use super::{HElem, VElem};
use crate::layout::{BlockElem, ParElem, Spacing};
use crate::prelude::*;
use crate::text::{SpaceElem, TextElem};

/// A list of terms and their descriptions.
///
/// Displays a sequence of terms and their descriptions vertically. When the
/// descriptions span over multiple lines, they use hanging indent to
/// communicate the visual hierarchy.
///
/// ## Syntax
/// This function also has dedicated syntax: Starting a line with a slash,
/// followed by a term, a colon and a description creates a term list item.
///
/// ## Example
/// ```example
/// / Ligature: A merged glyph.
/// / Kerning: A spacing adjustment
///   between two adjacent letters.
/// ```
///
/// Display: Term List
/// Category: layout
#[element(Layout)]
pub struct TermsElem {
    /// If this is `{false}`, the items are spaced apart with [term list
    /// spacing]($func/terms.spacing). If it is `{true}`, they use normal
    /// [leading]($func/par.leading) instead. This makes the term list more
    /// compact, which can look better if the items are short.
    ///
    /// ```example
    /// / Fact: If a term list has a lot
    ///   of text, and maybe other inline
    ///   content, it should not be tight
    ///   anymore.
    ///
    /// / Tip: To make it wide, simply
    ///   insert a blank line between the
    ///   items.
    /// ```
    #[default(true)]
    pub tight: bool,

    /// The indentation of each item's term.
    pub indent: Length,

    /// The hanging indent of the description.
    ///
    /// ```example
    /// #set terms(hanging-indent: 0pt)
    /// / Term: This term list does not
    ///   make use of hanging indents.
    /// ```
    #[default(Em::new(1.0).into())]
    pub hanging_indent: Length,

    /// The spacing between the items of a wide (non-tight) term list.
    ///
    /// If set to `{auto}`, uses the spacing [below blocks]($func/block.below).
    pub spacing: Smart<Spacing>,

    /// The term list's children.
    ///
    /// When using the term list syntax, adjacent items are automatically
    /// collected into term lists, even through constructs like for loops.
    ///
    /// ```example
    /// #for year, product in (
    ///   "1978": "TeX",
    ///   "1984": "LaTeX",
    ///   "2019": "Typst",
    /// ) [/ #product: Born in #year.]
    /// ```
    #[variadic]
    pub children: Vec<TermItem>,
}

impl Layout for TermsElem {
    fn layout(
        &self,
        vt: &mut Vt,
        styles: StyleChain,
        regions: Regions,
    ) -> SourceResult<Fragment> {
        let indent = self.indent(styles);
        let hanging_indent = self.hanging_indent(styles);
        let gutter = if self.tight(styles) {
            ParElem::leading_in(styles).into()
        } else {
            self.spacing(styles)
                .unwrap_or_else(|| BlockElem::below_in(styles).amount())
        };

        let mut seq = vec![];
        for (i, child) in self.children().into_iter().enumerate() {
            if i > 0 {
                seq.push(VElem::new(gutter).with_weakness(1).pack());
            }
            if indent.is_zero() {
                seq.push(HElem::new(indent.into()).pack());
            }
            seq.push((child.term() + TextElem::packed(':')).strong());
            seq.push(SpaceElem::new().pack());
            seq.push(child.description());
        }

        Content::sequence(seq)
            .styled(ParElem::set_hanging_indent(hanging_indent + indent))
            .layout(vt, styles, regions)
    }
}

/// A term list item.
///
/// Display: Term List Item
/// Category: layout
#[element]
pub struct TermItem {
    /// The term described by the list item.
    #[required]
    pub term: Content,

    /// The description of the term.
    #[required]
    pub description: Content,
}

cast_from_value! {
    TermItem,
    array: Array => {
        let mut iter = array.into_iter();
        let (term, description) = match (iter.next(), iter.next(), iter.next()) {
            (Some(a), Some(b), None) => (a.cast()?, b.cast()?),
            _ => Err("array must contain exactly two entries")?,
        };
        Self::new(term, description)
    },
    v: Content => v.to::<Self>().cloned().ok_or("expected term item or array")?,
}