egui/emigui/src/font.rs

use std::{collections::BTreeMap, sync::Arc};

use parking_lot::Mutex;
use rusttype::{point, Scale};

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

/// A typeset piece of text on a single line.
pub struct Fragment {
    /// The start of each character, probably starting at zero.
    /// The last element is the end of the last character.
    /// x_offsets.len() == text.chars().count() + 1
    /// This is never empty.
    /// Unit: points.
    pub x_offsets: Vec<f32>,

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

    // TODO: make this a str reference into a String owned by Galley
    /// The actual characters.
    pub text: String,
}

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

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

// pub fn fn_text_width(fragmens: &[Fragment]) -> f32 {
//     if fragmens.is_empty() {
//         0.0
//     } else {
//         fragmens.last().unwrap().max_x() - fragmens.first().unwrap().min_x()
//     }
// }

/// A collection of text locked into place.
#[derive(Default)]
pub struct Galley {
    // TODO: maybe rename/refactor this as `lines`?
    pub fragments: Vec<Fragment>,
    pub size: Vec2,
}

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

#[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_rect: Option<UvRect>,
}

/// 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,
    glyph_infos: BTreeMap<char, 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::try_from_bytes(font_data).expect("Error constructing Font");
        let scale_in_pixels = pixels_per_point * scale_in_points;

        let mut font = Font {
            font,
            scale_in_pixels,
            pixels_per_point,
            glyph_infos: Default::default(),
            atlas,
        };

        /// Printable ASCII characters [32, 126], which excludes control codes.
        const FIRST_ASCII: usize = 32; // 32 == space
        const LAST_ASCII: usize = 126;
        for c in (FIRST_ASCII..=LAST_ASCII).map(|c| c as u8 as char) {
            font.add_char(c);
        }
        font
    }

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

    /// Height of one line of text. In points
    /// TODO: rename height ?
    pub fn line_spacing(&self) -> f32 {
        self.scale_in_pixels / self.pixels_per_point
    }
    pub fn height(&self) -> f32 {
        self.scale_in_pixels / self.pixels_per_point
    }

    pub fn uv_rect(&self, c: char) -> Option<UvRect> {
        self.glyph_infos.get(&c).and_then(|gi| gi.uv_rect)
    }

    fn glyph_info(&self, c: char) -> Option<&GlyphInfo> {
        self.glyph_infos.get(&c)
    }

    fn add_char(&mut self, c: char) {
        let glyph = self.font.glyph(c);
        assert_ne!(
            glyph.id().0,
            0,
            "Failed to find a glyph for the character '{}'",
            c
        );
        let glyph = glyph.scaled(Scale::uniform(self.scale_in_pixels));
        let glyph = glyph.positioned(point(0.0, 0.0));

        let uv_rect = 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 mut atlas_lock = self.atlas.lock();
            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 =
                self.scale_in_pixels as f32 + bb.min.y as f32 - 4.0 * self.pixels_per_point; // TODO: use font.v_metrics
            Some(UvRect {
                offset: vec2(
                    bb.min.x as f32 / self.pixels_per_point,
                    offset_y_in_pixels / self.pixels_per_point,
                ),
                size: vec2(glyph_width as f32, glyph_height as f32) / self.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 / self.pixels_per_point;

        self.glyph_infos.insert(
            c,
            GlyphInfo {
                id: glyph.id(),
                advance_width: advance_width_in_points,
                uv_rect,
            },
        );
    }

    /// Typeset the given text onto one line.
    /// Assumes there are no \n in the text.
    /// Return x_offsets, one longer than the number of characters in the text.
    fn layout_single_line_fragment(&self, text: &str) -> Vec<f32> {
        let scale_in_pixels = Scale::uniform(self.scale_in_pixels);

        let mut x_offsets = vec![0.0];
        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);
            } else {
                // Ignore unknown glyph
            }
            x_offsets.push(cursor_x_in_points);
        }

        x_offsets
    }

    /// Typeset the given text onto one line.
    /// Assumes there are no \n in the text.
    /// Always returns exactly one frament.
    pub fn layout_single_line(&self, text: &str) -> Galley {
        let x_offsets = self.layout_single_line_fragment(text);
        let fragment = Fragment {
            x_offsets,
            y_offset: 0.0,
            text: text.to_owned(),
        };
        assert_eq!(fragment.x_offsets.len(), fragment.text.chars().count() + 1);
        let width = fragment.max_x();
        let size = vec2(width, self.height());
        Galley {
            fragments: vec![fragment],
            size,
        }
    }

