Thoroughly improve documentation 📝

11 files changed, 457 insertions, 351 deletions

diff --git a/src/bin/main.rs b/src/bin/main.rs
index 9e1403b2..6ca1ba56 100644
--- a/src/bin/main.rs
+++ b/src/bin/main.rs
@@ -1,16 +1,25 @@
 use std::env;
-use std::fs::File;
 use std::error::Error;
-use std::process;
+use std::fs::File;
 use std::io::Read;
 use std::path::{Path, PathBuf};
+use std::process;
 
 use typeset::Typesetter;
 use typeset::{font::FileSystemFontProvider, font_info};
 use typeset::export::pdf::PdfExporter;
 
 
+fn main() {
+    if let Err(err) = run() {
+        eprintln!("error: {}", err);
+        process::exit(1);
+    }
+}
+
+/// The actual main function.
 fn run() -> Result<(), Box<Error>> {
+    // Check the command line arguments.
     let args: Vec<String> = env::args().collect();
     if args.len() < 2 || args.len() > 3 {
         help_and_quit();
@@ -19,9 +28,10 @@ fn run() -> Result<(), Box<Error>> {
     // Open the input file.
     let mut file = File::open(&args[1]).map_err(|_| "failed to open source file")?;
 
-    // The output file name.
-    let output_filename = if args.len() <= 2 {
-        let source_path = Path::new(&args[1]);
+    let source_path = Path::new(&args[1]);
+
+    // Compute the output filename from the input filename by replacing the extension.
+    let dest_path = if args.len() <= 2 {
         let stem = source_path.file_stem().ok_or_else(|| "missing destation file name")?;
         let base = source_path.parent().ok_or_else(|| "missing destation folder")?;
         base.join(format!("{}.pdf", stem.to_string_lossy()))
@@ -29,6 +39,11 @@ fn run() -> Result<(), Box<Error>> {
         PathBuf::from(&args[2])
     };
 
+    // We do not want to overwrite the source file.
+    if dest_path == source_path {
+        return Err("source and destination path are the same".into());
+    }
+
     // Read the input file.
     let mut src = String::new();
     file.read_to_string(&mut src).map_err(|_| "failed to read from source file")?;
@@ -50,22 +65,15 @@ fn run() -> Result<(), Box<Error>> {
 
     // Export the document into a PDF file.
     let exporter = PdfExporter::new();
-    let output_file = File::create(&output_filename)?;
+    let output_file = File::create(&dest_path)?;
     exporter.export(&document, output_file)?;
 
     Ok(())
 }
 
-fn main() {
-    if let Err(err) = run() {
-        eprintln!("error: {}", err);
-        process::exit(1);
-    }
-}
-
 /// Print a usage message and quit.
 fn help_and_quit() {
-    let name = env::args().next().unwrap_or("help".to_string());
-    println!("usage: {} <source> [<destination>]", name);
+    let name = env::args().next().unwrap_or("typeset".to_string());
+    println!("usage: {} source [destination]", name);
     process::exit(0);
 }
diff --git a/src/doc.rs b/src/doc.rs
index 51583bdc..4f87a7f2 100644
--- a/src/doc.rs
+++ b/src/doc.rs
@@ -9,35 +9,28 @@ use crate::layout::Size;
 pub struct Document {
     /// The pages of the document.
     pub pages: Vec<Page>,
-    /// The fonts used in the document.
+    /// The fonts used (the page contents index into this).
     pub fonts: Vec<Font>,
 }
 
-/// A page with text contents in a document.
+/// A page of a document.
 #[derive(Debug, Clone)]
 pub struct Page {
     /// The width of the page.
     pub width: Size,
     /// The height of the page.
     pub height: Size,
-    /// Text content on the page.
-    pub text: Vec<Text>,
+    /// Text actions specifying how to draw text content on the page.
+    pub actions: Vec<TextAction>,
 }
 
-/// A series of text command, that can be written on to a page.
+/// A series of text layouting actions.
 #[derive(Debug, Clone)]
-pub struct Text {
-    /// The text commands.
-    pub commands: Vec<TextCommand>,
-}
-
-/// Different commands for rendering text.
-#[derive(Debug, Clone)]
-pub enum TextCommand {
-    /// Writing of the text.
-    Text(String),
-    /// Moves from the *start* of the current line by an (x,y) offset.
-    Move(Size, Size),
-    /// Use the indexed font in the documents font list with a given font size.
+pub enum TextAction {
+    /// Move from the _start_ of the current line by an (x, y) offset.
+    MoveNewline(Size, Size),
+    /// Write text starting at the current position.
+    WriteText(String),
+    /// Set the font by index and font size.
     SetFont(usize, f32),
 }
diff --git a/src/error.rs b/src/error.rs
index 442971d0..514eb1a8 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -2,13 +2,19 @@
 
 /// Create an error type.
 macro_rules! error_type {
-    (
+    (   // The variable used instead of self in functions
+        // followed by the error type things are happening on.
         $var:ident: $err:ident,
+        // Optionally the name of a result type to generate.
         $(res: $res:ident,)*
+        // A `Display` and `Debug` implementation.
         show: $f:ident => $show:expr,
+        // Optionally a `source` function for the `std::error::Error` trait.
         $(source: $source:expr,)*
+        // Any number of `From` implementations.
         $(from: ($from:path, $conv:expr),)*
     ) => {
+        // Possibly create a result type.
         $(type $res<T> = std::result::Result<T, $err>;)*
 
         impl std::fmt::Display for $err {
@@ -25,12 +31,14 @@ macro_rules! error_type {
         }
 
         impl std::error::Error for $err {
+            // The source method is only generated if an implementation was given.
             $(fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                 let $var = self;
                 $source
             })*
         }
 
+        // Create any number of from implementations.
         $(impl From<$from> for $err {
             fn from($var: $from) -> $err {
                 $conv
diff --git a/src/export/pdf.rs b/src/export/pdf.rs
index 55e00abb..87b9d52c 100644
--- a/src/export/pdf.rs
+++ b/src/export/pdf.rs
@@ -8,12 +8,13 @@ use pdf::doc::{Catalog, PageTree, Page, Resource, Text};
 use pdf::font::{Type0Font, CIDFont, CIDFontType, CIDSystemInfo, FontDescriptor, FontFlags};
 use pdf::font::{GlyphUnit, CMap, CMapEncoding, WidthRecord, FontStream};
 
-use crate::doc::{Document, Text as DocText, TextCommand};
+use crate::doc::{Document, TextAction};
 use crate::font::{Font, FontError};
 use crate::layout::Size;
 
 
 /// Exports documents into _PDFs_.
+#[derive(Debug)]
 pub struct PdfExporter {}
 
 impl PdfExporter {
@@ -32,6 +33,7 @@ impl PdfExporter {
 }
 
 /// Writes documents in the _PDF_ format.
+#[derive(Debug)]
 struct PdfEngine<'d, W: Write> {
     writer: PdfWriter<W>,
     doc: &'d Document,
@@ -40,7 +42,7 @@ struct PdfEngine<'d, W: Write> {
 }
 
