egui/emigui/src/font.rs

use std::sync::{Arc, Mutex};

use rusttype::{point, Scale};

use crate::{
    math::{vec2, Vec2},
    texture_atlas::TextureAtlas,
};

pub struct TextFragment {
    /// The start of each character, starting at zero.
    /// Unit: points.
    pub x_offsets: Vec<f32>,

    /// 0 for the first line, n * line_spacing for the rest
    /// Unit: points.
    pub y_offset: f32,
    pub text: String,
}

impl TextFragment {
    pub fn min_x(&self) -> f32 {
        *self.x_offsets.first().unwrap()
    }

    pub fn max_x(&self) -> f32 {
        *self.x_offsets.last().unwrap()
    }
}

// ----------------------------------------------------------------------------

#[derive(Clone, Copy, Debug)]
pub struct UvRect {
    /// X/Y offset for nice rendering (unit: points).
    pub offset: Vec2,
    pub size: Vec2,

    /// Top left corner UV in texture.
    pub min: (u16, u16),

    /// Bottom right corner (exclusive).
    pub max: (u16, u16),
}

#[derive(Clone, Copy, Debug)]
pub struct GlyphInfo {
    id: rusttype::GlyphId,

    /// Unit: points.
    pub advance_width: f32,

    /// Texture coordinates. None for space.
    pub uv: Option<UvRect>,
}

/// Printable ASCII characters [32, 126], which excludes control codes.
const FIRST_ASCII: usize = 32; // 32 == space
const LAST_ASCII: usize = 126;

/// The interface uses points as the unit for everything.
#[derive(Clone)]
pub struct Font {
    font: rusttype::Font<'static>,
    /// Maximum character height
    scale_in_pixels: f32,
    pixels_per_point: f32,
    /// NUM_CHARS big
    glyph_infos: Vec<GlyphInfo>,
    atlas: Arc<Mutex<TextureAtlas>>,
}

impl Font {
    pub fn new(
        atlas: Arc<Mutex<TextureAtlas>>,
        font_data: &'static [u8],
        scale_in_points: f32,
        pixels_per_point: f32,
    ) -> Font {
        let font = rusttype::Font::from_bytes(font_data).expect("Error constructing Font");

        let scale_in_pixels = pixels_per_point * scale_in_points;

        let glyphs: Vec<_> = Self::supported_characters()
            .map(|c| {
                let glyph = font.glyph(c);
                assert_ne!(
                    glyph.id().0,
                    0,
                    "Failed to find a glyph for the character '{}'",
                    c
                );
                let glyph = glyph.scaled(Scale::uniform(scale_in_pixels));
                glyph.positioned(point(0.0, 0.0))
            })
            .collect();

        let mut glyph_infos = vec![];
        let mut atlas_lock = atlas.lock().unwrap();

        for glyph in glyphs {
            let uv = if let Some(bb) = glyph.pixel_bounding_box() {
                let glyph_width = bb.width() as usize;
                let glyph_height = bb.height() as usize;
                assert!(glyph_width >= 1);
                assert!(glyph_height >= 1);

                let glyph_pos = atlas_lock.allocate((glyph_width, glyph_height));

                let texture = atlas_lock.texture_mut();
                glyph.draw(|x, y, v| {
                    if v > 0.0 {
                        let px = glyph_pos.0 + x as usize;
                        let py = glyph_pos.1 + y as usize;
                        texture[(px, py)] = (v * 255.0).round() as u8;
                    }
                });

                let offset_y_in_pixels =
                    scale_in_pixels as f32 + bb.min.y as f32 - 4.0 * pixels_per_point; // TODO: use font.v_metrics
                Some(UvRect {
                    offset: vec2(
                        bb.min.x as f32 / pixels_per_point,
                        offset_y_in_pixels / pixels_per_point,
                    ),
                    size: vec2(glyph_width as f32, glyph_height as f32) / pixels_per_point,
                    min: (glyph_pos.0 as u16, glyph_pos.1 as u16),
                    max: (
                        (glyph_pos.0 + glyph_width) as u16,
                        (glyph_pos.1 + glyph_height) as u16,
                    ),
                })
            } else {
                // No bounding box. Maybe a space?
                None
            };

            let advance_width_in_points =
                glyph.unpositioned().h_metrics().advance_width / pixels_per_point;

            glyph_infos.push(GlyphInfo {
                id: glyph.id(),
                advance_width: advance_width_in_points,
                uv,
            });
        }

        drop(atlas_lock);