    /// A paragraph is text with no line break character in it.
    /// The text will be linebreaked by the given `max_width_in_points`.
    /// TODO: return Galley ?
    pub fn layout_paragraph_max_width(
        &self,
        text: &str,
        max_width_in_points: f32,
    ) -> Vec<Fragment> {
        let full_x_offsets = self.layout_single_line_fragment(text);

        let mut line_start_x = full_x_offsets[0];
        assert_eq!(line_start_x, 0.0);
        let mut cursor_y = 0.0;
        let mut line_start_idx = 0;

        // start index of the last space. A candidate for a new line.
        let mut last_space = None;

        let mut out_fragments = vec![];

        for (i, (x, chr)) in full_x_offsets.iter().skip(1).zip(text.chars()).enumerate() {
            let line_width = x - line_start_x;

            if line_width > max_width_in_points {
                if let Some(last_space_idx) = last_space {
                    let include_trailing_space = true;
                    let fragment = if include_trailing_space {
                        Fragment {
                            x_offsets: full_x_offsets[line_start_idx..=last_space_idx + 1]
                                .iter()
                                .map(|x| x - line_start_x)
                                .collect(),
                            y_offset: cursor_y,
                            text: text
                                .chars()
                                .skip(line_start_idx)
                                .take(last_space_idx + 1 - line_start_idx)
                                .collect(),
                        }
                    } else {
                        Fragment {
                            x_offsets: full_x_offsets[line_start_idx..=last_space_idx]
                                .iter()
                                .map(|x| x - line_start_x)
                                .collect(),
                            y_offset: cursor_y,
                            text: text
                                .chars()
                                .skip(line_start_idx)
                                .take(last_space_idx - line_start_idx)
                                .collect(),
                        }
                    };
                    assert_eq!(fragment.x_offsets.len(), fragment.text.chars().count() + 1);
                    out_fragments.push(fragment);

                    line_start_idx = last_space_idx + 1;
                    line_start_x = full_x_offsets[line_start_idx];
                    last_space = None;
                    cursor_y += self.line_spacing();
                    cursor_y = self.round_to_pixel(cursor_y);
                }
            }

            const NON_BREAKING_SPACE: char = '\u{A0}';
            if chr.is_whitespace() && chr != NON_BREAKING_SPACE {
                last_space = Some(i);
            }
        }

        if line_start_idx + 1 < full_x_offsets.len() {
            let fragment = Fragment {
                x_offsets: full_x_offsets[line_start_idx..]
                    .iter()
                    .map(|x| x - line_start_x)
                    .collect(),
                y_offset: cursor_y,
                text: text.chars().skip(line_start_idx).collect(),
            };
            assert_eq!(fragment.x_offsets.len(), fragment.text.chars().count() + 1);
            out_fragments.push(fragment);
        }

        out_fragments
    }

    pub fn layout_multiline(&self, text: &str, max_width_in_points: f32) -> Galley {
        let line_spacing = self.line_spacing();
        let mut cursor_y = 0.0;
        let mut fragments = Vec::new();
        for line in text.split('\n') {
            let mut paragraph_fragments =
                self.layout_paragraph_max_width(line, max_width_in_points);
            if let Some(last_fragment) = paragraph_fragments.last() {
                let line_height = last_fragment.y_offset + line_spacing;
                for fragment in &mut paragraph_fragments {
                    fragment.y_offset += cursor_y;
                }
                fragments.append(&mut paragraph_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 &fragments {
            widest_line = fragment.max_x().max(widest_line);
        }

        Galley {
            fragments,
            size: vec2(widest_line, cursor_y),
        }
    }
}