 /// Offsets for the various groups of ids.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone)]
 struct Offsets {
     catalog: Ref,
     page_tree: Ref,
@@ -50,14 +52,13 @@ struct Offsets {
 }
 
 impl<'d, W: Write> PdfEngine<'d, W> {
-    /// Create a new _PDF_ Creator.
+    /// Create a new _PDF_ engine.
     fn new(doc: &'d Document, target: W) -> PdfResult<PdfEngine<'d, W>> {
         // Calculate a unique id for all objects that will be written.
         let catalog = 1;
         let page_tree = catalog + 1;
         let pages = (page_tree + 1, page_tree + doc.pages.len() as Ref);
-        let content_count = doc.pages.iter().flat_map(|p| p.text.iter()).count() as Ref;
-        let contents = (pages.1 + 1, pages.1 + content_count);
+        let contents = (pages.1 + 1, pages.1 + doc.pages.len() as Ref);
         let fonts = (contents.1 + 1, contents.1 + 5 * doc.fonts.len() as Ref);
         let offsets = Offsets { catalog, page_tree, pages, contents, fonts };
 
@@ -66,13 +67,13 @@ impl<'d, W: Write> PdfEngine<'d, W> {
             let mut font = 0usize;
             let mut chars = vec![HashSet::new(); doc.fonts.len()];
 
-            // Iterate through every text object on every page and find out
-            // which characters they use.
-            for text in doc.pages.iter().flat_map(|page| page.text.iter()) {
-                for command in &text.commands {
-                    match command {
-                        TextCommand::Text(string) => chars[font].extend(string.chars()),
-                        TextCommand::SetFont(id, _) => font = *id,
+            // Iterate through every text object on every page and find out which characters they
+            // use.
+            for page in &doc.pages {
+                for action in &page.actions {
+                    match action {
+                        TextAction::WriteText(string) => chars[font].extend(string.chars()),
+                        TextAction::SetFont(id, _) => font = *id,
                         _ => {},
                     }
                 }
@@ -94,7 +95,6 @@ impl<'d, W: Write> PdfEngine<'d, W> {
 
     /// Write the complete document.
     fn write(&mut self) -> PdfResult<usize> {
-        // Write all the things!
         self.writer.write_header(&Version::new(1, 7))?;
         self.write_pages()?;
         self.write_contents()?;
@@ -106,20 +106,20 @@ impl<'d, W: Write> PdfEngine<'d, W> {
 
     /// Write the document catalog and page tree.
     fn write_pages(&mut self) -> PdfResult<()> {
-        // The document catalog.
+        // The document catalog
         self.writer.write_obj(self.offsets.catalog, &Catalog::new(self.offsets.page_tree))?;
 
-        // The font resources.
+        // The font resources
         let fonts = (0 .. self.fonts.len())
             .map(|i| Resource::Font((i + 1) as u32, self.offsets.fonts.0 + 5 * i as u32));
 
-        // The root page tree.
+        // The root page tree
         self.writer.write_obj(self.offsets.page_tree, PageTree::new()
             .kids(ids(self.offsets.pages))
             .resources(fonts)
         )?;
 
-        // The page objects.
+        // The page objects
         for (id, page) in ids(self.offsets.pages).zip(&self.doc.pages) {
             self.writer.write_obj(id, Page::new(self.offsets.page_tree)
                 .media_box(Rect::new(0.0, 0.0, page.width.to_points(), page.height.to_points()))
@@ -132,26 +132,22 @@ impl<'d, W: Write> PdfEngine<'d, W> {
 
     /// Write the contents of all pages.
     fn write_contents(&mut self) -> PdfResult<()> {
-        let mut id = self.offsets.contents.0;
-        for page in &self.doc.pages {
-            for text in &page.text {
-                self.write_text(id, text)?;
-                id += 1;
-            }
+        for (id, page) in ids(self.offsets.contents).zip(&self.doc.pages) {
+            self.write_text_actions(id, &page.actions)?;
         }
         Ok(())
     }
 
-    /// Write one text object.
-    fn write_text(&mut self, id: u32, doc_text: &DocText) -> PdfResult<()> {
+    /// Write a series of text actions.
+    fn write_text_actions(&mut self, id: u32, actions: &[TextAction]) -> PdfResult<()> {
         let mut font = 0;
         let mut text = Text::new();
 
-        for command in &doc_text.commands {
-            match command {
-                TextCommand::Text(string) => { text.tj(self.fonts[font].encode(&string)); },
-                TextCommand::Move(x, y) => { text.td(x.to_points(), y.to_points()); },
-                TextCommand::SetFont(id, size) => {
+        for action in actions {
+            match action {
+                TextAction::MoveNewline(x, y) => { text.td(x.to_points(), y.to_points()); },
+                TextAction::WriteText(string) => { text.tj(self.fonts[font].encode(&string)); },
+                TextAction::SetFont(id, size) => {
                     font = *id;
                     text.tf(*id as u32 + 1, *size);
                 },
@@ -220,7 +216,7 @@ impl<'d, W: Write> PdfEngine<'d, W> {
     }
 }
 
-/// Create an iterator from reference pair.
+/// Create an iterator from a reference pair.
 fn ids((start, end): (Ref, Ref)) -> impl Iterator<Item=Ref> {
     start ..= end
 }
@@ -240,30 +236,30 @@ struct PdfFont {
 }
 
 impl PdfFont {
-    /// Create a subetted version of the font and calculate some information
-    /// needed for creating the _PDF_.
+    /// Create a subetted version of the font and calculate some information needed for creating the
+    /// _PDF_.
     fn new(font: &Font, chars: &HashSet<char>) -> PdfResult<PdfFont> {
         /// Convert a size into a _PDF_ glyph unit.
         fn size_to_glyph_unit(size: Size) -> GlyphUnit {
             (1000.0 * size.to_points()).round() as GlyphUnit
         }
 
-        // Subset the font using the selected characters
+        // Subset the font using the selected characters.
         let subsetted = font.subsetted(
             chars.iter().cloned(),
             &["head", "hhea", "maxp", "hmtx", "loca", "glyf"][..],
             &["cvt ", "prep", "fpgm", /* "OS/2", "cmap", "name", "post" */][..],
         )?;
 
-        // Specify flags for the font
+        // Specify flags for the font.
         let mut flags = FontFlags::empty();
-        flags.set(FontFlags::FIXED_PITCH, font.metrics.is_fixed_pitch);
+        flags.set(FontFlags::FIXED_PITCH, font.metrics.monospace);
         flags.set(FontFlags::SERIF, font.name.contains("Serif"));
         flags.insert(FontFlags::SYMBOLIC);
-        flags.set(FontFlags::ITALIC, font.metrics.is_italic);
+        flags.set(FontFlags::ITALIC, font.metrics.italic);
         flags.insert(FontFlags::SMALL_CAP);
 
-        // Transform the widths
+        // Transform the widths.
         let widths = subsetted.widths.iter().map(|&x| size_to_glyph_unit(x)).collect();
 
         Ok(PdfFont {
diff --git a/src/font.rs b/src/font.rs
index 17331234..1a31c681 100644
--- a/src/font.rs
+++ b/src/font.rs
@@ -25,7 +25,7 @@ use opentype::global::{MacStyleFlags, NameEntry};
 use crate::layout::Size;
 