        Font {
            font,
            scale_in_pixels,
            pixels_per_point,
            glyph_infos,
            atlas,
        }
    }

    pub fn round_to_pixel(&self, point: f32) -> f32 {
        (point * self.pixels_per_point).round() / self.pixels_per_point
    }

    /// In points
    pub fn line_spacing(&self) -> f32 {
        self.scale_in_pixels / self.pixels_per_point
    }

    pub fn supported_characters() -> impl Iterator<Item = char> {
        (FIRST_ASCII..=LAST_ASCII).map(|c| c as u8 as char)
    }

    pub fn uv_rect(&self, c: char) -> Option<UvRect> {
        let c = c as usize;
        if FIRST_ASCII <= c && c <= LAST_ASCII {
            self.glyph_infos[c - FIRST_ASCII].uv
        } else {
            None
        }
    }

    fn glyph_info(&self, c: char) -> Option<GlyphInfo> {
        let c = c as usize;
        if FIRST_ASCII <= c && c <= LAST_ASCII {
            Some(self.glyph_infos[c - FIRST_ASCII])
        } else {
            None
        }
    }

    /// Returns the a single line of characters separated into words
    pub fn layout_single_line(&self, text: &str) -> Vec<TextFragment> {
        let scale_in_pixels = Scale::uniform(self.scale_in_pixels);

        let mut current_fragment = TextFragment {
            x_offsets: vec![0.0],
            y_offset: 0.0,
            text: String::new(),
        };
        let mut all_fragments = vec![];
        let mut cursor_x_in_points = 0.0f32;
        let mut last_glyph_id = None;

        for c in text.chars() {
            if let Some(glyph) = self.glyph_info(c) {
                if let Some(last_glyph_id) = last_glyph_id {
                    cursor_x_in_points +=
                        self.font
                            .pair_kerning(scale_in_pixels, last_glyph_id, glyph.id)
                            / self.pixels_per_point
                }
                cursor_x_in_points += glyph.advance_width;
                cursor_x_in_points = self.round_to_pixel(cursor_x_in_points);
                last_glyph_id = Some(glyph.id);

                let is_space = glyph.uv.is_none();
                if is_space {
                    // TODO: also break after hyphens etc
                    if !current_fragment.text.is_empty() {
                        all_fragments.push(current_fragment);
                        current_fragment = TextFragment {
                            x_offsets: vec![cursor_x_in_points],
                            y_offset: 0.0,
                            text: String::new(),
                        }
                    }
                } else {
                    current_fragment.text.push(c);
                    current_fragment.x_offsets.push(cursor_x_in_points);
                }
            } else {
                // Ignore unknown glyph
            }
        }

        if !current_fragment.text.is_empty() {
            all_fragments.push(current_fragment)
        }
        all_fragments
    }

    pub fn layout_single_line_max_width(
        &self,
        text: &str,
        max_width_in_points: f32,
    ) -> Vec<TextFragment> {
        let mut words = self.layout_single_line(text);
        if words.is_empty() || words.last().unwrap().max_x() <= max_width_in_points {
            return words; // Early-out
        }

        let line_spacing = self.line_spacing();

        // Break up lines:
        let mut line_start_x = 0.0;
        let mut cursor_y = 0.0;

        for word in words.iter_mut().skip(1) {
            if word.max_x() - line_start_x >= max_width_in_points {
                // Time for a new line:
                cursor_y += line_spacing;
                line_start_x = word.min_x();
            }

            word.y_offset += cursor_y;
            for x in &mut word.x_offsets {
                *x -= line_start_x;
            }
        }

        words
    }

    /// Returns each line + total bounding box size.
    pub fn layout_multiline(
        &self,
        text: &str,
        max_width_in_points: f32,
    ) -> (Vec<TextFragment>, Vec2) {
        let line_spacing = self.line_spacing();
        let mut cursor_y = 0.0;
        let mut text_fragments = Vec::new();
        for line in text.split('\n') {
            let mut line_fragments = self.layout_single_line_max_width(&line, max_width_in_points);
            if let Some(last_word) = line_fragments.last() {
                let line_height = last_word.y_offset + line_spacing;
                for fragment in &mut line_fragments {
                    fragment.y_offset += cursor_y;
                }
                text_fragments.append(&mut line_fragments);
                cursor_y += line_height; // TODO: add extra spacing between paragraphs
            } else {
                cursor_y += line_spacing;
            }
            cursor_y = self.round_to_pixel(cursor_y);
        }

        let mut widest_line = 0.0;
        for fragment in &text_fragments {
            widest_line = fragment.max_x().max(widest_line);
        }

        let bounding_size = vec2(widest_line, cursor_y);
        (text_fragments, bounding_size)
    }
}