Devise text layouter 📑

4 files changed, 278 insertions, 15 deletions

diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index f2ca3029..1af3156d 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -2,20 +2,42 @@
 
 use crate::doc::{Document, Page, TextAction};
 use crate::font::{Font, FontLoader, FontFamily, FontError};
-use crate::syntax::SyntaxTree;
+use crate::syntax::{SyntaxTree, Node};
 
 mod size;
+mod text;
 
 pub use size::Size;
+pub use text::TextLayouter;
 
 
 /// Layout a syntax tree in a given context.
-#[allow(unused_variables)]
 pub fn layout(tree: &SyntaxTree, ctx: &LayoutContext) -> LayoutResult<Layout> {
-    Ok(Layout {
-        extent: LayoutDimensions { width: Size::zero(), height: Size::zero() },
-        actions: vec![],
-    })
+    let mut layouter = TextLayouter::new(ctx);
+
+    let mut italic = false;
+    let mut bold = false;
+
+    for node in &tree.nodes {
+        match node {
+            Node::Text(text) => layouter.add_text(text)?,
+            Node::Space => layouter.add_space()?,
+            Node::Newline => layouter.add_paragraph()?,
+
+            Node::ToggleItalics => {
+                italic = !italic;
+                layouter.set_italic(italic);
+            },
+            Node::ToggleBold => {
+                bold = !bold;
+                layouter.set_bold(bold);
+            }
+
+            Node::Func(_) => unimplemented!(),
+        }
+    }
+
+    layouter.finish()
 }
 