 
-/// A loaded font, containing relevant information for typesetting.
+/// A loaded and parsed font program.
 #[derive(Debug, Clone)]
 pub struct Font {
     /// The base name of the font.
@@ -38,19 +38,19 @@ pub struct Font {
     pub widths: Vec<Size>,
     /// The fallback glyph.
     pub default_glyph: u16,
-    /// The relevant metrics of this font.
+    /// The typesetting-relevant metrics of this font.
     pub metrics: FontMetrics,
 }
 
 impl Font {
-    /// Create a new font from a font program.
+    /// Create a new font from a raw font program.
     pub fn new(program: Vec<u8>) -> FontResult<Font> {
-        // Create opentype reader to parse font tables
+        // Create an OpentypeReader to parse the font tables.
         let cursor = Cursor::new(&program);
         let mut reader = OpenTypeReader::new(cursor);
 
         // Read the relevant tables
-        // (all of these are required by the OpenType specification)
+        // (all of these are required by the OpenType specification, so we expect them).
         let head = reader.read_table::<Header>()?;
         let name = reader.read_table::<Name>()?;
         let os2 = reader.read_table::<OS2>()?;
@@ -58,21 +58,21 @@ impl Font {
         let hmtx = reader.read_table::<HorizontalMetrics>()?;
         let post = reader.read_table::<Post>()?;
 
-        // Create conversion function between font units and sizes
+        // Create a conversion function between font units and sizes.
         let font_unit_ratio = 1.0 / (head.units_per_em as f32);
         let font_unit_to_size = |x| Size::from_points(font_unit_ratio * x as f32);
 
-        // Find out the name of the font
+        // Find out the name of the font.
         let font_name = name.get_decoded(NameEntry::PostScriptName)
             .unwrap_or_else(|| "unknown".to_owned());
 
-        // Convert the widths
+        // Convert the widths from font units to sizes.
         let widths = hmtx.metrics.iter().map(|m| font_unit_to_size(m.advance_width)).collect();
 
-        // Calculate some metrics
+        // Calculate the typesetting-relevant metrics.
         let metrics = FontMetrics {
-            is_italic: head.mac_style.contains(MacStyleFlags::ITALIC),
-            is_fixed_pitch: post.is_fixed_pitch,
+            italic: head.mac_style.contains(MacStyleFlags::ITALIC),
+            monospace: post.is_fixed_pitch,
             italic_angle: post.italic_angle.to_f32(),
             bounding_box: [
                 font_unit_to_size(head.x_min),
@@ -105,6 +105,7 @@ impl Font {
     /// Encode the given text for this font (into glyph ids).
     #[inline]
     pub fn encode(&self, text: &str) -> Vec<u8> {
+        // Each glyph id takes two bytes that we encode in big endian.
         let mut bytes = Vec::with_capacity(2 * text.len());
         for glyph in text.chars().map(|c| self.map(c)) {
             bytes.push((glyph >> 8) as u8);
@@ -113,62 +114,37 @@ impl Font {
         bytes
     }
 
-    /// Generate a subsetted version of this font including only the chars listed in
-    /// `chars`.
+    /// Generate a subsetted version of this font including only the chars listed in `chars`.
     ///
-    /// All needed tables will be included (returning an error if a table was not present
-    /// in the  source font) and optional tables will be included if there were present
-    /// in the source font. All other tables will be dropped.
-    pub fn subsetted<C, I, S>(
-        &self,
-        chars: C,
-        needed_tables: I,
-        optional_tables: I,
-    ) -> Result<Font, FontError>
+    /// All needed tables will be included (returning an error if a table was not present in the
+    /// source font) and optional tables will be included if they were present in the source font.
+    /// All other tables will be dropped.
+    #[inline]
+    pub fn subsetted<C, I, S>(&self, chars: C, needed_tables: I, optional_tables: I)
+        -> Result<Font, FontError>
     where
         C: IntoIterator<Item=char>,
         I: IntoIterator<Item=S>,
-        S: AsRef<str>,
+        S: AsRef<str>
     {
-        let mut chars: Vec<char> = chars.into_iter().collect();
-        chars.sort();
-
-        let mut reader = OpenTypeReader::from_slice(&self.program);
-        let outlines = reader.outlines()?;
-        let tables = reader.tables()?.to_vec();
-
-        let subsetter = Subsetter {
-            font: &self,
-            reader,
-            outlines,
-            tables,
-            cmap: None,
-            hmtx: None,
-            loca: None,
-            glyphs: Vec::with_capacity(1 + chars.len()),
-            chars,
-            records: vec![],
-            body: vec![],
-        };
-
-        subsetter.subset(needed_tables, optional_tables)
+        Subsetter::subset(self, chars, needed_tables, optional_tables)
     }
 }
 
-/// Font metrics relevant to the typesetting engine.
+/// Font metrics relevant to the typesetting or exporting processes.
 #[derive(Debug, Copy, Clone)]
 pub struct FontMetrics {
     /// Whether the font is italic.
-    pub is_italic: bool,
-    /// Whether font is fixed pitch.
-    pub is_fixed_pitch: bool,
-    /// The angle of italics.
+    pub italic: bool,
+    /// Whether font is monospace.
+    pub monospace: bool,
+    /// The angle of text in italics.
     pub italic_angle: f32,
     /// The glyph bounding box: [x_min, y_min, x_max, y_max],
     pub bounding_box: [Size; 4],
-    /// The typographics ascender relevant for line spacing.
+    /// The typographics ascender.
     pub ascender: Size,
-    /// The typographics descender relevant for line spacing.
+    /// The typographics descender.
     pub descender: Size,
     /// The approximate height of capital letters.
     pub cap_height: Size,
@@ -176,25 +152,34 @@ pub struct FontMetrics {
     pub weight_class: u16,
 }
 
-/// Describes a font.
+/// Categorizes a font.
 ///
 /// Can be constructed conveniently with the [`font_info`] macro.
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
 pub struct FontInfo {
     /// The font families this font is part of.
     pub families: Vec<FontFamily>,
-    /// Whether the font is in italics.
+    /// Whether the font is italic.
     pub italic: bool,
-    /// Whether the font is bold.
+    /// Whether the font bold.
     pub bold: bool,
 }
 
+/// A family of fonts.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub enum FontFamily {
+    Serif,
+    SansSerif,
+    Monospace,
+    /// A custom class like _Arial_ or _Times_.
+    Named(String),
+}
+
 /// A macro to create [FontInfos](crate::font::FontInfo) easily.
 ///
-/// Accepts first a bracketed (ordered) list of font families. Allowed are string expressions
-/// as well as the three base families `SansSerif`, `Serif` and `Monospace`.
-///
-/// Then there may follow (separated by commas) the keywords `italic` and/or `bold`.
+/// Accepts first a bracketed, ordered list of font families. Allowed are string expressions as well
+/// as the three base families `SansSerif`, `Serif` and `Monospace`. Then there may follow
+/// (separated by commas) the keywords `italic` and/or `bold`.
 ///
 /// # Examples
 /// The font _Noto Sans_ in regular typeface.
@@ -259,48 +244,43 @@ macro_rules! font_info {
     (@__gen Monospace) => {  $crate::font::FontFamily::Monospace };
 }
 
-/// A family of fonts (either generic or named).
-#[derive(Debug, Clone, Eq, PartialEq, Hash)]
-pub enum FontFamily {
-    SansSerif,
-    Serif,
-    Monospace,
-    Named(String),
-}
+//------------------------------------------------------------------------------------------------//
 
