typst/src/layout/text.rs

//! Layouting of continous pieces of text into boxes.
//!
//! The layouter picks the most suitable font for each individual character.
//! When the primary layouting axis horizontally inversed, the word is spelled
//! backwards. Vertical word layout is not yet supported.

use fontdock::{FaceId, FaceQuery, FontStyle};
use ttf_parser::GlyphId;

use super::elements::{LayoutElement, Shaped};
use super::*;
use crate::font::FontLoader;
use crate::geom::Size;
use crate::style::TextStyle;

/// Layouts text into a box.
pub async fn layout_text(text: &str, ctx: TextContext<'_>) -> BoxLayout {
    TextLayouter::new(text, ctx).layout().await
}

/// Performs the text layouting.
#[derive(Debug)]
struct TextLayouter<'a> {
    ctx: TextContext<'a>,
    text: &'a str,
    shaped: Shaped,
    elements: LayoutElements,
    start: f64,
    width: f64,
}

/// The context for text layouting.
#[derive(Debug)]
pub struct TextContext<'a> {
    /// The font loader to retrieve fonts from when typesetting text with
    /// `layout_text`.
    pub loader: &'a mut FontLoader,
    /// The style for text: Font selection with classes, weights and variants,
    /// font sizes, spacing and so on.
    pub style: &'a TextStyle,
    /// The direction into which the word is laid out. For now, only horizontal
    /// directions are supported.
    pub dir: Dir,
    /// The alignment of the _resulting_ layout. This does not effect the line
    /// layouting itself, but rather how the finished layout will be positioned
    /// in a parent layout.
    pub align: LayoutAlign,
}

impl<'a> TextLayouter<'a> {
    fn new(text: &'a str, ctx: TextContext<'a>) -> Self {
        Self {
            text,
            shaped: Shaped::new(FaceId::MAX, ctx.style.font_size()),
            elements: LayoutElements::new(),
            start: 0.0,
            width: 0.0,
            ctx,
        }
    }

    async fn layout(mut self) -> BoxLayout {
        // If the primary axis is negative, we layout the characters reversed.
        if self.ctx.dir.is_positive() {
            for c in self.text.chars() {
                self.layout_char(c).await;
            }
        } else {
            for c in self.text.chars().rev() {
                self.layout_char(c).await;
            }
        }

        // Flush the last buffered parts of the word.
        if !self.shaped.text.is_empty() {
            let pos = Size::new(self.start, 0.0);
            self.elements.push(pos, LayoutElement::Text(self.shaped));
        }

        BoxLayout {
            size: Size::new(self.width, self.ctx.style.font_size()),
            align: self.ctx.align,
            elements: self.elements,
        }
    }

    async fn layout_char(&mut self, c: char) {
        let (index, glyph, char_width) = match self.select_font(c).await {
            Some(selected) => selected,
            // TODO: Issue warning about missing character.
            None => return,
        };

        // Flush the buffer and issue a font setting action if the font differs
        // from the last character's one.
        if self.shaped.face != index {
            if !self.shaped.text.is_empty() {
                let pos = Size::new(self.start, 0.0);
                let shaped = std::mem::replace(
                    &mut self.shaped,
                    Shaped::new(FaceId::MAX, self.ctx.style.font_size()),
                );

                self.elements.push(pos, LayoutElement::Text(shaped));
                self.start = self.width;
            }

            self.shaped.face = index;
        }

        self.shaped.text.push(c);
        self.shaped.glyphs.push(glyph);
        self.shaped.offsets.push(self.width - self.start);

        self.width += char_width;
    }

    async fn select_font(&mut self, c: char) -> Option<(FaceId, GlyphId, f64)> {
        let mut variant = self.ctx.style.variant;

        if self.ctx.style.bolder {
            variant.weight = variant.weight.thicken(300);
        }

        if self.ctx.style.italic {
            variant.style = match variant.style {
                FontStyle::Normal => FontStyle::Italic,
                FontStyle::Italic => FontStyle::Normal,
                FontStyle::Oblique => FontStyle::Normal,
            }
        }

        let query = FaceQuery {
            fallback: self.ctx.style.fallback.iter(),
            variant,
            c,
        };

        if let Some((id, owned_face)) = self.ctx.loader.query(query).await {
            let face = owned_face.get();

            // Determine the width of the char.
            let units_per_em = face.units_per_em().unwrap_or(1000) as f64;
            let ratio = 1.0 / units_per_em;
            let to_raw = |x| ratio * x as f64;

            let glyph = face.glyph_index(c)?;
            let glyph_width = face.glyph_hor_advance(glyph)?;
            let char_width = to_raw(glyph_width) * self.ctx.style.font_size();

            Some((id, glyph, char_width))
        } else {
            None
        }
    }
}