 /// A collection of layouted content.
diff --git a/src/layout/text.rs b/src/layout/text.rs
new file mode 100644
index 00000000..15da8373
--- /dev/null
+++ b/src/layout/text.rs
@@ -0,0 +1,248 @@
+//! Layouting of text.
+
+use std::cell::Ref;
+use std::mem;
+
+use smallvec::SmallVec;
+
+use crate::doc::TextAction;
+use crate::font::{Font, FontQuery};
+use super::{Layouter, Layout, LayoutError, LayoutContext, Size, LayoutResult};
+
+
+/// Layouts text within the constraints of a layouting context.
+#[derive(Debug)]
+pub struct TextLayouter<'a, 'p> {
+    ctx: &'a LayoutContext<'a, 'p>,
+    units: Vec<Unit>,
+    italic: bool,
+    bold: bool,
+}
+
+/// A units that is arranged by the text layouter.
+#[derive(Debug, Clone)]
+enum Unit {
+    /// A paragraph.
+    Paragraph,
+    /// A space with its font index and width.
+    Space(usize, Size),
+    /// One logical tex  unit.
+    Text(TextUnit),
+}
+
+/// A logical unit of text (a word, syllable or a similar construct).
+#[derive(Debug, Clone)]
+struct TextUnit {
+    /// Contains pairs of (characters, font_index, char_width) for each character of the text.
+    chars_with_widths: SmallVec<[(char, usize, Size); 12]>,
+    /// The total width of the unit.
+    width: Size,
+}
+
+impl<'a, 'p> TextLayouter<'a, 'p> {
+    /// Create a new text layouter.
+    pub fn new(ctx: &'a LayoutContext<'a, 'p>) -> TextLayouter<'a, 'p> {
+        TextLayouter {
+            ctx,
+            italic: false,
+            bold: false,
+            units: vec![],
+        }
+    }
+
+    /// Add more text to the layout.
+    pub fn add_text(&mut self, text: &str) -> LayoutResult<()> {
+        let mut chars_with_widths = SmallVec::<[(char, usize, Size); 12]>::new();
+
+        // Find out which font to use for each character in the text and meanwhile calculate the
+        // width of the text.
+        let mut text_width = Size::zero();
+        for c in text.chars() {
+            // Find out the width and add it to the total width.
+            let (index, font) = self.get_font_for(c)?;
+            let char_width = self.width_of(c, &font);
+            text_width += char_width;
+
+            chars_with_widths.push((c, index, char_width));
+        }
+
+        self.units.push(Unit::Text(TextUnit {
+            chars_with_widths,
+            width: text_width,
+        }));
+
+        Ok(())
+    }
+
+    /// Add a single space character.
+    pub fn add_space(&mut self) -> LayoutResult<()> {
+        let (index, font) = self.get_font_for(' ')?;
+        let width = self.width_of(' ', &font);
+        drop(font);
+        Ok(self.units.push(Unit::Space(index, width)))
+    }
+
+    /// Start a new paragraph.
+    pub fn add_paragraph(&mut self) -> LayoutResult<()> {
+        Ok(self.units.push(Unit::Paragraph))
+    }
+
+    /// Enable or disable italics.
+    pub fn set_italic(&mut self, italic: bool) {
+        self.italic = italic;
+    }
+
+    /// Enable or disable boldface.
+    pub fn set_bold(&mut self, bold: bool) {
+        self.bold = bold;
+    }
+
+    /// Load a font that has the character we need.
+    fn get_font_for(&self, character: char) -> LayoutResult<(usize, Ref<Font>)> {
+        self.ctx.loader.get(FontQuery {
+            families: self.ctx.text_style.font_families.clone(),
+            italic: self.italic,
+            bold: self.bold,
+            character,
+        }).ok_or_else(|| LayoutError::NoSuitableFont)
+    }
+
+    /// The width of a char in a specific font.
+    fn width_of(&self, character: char, font: &Font) -> Size {
+        font.widths[font.map(character) as usize] * self.ctx.text_style.font_size
+    }
+}
+
+impl Layouter for TextLayouter<'_, '_> {
+    fn finish(self) -> LayoutResult<Layout> {
+        TextFinisher {
+            actions: vec![],
+            buffered_text: String::new(),
+            current_width: Size::zero(),
+            active_font: std::usize::MAX,
+            max_width: self.ctx.max_extent.width,
+            layouter: self,
+        }.finish()
+    }
+}
+
+/// Finishes a text layout by converting the text units into a stream of text actions.
+#[derive(Debug)]
+struct TextFinisher<'a, 'p> {
+    layouter: TextLayouter<'a, 'p>,
+    actions: Vec<TextAction>,
+    buffered_text: String,
+    current_width: Size,
+    active_font: usize,
+    max_width: Size,
+}
+
+impl<'a, 'p> TextFinisher<'a, 'p> {
+    /// Finish the layout.
+    fn finish(mut self) -> LayoutResult<Layout> {
+        // Move the units out of the layouter leaving an empty vector in place. This is needed to
+        // move the units out into the for loop while keeping the borrow checker happy.
+        let mut units = Vec::new();
+        mem::swap(&mut self.layouter.units, &mut units);
+
+        // Move to the top-left corner of the layout space.
+        self.move_start();
+
+        for unit in units {
+            match unit {
+                Unit::Paragraph => self.write_paragraph(),
+                Unit::Space(index, width) => self.write_space(index, width),
+                Unit::Text(text) => self.write_text_unit(text),
+            }
+        }
+
+        self.write_buffered_text();
+
+        Ok(Layout {
+            extent: self.layouter.ctx.max_extent.clone(),
+            actions: self.actions,
+        })
+    }
+
+    /// Add a paragraph to the output.
+    fn write_paragraph(&mut self) {
+        self.write_buffered_text();
+        self.move_newline(self.layouter.ctx.text_style.paragraph_spacing);
+    }
+
+    /// Add a single space to the output if it is not eaten by a line break.
+    fn write_space(&mut self, font: usize, width: Size) {
+        if self.would_overflow(width) {
+            self.write_buffered_text();
+            self.move_newline(1.0);
+        } else if self.current_width > Size::zero() {
+            if font != self.active_font {
+                self.write_buffered_text();
+                self.set_font(font);
+            }
+
+            self.buffered_text.push(' ');
+            self.current_width += width;
+        }
+    }
+
+    /// Add a single unit of text without breaking it apart.
+    fn write_text_unit(&mut self, text: TextUnit) {
+        if self.would_overflow(text.width) {
+            self.write_buffered_text();
+            self.move_newline(1.0);
+        }
+
+        // Finally write the word.
+        for (c, font, width) in text.chars_with_widths {
+            if font != self.active_font {
+                // If we will change the font, first write the remaining things.
+                self.write_buffered_text();
+                self.set_font(font);
+            }
+
+            self.buffered_text.push(c);
+            self.current_width += width;
+        }
+    }
+
+    /// Move to the top-left corner of the layout space.
+    fn move_start(&mut self) {
+        self.actions.push(TextAction::MoveNewline(
+            Size::zero(), self.layouter.ctx.max_extent.height
+                - Size::from_points(self.layouter.ctx.text_style.font_size)
+        ));
+    }
+
+    /// Move to the next line. A factor of 1.0 uses the default line spacing.
+    fn move_newline(&mut self, factor: f32) {
+        if self.active_font != std::usize::MAX {
+            let vertical = Size::from_points(self.layouter.ctx.text_style.font_size)
+                * self.layouter.ctx.text_style.line_spacing
+                * factor;
+
+            self.actions.push(TextAction::MoveNewline(Size::zero(), -vertical));
+            self.current_width = Size::zero();
+        }
+    }
+
+    /// Output a text action containing the buffered text and reset the buffer.
+    fn write_buffered_text(&mut self) {
+        if !self.buffered_text.is_empty() {
+            let mut buffered = String::new();
+            mem::swap(&mut self.buffered_text, &mut buffered);
+            self.actions.push(TextAction::WriteText(buffered));
+        }
+    }
+
+    /// Output an action setting a new font and update the active font.
+    fn set_font(&mut self, index: usize) {
+        self.active_font = index;
+        self.actions.push(TextAction::SetFont(index, self.layouter.ctx.text_style.font_size));
+    }
+
+    /// Check whether additional text with the given width would overflow the current line.
+    fn would_overflow(&self, width: Size) -> bool {
+        self.current_width + width > self.max_width
+    }
+}
diff --git a/src/parsing.rs b/src/parsing.rs
index 5ee8b382..727a06d8 100644
--- a/src/parsing.rs
+++ b/src/parsing.rs
@@ -172,12 +172,11 @@ impl<'s> Iterator for Tokens<'s> {
             ':' if self.state == TS::Function => Token::Colon,
             '=' if self.state == TS::Function => Token::Equals,
 