 /// A type that provides fonts.
 pub trait FontProvider {
-    /// Returns the font with the given info if this provider has it.
+    /// Returns a font with the given info if this provider has one.
     fn get(&self, info: &FontInfo) -> Option<Box<dyn FontData>>;
 
     /// The available fonts this provider can serve. While these should generally be retrievable
-    /// through the `get` method, it is not guaranteed that a font info that is contained here
-    /// yields a `Some` value when passed into `get`.
-    fn available<'a>(&'a self) -> &'a [FontInfo];
+    /// through the `get` method, it does not have to be guaranteed that a font info, that is
+    /// contained, here yields a `Some` value when passed into `get`.
+    fn available<'p>(&'p self) -> &'p [FontInfo];
 }
 
 /// A wrapper trait around `Read + Seek`.
 ///
-/// This type is needed because currently you can't make a trait object
-/// with two traits, like `Box<dyn Read + Seek>`.
-/// Automatically implemented for all types that are [`Read`] and [`Seek`].
+/// This type is needed because currently you can't make a trait object with two traits, like
+/// `Box<dyn Read + Seek>`. Automatically implemented for all types that are [`Read`] and [`Seek`].
 pub trait FontData: Read + Seek {}
 impl<T> FontData for T where T: Read + Seek {}
 
 /// A font provider serving fonts from a folder on the local file system.
+#[derive(Debug)]
 pub struct FileSystemFontProvider {
+    /// The root folder.
     base: PathBuf,
+    /// Paths of the fonts relative to the `base` path.
     paths: Vec<PathBuf>,
+    /// The information for the font with the same index in `paths`.
     infos: Vec<FontInfo>,
 }
 
 impl FileSystemFontProvider {
-    /// Create a new provider from a folder and an iterator of pairs of
-    /// font paths and font infos.
+    /// Create a new provider from a folder and an iterator of pairs of font paths and font infos.
     ///
     /// # Example
-    /// Serve the two fonts `NotoSans-Regular` and `NotoSans-Italic` from the local
-    /// folder `../fonts`.
+    /// Serve the two fonts `NotoSans-Regular` and `NotoSans-Italic` from the local folder
+    /// `../fonts`.
     /// ```
     /// # use typeset::{font::FileSystemFontProvider, font_info};
     /// FileSystemFontProvider::new("../fonts", vec![
@@ -315,8 +295,8 @@ impl FileSystemFontProvider {
         I: IntoIterator<Item = (P, FontInfo)>,
         P: Into<PathBuf>,
     {
-        // Find out how long the iterator is at least, to reserve the correct
-        // capacity for the vectors.
+        // Find out how long the iterator is at least, to reserve the correct capacity for the
+        // vectors.
         let iter = infos.into_iter();
         let min = iter.size_hint().0;
 
@@ -339,42 +319,52 @@ impl FileSystemFontProvider {
 impl FontProvider for FileSystemFontProvider {
     #[inline]
     fn get(&self, info: &FontInfo) -> Option<Box<dyn FontData>> {
+        // Find the index of the font in both arrays (early exit if there is no match).
         let index = self.infos.iter().position(|i| i == info)?;
+
+        // Open the file and return a boxed reader operating on it.
         let path = &self.paths[index];
         let file = File::open(self.base.join(path)).ok()?;
         Some(Box::new(BufReader::new(file)) as Box<FontData>)
     }
 
     #[inline]
-    fn available<'a>(&'a self) -> &'a [FontInfo] {
+    fn available<'p>(&'p self) -> &'p [FontInfo] {
         &self.infos
     }
 }
 
-/// Serves matching fonts given a query.
+//------------------------------------------------------------------------------------------------//
+
+/// Serves fonts matching queries.
 pub struct FontLoader<'p> {
     /// The font providers.
     providers: Vec<&'p (dyn FontProvider + 'p)>,
-    /// All available fonts indexed by provider.
+    /// The fonts available from each provider (indexed like `providers`).
     provider_fonts: Vec<&'p [FontInfo]>,
-    /// The internal state.
+    /// The internal state. Uses interior mutability because the loader works behind
+    /// an immutable reference to ease usage.
     state: RefCell<FontLoaderState<'p>>,
 }
 
-/// Internal state of the font loader (wrapped in a RefCell).
+/// Internal state of the font loader (seperated to wrap it in a `RefCell`).
 struct FontLoaderState<'p> {
-    /// The loaded fonts along with their external indices.
+    /// The loaded fonts alongside their external indices. Some fonts may not have external indices
+    /// because they were loaded but did not contain the required character. However, these are
+    /// still stored because they may be needed later. The index is just set to `None` then.
     fonts: Vec<(Option<usize>, Font)>,
-    /// Allows to retrieve cached results for queries.
-    query_cache: HashMap<FontQuery<'p>, usize>,
-    /// Allows to lookup fonts by their infos.
+    /// Allows to retrieve a font (index) quickly if a query was submitted before.
+    query_cache: HashMap<FontQuery, usize>,
+    /// Allows to re-retrieve loaded fonts by their info instead of loading them again.
     info_cache: HashMap<&'p FontInfo, usize>,
-    /// Indexed by outside and indices maps to internal indices.
+    /// Indexed by external indices (the ones inside the tuples in the `fonts` vector) and maps to
+    /// internal indices (the actual indices into the vector).
     inner_index: Vec<usize>,
 }
 
 impl<'p> FontLoader<'p> {
-    /// Create a new font loader.
+    /// Create a new font loader using a set of providers.
+    #[inline]
     pub fn new<P: 'p>(providers: &'p [P]) -> FontLoader<'p> where P: AsRef<dyn FontProvider + 'p> {
         let providers: Vec<_> = providers.iter().map(|p| p.as_ref()).collect();
         let provider_fonts = providers.iter().map(|prov| prov.available()).collect();
@@ -391,12 +381,13 @@ impl<'p> FontLoader<'p> {
         }
     }
 
-    /// Return the best matching font and it's index (if there is any) given the query.
-    pub fn get(&self, query: FontQuery<'p>) -> Option<(usize, Ref<Font>)> {
-        // Check if we had the exact same query before.
+    /// Returns the font (and its index) best matching the query, if there is any.
+    pub fn get(&self, query: FontQuery) -> Option<(usize, Ref<Font>)> {
+        // Load results from the cache, if we had the exact same query before.
         let state = self.state.borrow();
         if let Some(&index) = state.query_cache.get(&query) {
-            // That this is the query cache means it must has an index as we've served it before.
+            // The font must have an external index already because it is in the query cache.
+            // It has been served before.
             let extern_index = state.fonts[index].0.unwrap();
             let font = Ref::map(state, |s| &s.fonts[index].1);
 
@@ -404,94 +395,99 @@ impl<'p> FontLoader<'p> {
         }
         drop(state);
 
-        // Go over all font infos from all font providers that match the query.
-        for family in query.families {
+        // The outermost loop goes over the families because we want to serve
+        // the font that matches the first possible family.
+        for family in &query.families {
+            // For each family now go over all font infos from all font providers.
             for (provider, infos) in self.providers.iter().zip(&self.provider_fonts) {
                 for info in infos.iter() {
-                    // Check whether this info matches the query.
-                    if Self::matches(query, family, info) {
+                    // Proceed only if this font matches the query.
+                    if Self::matches(&query, family, info) {
                         let mut state = self.state.borrow_mut();
 
