// ```example
/// #set enum(numbering: "a)")
///
/// + Starting off ...
/// + Don't forget step two
/// ```
///
/// You can also use [`enum.item`]($enum.item) to programmatically customize the
/// number of each item in the enumeration:
///
/// ```example
/// #enum(
///   enum.item(1)[First step],
///   enum.item(5)[Fifth step],
///   enum.item(10)[Tenth step]
/// )
/// ```
///
/// # Syntax
/// This functions also has dedicated syntax:
///
/// - Starting a line with a plus sign creates an automatically numbered
///   enumeration item.
/// - Starting a line with a number followed by a dot creates an explicitly
///   numbered enumeration item.
///
/// Enumeration items can contain multiple paragraphs and other block-level
/// content. All content that is indented more than an item's marker becomes
/// part of that item.
#[elem(scope, title = "Numbered List", Show)]
pub struct EnumElem {
    /// Defines the default [spacing]($enum.spacing) of the enumeration. If it
    /// is `{false}`, the items are spaced apart with
    /// [paragraph spacing]($par.spacing). If it is `{true}`, they use
    /// [paragraph leading]($par.leading) instead. This makes the list more
    /// compact, which can look better if the items are short.
    ///
    /// In markup mode, the value of this parameter is determined based on
    /// whether items are separated with a blank line. If items directly follow
    /// each other, this is set to `{true}`; if items are separated by a blank
    /// line, this is set to `{false}`. The markup-defined tightness cannot be
    /// overridden with set rules.
    ///
    /// ```example
    /// + If an enum has a lot of text, and
    ///   maybe other inline content, it
    ///   should not be tight anymore.
    ///
    /// + To make an enum wide, simply
    ///   insert a blank line between the
    ///   items.
    /// ```
    #[default(true)]
    pub tight: bool,

    /// How to number the enumeration. Accepts a
    /// [numbering pattern or function]($numbering).
    ///
    /// If the numbering pattern contains multiple counting symbols, they apply
    /// to nested enums. If given a function, the function receives one argument
    /// if `full` is `{false}` and multiple arguments if `full` is `{true}`.
    ///
    /// ```example
    /// #set enum(numbering: "1.a)")
    /// + Different
    /// + Numbering
    ///   + Nested
    ///   + Items
    /// + Style
    ///
    /// #set enum(numbering: n => super[#n])
    /// + Superscript
    /// + Numbering!
    /// ```
    #[default(Numbering::Pattern(NumberingPattern::from_str("1.").unwrap()))]
    #[borrowed]
    pub numbering: Numbering,

    /// Which number to start the enumeration with.
    ///
    /// ```example
    /// #enum(
    ///   start: 3,
    ///   [Skipping],
    ///   [Ahead],
    /// )
    /// ```
    pub start: Smart<usize>,

    /// Whether to display the full numbering, including the numbers of
    /// all parent enumerations.
    ///
    ///
    /// ```example
    /// #set enum(numbering: "1.a)", full: true)
    /// + Cook
    ///   + Heat water
    ///   + Add ingredients
    /// + Eat
    /// ```
    #[default(false)]
    pub full: bool,

    /// Whether to reverse the numbering for this enumeration.
    ///
    /// ```example
    /// #set enum(reversed: true)
    /// + Coffee
    /// + Tea
    /// + Milk
    /// ```
    #[default(false)]
    pub reversed: bool,

    /// The indentation of each item.
    #[resolve]
    pub indent: Length,

    /// The space between the numbering and the body of each item.
    #[resolve]
    #[default(Em::new(0.5).into())]
    pub body_indent: Length,

    /// The spacing between the items of the enumeration.
    ///
    /// If set to `{auto}`, uses paragraph [`leading`]($par.leading) for tight
    /// enumerations and paragraph [`spacing`]($par.spacing) for wide
    /// (non-tight) enumerations.
    pub spacing: Smart<Length>,