-            // Double star/underscore and dollar in bodies
+            // Double star/underscore in bodies
             '*' if self.state == TS::Body && afterwards == Some('*')
                 => self.consumed(Token::DoubleStar),
             '_' if self.state == TS::Body && afterwards == Some('_')
                 => self.consumed(Token::DoubleUnderscore),
-            '$' if self.state == TS::Body => Token::Dollar,
 
             // Escaping
             '\\' => {
@@ -393,7 +392,6 @@ impl<'s> Parser<'s> {
                 // Modifiers
                 Token::DoubleUnderscore => self.append_consumed(Node::ToggleItalics),
                 Token::DoubleStar => self.append_consumed(Node::ToggleBold),
-                Token::Dollar => self.append_consumed(Node::ToggleMath),
 
                 // Normal text
                 Token::Text(word) => self.append_consumed(Node::Text(word.to_owned())),
@@ -678,7 +676,7 @@ mod token_tests {
     use super::*;
     use Token::{Space as S, Newline as N, LeftBracket as L, RightBracket as R,
                 Colon as C, Equals as E, DoubleUnderscore as DU, DoubleStar as DS,
-                Dollar as D, Text as T, LineComment as LC, BlockComment as BC, StarSlash as SS};
+                Text as T, LineComment as LC, BlockComment as BC, StarSlash as SS};
 
     /// Test if the source code tokenizes to the tokens.
     fn test(src: &str, tokens: Vec<Token>) {
@@ -692,7 +690,6 @@ mod token_tests {
         test("Hallo", vec![T("Hallo")]);
         test("[", vec![L]);
         test("]", vec![R]);
-        test("$", vec![D]);
         test("**", vec![DS]);
         test("__", vec![DU]);
         test("\n", vec![N]);
diff --git a/src/syntax.rs b/src/syntax.rs
index cfd41719..c469686b 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -28,8 +28,6 @@ pub enum Token<'s> {
     DoubleUnderscore,
     /// Two stars, indicating bold text.
     DoubleStar,
-    /// A dollar sign, indicating mathematical content.
-    Dollar,
     /// A line comment.
     LineComment(&'s str),
     /// A block comment.
@@ -67,8 +65,6 @@ pub enum Node {
     ToggleItalics,
     /// Indicates that boldface was enabled / disabled.
     ToggleBold,
-    /// Indicates that math mode was enabled / disabled.
-    ToggleMath,
     /// Literal text.
     Text(String),
     /// A function invocation.