-                        // Check if we have already loaded this font before.
-                        // Otherwise we'll fetch the font from the provider.
+                        // Check if we have already loaded this font before, otherwise, we will
+                        // load it from the provider. Anyway, have it stored and find out its
+                        // internal index.
                         let index = if let Some(&index) = state.info_cache.get(info) {
                             index
                         } else if let Some(mut source) = provider.get(info) {
-                            // Read the font program into a vec.
+                            // Read the font program into a vector and parse it.
                             let mut program = Vec::new();
                             source.read_to_end(&mut program).ok()?;
-
-                            // Create a font from it.
                             let font = Font::new(program).ok()?;
 
-                            // Insert it into the storage.
+                            // Insert it into the storage and cache it by its info.
                             let index = state.fonts.len();
                             state.info_cache.insert(info, index);
                             state.fonts.push((None, font));
 
                             index
                         } else {
+                            // Strangely, this provider lied and cannot give us the promised font.
                             continue;
                         };
 
-                        // Check whether this font has the character we need.
+                        // Proceed if this font has the character we need.
                         let has_char = state.fonts[index].1.mapping.contains_key(&query.character);
                         if has_char {
-                            // We can take this font, so we store the query.
+                            // This font is suitable, thus we cache the query result.
                             state.query_cache.insert(query, index);
 
-                            // Now we have to find out the external index of it, or assign a new
-                            // one if it has not already one.
-                            let maybe_extern_index = state.fonts[index].0;
-                            let extern_index = maybe_extern_index.unwrap_or_else(|| {
+                            // Now we have to find out the external index of it or assign a new one
+                            // if it has none.
+                            let external_index = state.fonts[index].0.unwrap_or_else(|| {
                                 // We have to assign an external index before serving.
-                                let extern_index = state.inner_index.len();
+                                let new_index = state.inner_index.len();
                                 state.inner_index.push(index);
-                                state.fonts[index].0 =  Some(extern_index);
-                                extern_index
+                                state.fonts[index].0 =  Some(new_index);
+                                new_index
                             });
 
-                            // Release the mutable borrow and borrow immutably.
+                            // Release the mutable borrow to be allowed to borrow immutably.
                             drop(state);
-                            let font = Ref::map(self.state.borrow(), |s| &s.fonts[index].1);
 
-                            // Finally we can return it.
-                            return Some((extern_index, font));
+                            // Finally, get a reference to the actual font.
+                            let font = Ref::map(self.state.borrow(), |s| &s.fonts[index].1);
+                            return Some((external_index, font));
                         }
                     }
                 }
             }
         }
 
+        // Not a single match!
         None
     }
 
-    /// Return a loaded font at an index. Panics if the index is out of bounds.
+    /// Return the font previously loaded at this index. Panics if the index is not assigned.
+    #[inline]
     pub fn get_with_index(&self, index: usize) -> Ref<Font> {
         let state = self.state.borrow();
         let internal = state.inner_index[index];
         Ref::map(state, |s| &s.fonts[internal].1)
     }
 
-    /// Return the list of fonts.
+    /// Move the whole list of fonts out.
     pub fn into_fonts(self) -> Vec<Font> {
-        // Sort the fonts by external key so that they are in the correct order.
+        // Sort the fonts by external index so that they are in the correct order.
+        // All fonts that were cached but not used by the outside are sorted to the back
+        // and are removed in the next step.
         let mut fonts = self.state.into_inner().fonts;
         fonts.sort_by_key(|&(maybe_index, _)| match maybe_index {
-            Some(index) => index as isize,
-            None => -1,
+            Some(index) => index,
+            None => std::usize::MAX,
         });
 
-        // Remove the fonts that are not used from the outside
+        // Remove the fonts that are not used from the outside.
         fonts.into_iter().filter_map(|(maybe_index, font)| {
-            maybe_index.map(|_| font)
+            if maybe_index.is_some() { Some(font) } else { None }
         }).collect()
     }
 
-    /// Check whether the query and the current family match the info.
-    fn matches(query: FontQuery, family: &FontFamily, info: &FontInfo) -> bool {
-        info.families.contains(family)
-          && info.italic == query.italic && info.bold == query.bold
+    /// Checks whether the query and the family match the info.
+    fn matches(query: &FontQuery, family: &FontFamily, info: &FontInfo) -> bool {
+        info.italic == query.italic && info.bold == query.bold
+            && info.families.contains(family)
     }
 }
 
@@ -510,21 +506,25 @@ impl Debug for FontLoader<'_> {
 }
 
 /// A query for a font with specific properties.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
-pub struct FontQuery<'a> {
-    /// A fallback list of font families to accept. The first family in this list, that also
-    /// satisfies the other conditions, shall be returned.
-    pub families: &'a [FontFamily],
-    /// Whether the font shall be in italics.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct FontQuery {
+    /// Which character is needed.
+    pub character: char,
+    /// Whether the font should be in italics.
     pub italic: bool,
-    /// Whether the font shall be in boldface.
+    /// Whether the font should be in boldface.
     pub bold: bool,
-    /// Which character we need.
-    pub character: char,
+    /// A fallback list of font families to accept. The font matching the first possible family in
+    /// this list satisfying all other constraints should be returned.
+    pub families: Vec<FontFamily>,
 }
 
+//------------------------------------------------------------------------------------------------//
+
+/// Subsets a font.
+#[derive(Debug)]
 struct Subsetter<'a> {
-    // Original font
+    // The original font
     font: &'a Font,
     reader: OpenTypeReader<Cursor<&'a [u8]>>,
     outlines: Outlines,
@@ -534,17 +534,53 @@ struct Subsetter<'a> {
     loca: Option<Vec<u32>>,
     glyphs: Vec<u16>,
 
-    // Subsetted font
+    // The subsetted font
     chars: Vec<char>,
     records: Vec<TableRecord>,
     body: Vec<u8>,
 }
 
 impl<'a> Subsetter<'a> {
-    fn subset<I, S>(mut self, needed_tables: I, optional_tables: I) -> FontResult<Font>
+    /// Subset a font. See [`Font::subetted`] for more details.
+    pub fn subset<C, I, S>(
+        font: &Font,
+        chars: C,
+        needed_tables: I,
+        optional_tables: I,
+    ) -> Result<Font, FontError>
+    where
+        C: IntoIterator<Item=char>,
+        I: IntoIterator<Item=S>,
+        S: AsRef<str>
+    {
+        // Parse some header information and keep the reading around.
+        let mut reader = OpenTypeReader::from_slice(&font.program);
+        let outlines = reader.outlines()?;
+        let tables = reader.tables()?.to_vec();
+
+        let chars: Vec<_> = chars.into_iter().collect();
+
+        let subsetter = Subsetter {
+            font,
+            reader,
+            outlines,
+            tables,
+            cmap: None,
+            hmtx: None,
+            loca: None,
+            glyphs: Vec::with_capacity(1 + chars.len()),
+            chars,
+            records: vec![],
+            body: vec![],
+        };
+
+        subsetter.run(needed_tables, optional_tables)
+    }
+
+    fn run<I, S>(mut self, needed_tables: I, optional_tables: I) -> FontResult<Font>
     where I: IntoIterator<Item=S>, S: AsRef<str> {
-        // Find out which glyphs to include based on which characters we want
-        // and which glyphs are used by composition.
+        // Find out which glyphs to include based on which characters we want and which glyphs are
+        // used by other composite glyphs.
         self.build_glyphs()?;
 