    /// The alignment that enum numbers should have.
    ///
    /// By default, this is set to `{end + top}`, which aligns enum numbers
    /// towards end of the current text direction (in left-to-right script,
    /// for example, this is the same as `{right}`) and at the top of the line.
    /// The choice of `{end}` for horizontal alignment of enum numbers is
    /// usually preferred over `{start}`, as numbers then grow away from the
    /// text instead of towards it, avoiding certain visual issues. This option
    /// lets you override this behaviour, however. (Also to note is that the
    /// [unordered list]($list) uses a different method for this, by giving the
    /// `marker` content an alignment directly.).
    ///
    /// ````example
    /// #set enum(number-align: start + bottom)
    ///
    /// Here are some powers of two:
    /// 1. One
    /// 2. Two
    /// 4. Four
    /// 8. Eight
    /// 16. Sixteen
    /// 32. Thirty two
    /// ````
    #[default(HAlignment::End + VAlignment::Top)]
    pub number_align: Alignment,

    /// The numbered list's items.
    ///
    /// When using the enum syntax, adjacent items are automatically collected
    /// into enumerations, even through constructs like for loops.
    ///
    /// ```example
    /// #for phase in (
    ///    "Launch",
    ///    "Orbit",
    ///    "Descent",
    /// ) [+ #phase]
    /// ```
    #[variadic]
    pub children: Vec<Packed<EnumItem>>,

    /// The numbers of parent items.
    #[internal]
    #[fold]
    #[ghost]
    pub parents: SmallVec<[usize; 4]>,
}

#[scope]
impl EnumElem {
    #[elem]
    type EnumItem;
}

impl Show for Packed<EnumElem> {
    fn show(&self, engine: &mut Engine, styles: StyleChain) -> SourceResult<Content> {
        if TargetElem::target_in(styles).is_html() {
            let mut elem = HtmlElem::new(tag::ol);
            if self.reversed(styles) {
                elem =
                    elem.with_attr(const { HtmlAttr::constant("reversed") }, "reversed");
            }
            return Ok(elem
                .with_body(Some(Content::sequence(self.children.iter().map(|item| {
                    let mut li = HtmlElem::new(tag::li);
                    if let Some(nr) = item.number(styles) {
                        li = li.with_attr(attr::value, eco_format!("{nr}"));
                    }
                    li.with_body(Some(item.body.clone())).pack().spanned(item.span())
                }))))
                .pack()
                .spanned(self.span()));
        }

        let mut realized =
            BlockElem::multi_layouter(self.clone(), engine.routines.layout_enum)
                .pack()
                .spanned(self.span());

        if self.tight(styles) {
            let leading = ParElem::leading_in(styles);
            let spacing =
                VElem::new(leading.into()).with_weak(true).with_attach(true).pack();
            realized = spacing + realized;
        }

        Ok(realized)
    }
}

/// An enumeration item.
#[elem(name = "item", title = "Numbered List Item")]
pub struct EnumItem {
    /// The item's number.
    #[positional]
    pub number: Option<usize>,

    /// The item's body.
    #[required]
    pub body: Content,
}

cast! {
    EnumItem,
    array: Array => {
        let mut iter = array.into_iter();
        let (number, body) = match (iter.next(), iter.next(), iter.next()) {
            (Some(a), Some(b), None) => (a.cast()?, b.cast()?),
            _ => bail!("array must contain exactly two entries"),
        };
        Self::new(body).with_number(number)
    },
    v: Content => v.unpack::<Self>().unwrap_or_else(Self::new),
}

impl ListLike for EnumElem {
    type Item = EnumItem;

    fn create(children: Vec<Packed<Self::Item>>, tight: bool) -> Self {
        Self::new(children).with_tight(tight)
    }
}

impl ListItemLike for EnumItem {
    fn styled(mut item: Packed<Self>, styles: Styles) -> Packed<Self> {
        item.body.style_in_place(styles);
        item
    }
}