         // Iterate through the needed tables first
@@ -553,7 +589,7 @@ impl<'a> Subsetter<'a> {
             let tag: Tag = table.parse()
                 .map_err(|_| FontError::UnsupportedTable(table.to_string()))?;
 
-            if self.contains(tag) {
+            if self.contains_table(tag) {
                 self.write_table(tag)?;
             } else {
                 return Err(FontError::MissingTable(tag.to_string()));
@@ -566,7 +602,7 @@ impl<'a> Subsetter<'a> {
             let tag: Tag = table.parse()
                 .map_err(|_| FontError::UnsupportedTable(table.to_string()))?;
 
-            if self.contains(tag) {
+            if self.contains_table(tag) {
                 self.write_table(tag)?;
             }
         }
@@ -598,16 +634,16 @@ impl<'a> Subsetter<'a> {
         self.read_cmap()?;
         let cmap = self.cmap.as_ref().unwrap();
 
-        // The default glyph should be always present.
+        // The default glyph should be always present, others only if used.
         self.glyphs.push(self.font.default_glyph);
         for &c in &self.chars {
-            self.glyphs.push(cmap.get(c).ok_or_else(|| FontError::MissingCharacter(c))?)
+            let glyph = cmap.get(c).ok_or_else(|| FontError::MissingCharacter(c))?;
+            self.glyphs.push(glyph);
         }
 
-        // Composite glyphs may need additional glyphs we have not yet in our list.
-        // So now we have a look at the glyf table to check that and add glyphs
-        // we need additionally.
-        if self.contains("glyf".parse().unwrap()) {
+        // Composite glyphs may need additional glyphs we do not have in our list yet. So now we
+        // have a look at the `glyf` table to check that and add glyphs we need additionally.
+        if self.contains_table("glyf".parse().unwrap()) {
             self.read_loca()?;
             let loca = self.loca.as_ref().unwrap();
             let table = self.get_table_data("glyf".parse().unwrap())?;
@@ -880,7 +916,8 @@ impl<'a> Subsetter<'a> {
             .take_bytes()
     }
 
-    fn contains(&self, tag: Tag) -> bool {
+    /// Whether this font contains some table.
+    fn contains_table(&self, tag: Tag) -> bool {
         self.tables.binary_search_by_key(&tag, |r| r.tag).is_ok()
     }
 
@@ -916,8 +953,8 @@ impl<'a> Subsetter<'a> {
     }
 }
 
-/// Calculate a checksum over the sliced data as sum of u32's.
-/// The data length has to be a multiple of four.
+/// Calculate a checksum over the sliced data as sum of u32's. The data length has to be a multiple
+/// of four.
 fn calculate_check_sum(data: &[u8]) -> u32 {
     let mut sum = 0u32;
     data.chunks_exact(4).for_each(|c| {
@@ -931,13 +968,12 @@ fn calculate_check_sum(data: &[u8]) -> u32 {
     sum
 }
 
+/// Helper trait to create subsetting errors more easily.
 trait TakeInvalid<T>: Sized {
-    /// Pull the type out of the option, returning a subsetting error
-    /// about an invalid font wrong.
+    /// Pull the type out of the option, returning an invalid font error if self was not valid.
     fn take_invalid<S: Into<String>>(self, message: S) -> FontResult<T>;
 
-    /// Pull the type out of the option, returning an error about missing
-    /// bytes if it is nothing.
+    /// Same as above with predefined message "expected more bytes".
     fn take_bytes(self) -> FontResult<T> {
         self.take_invalid("expected more bytes")
     }
@@ -949,15 +985,17 @@ impl<T> TakeInvalid<T> for Option<T> {
     }
 }
 
+//------------------------------------------------------------------------------------------------//
+
 /// The error type for font operations.
 pub enum FontError {
     /// The font file is incorrect.
     InvalidFont(String),
     /// A requested table was not present in the source font.
     MissingTable(String),
-    /// The table is unknown to the subsetting engine (unimplemented or invalid).
+    /// The table is unknown to the subsetting engine.
     UnsupportedTable(String),
-    /// A requested character was not present in the source font.
+    /// A character requested for subsetting was not present in the source font.
     MissingCharacter(char),
     /// An I/O Error occured while reading the font program.
     Io(io::Error),
diff --git a/src/func.rs b/src/func.rs
index ef5120df..402bf6fa 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -4,27 +4,27 @@ use std::any::Any;
 use std::collections::HashMap;
 use std::fmt::{self, Debug, Formatter};
 
-use crate::syntax::FuncHeader;
-use crate::parsing::{ParseContext, ParseResult};
 use crate::layout::{Layout, LayoutContext, LayoutResult};
+use crate::parsing::{ParseContext, ParseResult};
+use crate::syntax::FuncHeader;
 
 
-/// Types that act as functions.
+/// Typesetting function types.
 ///
-/// These types have to be able to parse tokens into themselves and store the
-/// relevant information from the parsing to do their role in typesetting later.
+/// These types have to be able to parse tokens into themselves and store the relevant information
+/// from the parsing to do their role in typesetting later.
 ///
-/// The trait `FunctionBounds` is automatically implemented for types which can be
-/// used as functions, that is they fulfill the bounds  `Debug + PartialEq + 'static`.
+/// The trait `FunctionBounds` is automatically implemented for types which can be used as
+/// functions, that is they fulfill the bounds `Debug + PartialEq + 'static`.
 pub trait Function: FunctionBounds {
-    /// Parse the header and body into this function given this context.
+    /// Parse the header and body into this function given a context.
     fn parse(header: &FuncHeader, body: Option<&str>, ctx: &ParseContext)
         -> ParseResult<Self> where Self: Sized;
 
     /// Layout this function given a context.
     ///
-    /// Returns optionally the resulting layout and a if changes to the context
-    /// should be made new context.
+    /// Returns optionally the resulting layout and a new context if changes to the context should
+    /// be made.
     fn layout(&self, ctx: &LayoutContext)
         -> LayoutResult<(Option<Layout>, Option<LayoutContext>)>;
 }
@@ -37,8 +37,8 @@ impl PartialEq for dyn Function {
 
 /// A helper trait that describes requirements for types that can implement [`Function`].
 ///
-/// Automatically implemented for all types which fulfill to the bounds
-/// `Debug + PartialEq + 'static`. There should be no need to implement this manually.
+/// Automatically implemented for all types which fulfill to the bounds `Debug + PartialEq +
+/// 'static`. There should be no need to implement this manually.
 pub trait FunctionBounds: Debug {
     /// Cast self into `Any`.
     fn help_cast_as_any(&self) -> &dyn Any;
@@ -66,7 +66,7 @@ pub struct Scope {
     parsers: HashMap<String, Box<ParseFunc>>,
 }
 
-/// A function which transforms a parsing context into a boxed function.
+/// A function which parses a function invocation into a function type.
 type ParseFunc = dyn Fn(&FuncHeader, Option<&str>, &ParseContext)
                        -> ParseResult<Box<dyn Function>>;
 
@@ -81,7 +81,7 @@ impl Scope {
         Scope::new()
     }
 
-    /// Add a function type to the scope with a given name.
+    /// Add a function type to the scope giving it a name.
     pub fn add<F: Function + 'static>(&mut self, name: &str) {
         self.parsers.insert(
             name.to_owned(),
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index 7bc62bd3..f2ca3029 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -1,73 +1,73 @@
-//! Layouting engine.
+//! The layouting engine.
 
-use crate::doc::Document;
+use crate::doc::{Document, Page, TextAction};
 use crate::font::{Font, FontLoader, FontFamily, FontError};
 use crate::syntax::SyntaxTree;
 
 mod size;
+
 pub use size::Size;
 
 
-/// Layout a syntax tree given a context.
+/// Layout a syntax tree in a given context.
 #[allow(unused_variables)]
 pub fn layout(tree: &SyntaxTree, ctx: &LayoutContext) -> LayoutResult<Layout> {
-    Ok(Layout {})
+    Ok(Layout {
+        extent: LayoutDimensions { width: Size::zero(), height: Size::zero() },
+        actions: vec![],
+    })
 }
 
 /// A collection of layouted content.
-pub struct Layout {}
+#[derive(Debug, Clone)]
+pub struct Layout {
+    /// The extent of this layout into all directions.
+    extent: LayoutDimensions,
+    /// Actions composing this layout.
+    actions: Vec<TextAction>,
+}
 
 impl Layout {
     /// Convert this layout into a document given the list of fonts referenced by it.
     pub fn into_document(self, fonts: Vec<Font>) -> Document {
         Document {
-            pages: vec![],
+            pages: vec![Page {
+                width: self.extent.width,
+                height: self.extent.height,
+                actions: self.actions,
+            }],
             fonts,
         }
     }
 }
 
+/// Types supporting some kind of layouting.
+pub trait Layouter {
+    /// Finishing the current layouting process and return a layout.
+    fn finish(self) -> LayoutResult<Layout>;
+}
+
 /// The context for layouting.
+#[derive(Debug, Clone)]
 pub struct LayoutContext<'a, 'p> {
+    /// Loads fonts matching queries.
     pub loader: &'a FontLoader<'p>,
+    /// The spacial constraints to layout in.
+    pub max_extent: LayoutDimensions,
+    /// Base style to set text with.
+    pub text_style: TextStyle,
 }
 
-/// Default styles for pages.
-#[derive(Debug, Clone, PartialEq)]
-pub struct PageStyle {
-    /// The width of the paper.
+#[derive(Debug, Clone)]
+pub struct LayoutDimensions {
+    /// Horizontal extent.
     pub width: Size,
-    /// The height of the paper.
+    /// Vertical extent.
     pub height: Size,
-
-    /// The left margin of the paper.
-    pub margin_left: Size,
-    /// The top margin of the paper.
-    pub margin_top: Size,
-    /// The right margin of the paper.
-    pub margin_right: Size,
-    /// The bottom margin of the paper.
-    pub margin_bottom: Size,
-}
-
-impl Default for PageStyle {
-    fn default() -> PageStyle {
-        PageStyle {
-            // A4 paper.
-            width: Size::from_mm(210.0),
-            height: Size::from_mm(297.0),
-
-            // Margins. A bit more on top and bottom.
-            margin_left: Size::from_cm(3.0),
-            margin_top: Size::from_cm(3.0),
-            margin_right: Size::from_cm(3.0),
-            margin_bottom: Size::from_cm(3.0),
-        }
-    }
 }
 
-/// Default styles for texts.
-#[derive(Debug, Clone, PartialEq)]
+/// Default styles for text.
+#[derive(Debug, Clone)]
 pub struct TextStyle {
     /// A fallback list of font families to use.
     pub font_families: Vec<FontFamily>,
@@ -75,7 +75,7 @@ pub struct TextStyle {
     pub font_size: f32,
     /// The line spacing (as a multiple of the font size).
     pub line_spacing: f32,
-    /// The spacing for paragraphs (as a multiple of the line spacing).
+    /// The paragraphs spacing (as a multiple of the line spacing).
     pub paragraph_spacing: f32,
 }
 
@@ -92,10 +92,44 @@ impl Default for TextStyle {
     }
 }
 
+/// Default styles for pages.
+#[derive(Debug, Clone)]
+pub struct PageStyle {
+    /// The width of the page.
+    pub width: Size,
+    /// The height of the page.
+    pub height: Size,
+
+    /// The amount of white space on the left side.
+    pub margin_left: Size,
+    /// The amount of white space on the top side.
+    pub margin_top: Size,
+    /// The amount of white space on the right side.
+    pub margin_right: Size,
+    /// The amount of white space on the bottom side.
+    pub margin_bottom: Size,
+}
+
+impl Default for PageStyle {
+    fn default() -> PageStyle {
+        PageStyle {
+            // A4 paper.
+            width: Size::from_mm(210.0),
+            height: Size::from_mm(297.0),
+
+            // All the same margins.
+            margin_left: Size::from_cm(3.0),
+            margin_top: Size::from_cm(3.0),
+            margin_right: Size::from_cm(3.0),
+            margin_bottom: Size::from_cm(3.0),
+        }
+    }
+}
+
 /// The error type for layouting.
 pub enum LayoutError {
     /// There was no suitable font.
-    MissingFont,
+    NoSuitableFont,
     /// An error occured while gathering font data.
     Font(FontError),
 }
@@ -106,7 +140,7 @@ pub type LayoutResult<T> = Result<T, LayoutError>;
 error_type! {
     err: LayoutError,
     show: f => match err {
-        LayoutError::MissingFont => write!(f, "missing font"),
+        LayoutError::NoSuitableFont => write!(f, "no suitable font"),
         LayoutError::Font(err) => write!(f, "font error: {}", err),
     },
     source: match err {
diff --git a/src/layout/size.rs b/src/layout/size.rs
index bf79a3c4..d0b557d7 100644
--- a/src/layout/size.rs
+++ b/src/layout/size.rs
@@ -1,10 +1,12 @@
+//! A general spacing type.
+
 use std::cmp::Ordering;
 use std::fmt::{self, Display, Debug, Formatter};
 use std::iter::Sum;
 use std::ops::*;
 
 
-/// A general size (unit of length) type.
+/// A general spacing type.
 #[derive(Copy, Clone, PartialEq, Default)]
 pub struct Size {
     /// The size in typographic points (1/72 inches).
@@ -12,39 +14,39 @@ pub struct Size {
 }
 
 impl Size {
-    /// Create an zeroed size.
+    /// Create a zeroed size.
     #[inline]
     pub fn zero() -> Size { Size { points: 0.0 } }
 
-    /// Create a size from a number of points.
+    /// Create a size from an amount of points.
     #[inline]
     pub fn from_points(points: f32) -> Size { Size { points } }
 
-    /// Create a size from a number of inches.
+    /// Create a size from an amount of inches.
     #[inline]
     pub fn from_inches(inches: f32) -> Size { Size { points: 72.0 * inches } }
 
-    /// Create a size from a number of millimeters.
+    /// Create a size from an amount of millimeters.
     #[inline]
     pub fn from_mm(mm: f32) -> Size { Size { points: 2.83465 * mm  } }
 
-    /// Create a size from a number of centimeters.
+    /// Create a size from an amount of centimeters.
     #[inline]
     pub fn from_cm(cm: f32) -> Size { Size { points: 28.3465 * cm } }
 
-    /// Create a size from a number of points.
+    /// Convert this size into points.
     #[inline]
     pub fn to_points(&self) -> f32 { self.points }
 
-    /// Create a size from a number of inches.
+    /// Convert this size into inches.
     #[inline]
     pub fn to_inches(&self) -> f32 { self.points * 0.0138889 }
 
-    /// Create a size from a number of millimeters.
+    /// Convert this size into millimeters.
     #[inline]
     pub fn to_mm(&self) -> f32 { self.points * 0.352778 }
 
-    /// Create a size from a number of centimeters.
+    /// Convert this size into centimeters.
     #[inline]
     pub fn to_cm(&self) -> f32 { self.points * 0.0352778 }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 9a1082c6..b088f633 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,13 +1,13 @@
 //! The compiler for the _Typeset_ typesetting language 📜.
 //!
 //! # Compilation
-//! - **Parsing:** The parsing step first transforms a plain string into an
-//!   [iterator of tokens](crate::parsing::Tokens). Then the [parser](crate::parsing::Parser)
-//!   operates on that to construct a syntax tree. The structures describing the tree can be found
-//!   in the [syntax] module.
-//! - **Layouting:** The next step is to transform the syntax tree into a portable representation
-//!   of the typesetted document. Types for these can be found in the [doc] and [layout] modules.
-//!   This representation contains already the finished layout.
+//! - **Parsing:** The parsing step first transforms a plain string into an [iterator of
+//!   tokens](crate::parsing::Tokens). Then the [parser](crate::parsing::Parser) operates on that to
+//!   construct a syntax tree. The structures describing the tree can be found in the [syntax]
+//!   module.
+//! - **Layouting:** The next step is to transform the syntax tree into a portable representation of
+//!   the typesetted document. Types for these can be found in the [doc] and [layout] modules. This
+//!   representation contains already the finished layout.
 //! - **Exporting:** The finished document can then be exported into supported formats. Submodules
 //!   for the supported formats are located in the [export] module. Currently the only supported
 //!   format is _PDF_.
@@ -43,11 +43,13 @@
 //! exporter.export(&document, file).unwrap();
 //! ```
 
+use std::fmt::{self, Debug, Formatter};
+
 use crate::doc::Document;
-use crate::func::Scope;
 use crate::font::{Font, FontLoader, FontProvider};
-use crate::layout::{layout, Layout, LayoutContext, LayoutResult, LayoutError};
-use crate::layout::{PageStyle, TextStyle};
+use crate::func::Scope;
+use crate::layout::{layout, Layout, LayoutContext, LayoutDimensions};
+use crate::layout::{PageStyle, TextStyle, LayoutResult, LayoutError};
 use crate::parsing::{parse, ParseContext, ParseResult, ParseError};
 use crate::syntax::SyntaxTree;
 
@@ -65,12 +67,12 @@ pub mod syntax;
 
 /// Transforms source code into typesetted documents.
 ///
-/// Holds the typesetting context, which can be configured through various methods.
+/// Can be configured through various methods.
 pub struct Typesetter<'p> {
     /// The default page style.
-    base_page_style: PageStyle,
+    page_style: PageStyle,
     /// The default text style.
-    base_text_style: TextStyle,
+    text_style: TextStyle,
     /// Font providers.
     font_providers: Vec<Box<dyn FontProvider + 'p>>,
 }
@@ -80,8 +82,8 @@ impl<'p> Typesetter<'p> {
     #[inline]
     pub fn new() -> Typesetter<'p> {
         Typesetter {
-            base_page_style: PageStyle::default(),
-            base_text_style: TextStyle::default(),
+            page_style: PageStyle::default(),
+            text_style: TextStyle::default(),
             font_providers: vec![],
         }
     }
@@ -89,13 +91,13 @@ impl<'p> Typesetter<'p> {
     /// Set the default page style for the document.
     #[inline]
     pub fn set_page_style(&mut self, style: PageStyle) {
-        self.base_page_style = style;
+        self.page_style = style;
     }
 
     /// Set the default text style for the document.
     #[inline]
     pub fn set_text_style(&mut self, style: TextStyle) {
-        self.base_text_style = style;
+        self.text_style = style;
     }
 
     /// Add a font provider to the context of this typesetter.
@@ -112,12 +114,23 @@ impl<'p> Typesetter<'p> {
         parse(src, &ctx)
     }
 
-    /// Layout a parsed syntax tree and return the layout and the referenced font list.
+    /// Layout a syntax tree and return the layout and the referenced font list.
     #[inline]
     pub fn layout(&self, tree: &SyntaxTree) -> LayoutResult<(Layout, Vec<Font>)> {
         let loader = FontLoader::new(&self.font_providers);
-        let ctx = LayoutContext { loader: &loader };
+
+        let page = &self.page_style;
+        let ctx = LayoutContext {
+            loader: &loader,
+            text_style: self.text_style.clone(),
+            max_extent: LayoutDimensions {
+                width: page.width - page.margin_left - page.margin_right,
+                height: page.height - page.margin_top - page.margin_bottom,
+            },
+        };
+
         let layout = layout(&tree, &ctx)?;
+
         Ok((layout, loader.into_fonts()))
     }
 
@@ -127,14 +140,25 @@ impl<'p> Typesetter<'p> {
         let tree = self.parse(src)?;
         let (layout, fonts) = self.layout(&tree)?;
         let document = layout.into_document(fonts);
+        println!("fonts = {}", document.fonts.len());
+        println!("document = {:?}", document.pages);
         Ok(document)
     }
 }
 
+impl Debug for Typesetter<'_> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_struct("Typesetter")
+            .field("page_style", &self.page_style)
+            .field("text_style", &self.text_style)
+            .field("font_providers", &self.font_providers.len())
+            .finish()
+    }
+}
+
 /// The general error type for typesetting.
 pub enum TypesetError {
-    /// An error that occured while transforming source code into
-    /// an abstract syntax tree.
+    /// An error that occured while parsing.
     Parse(ParseError),
     /// An error that occured while layouting.
     Layout(LayoutError),
@@ -163,7 +187,7 @@ mod test {
     use crate::export::pdf::PdfExporter;
     use crate::font::FileSystemFontProvider;
 
-    /// Create a pdf with a name from the source code.
+    /// Create a _PDF_ with a name from the source code.
     fn test(name: &str, src: &str) {
         let mut typesetter = Typesetter::new();
         typesetter.add_font_provider(FileSystemFontProvider::new("../fonts", vec![
@@ -175,10 +199,10 @@ mod test {
             ("NotoEmoji-Regular.ttf",    font_info!(["NotoEmoji", "Noto", SansSerif, Serif, Monospace])),
         ]));
 
-        // Typeset into document
+        // Typeset into document.
         let document = typesetter.typeset(src).unwrap();
 
-        // Write to file
+        // Write to file.
         let path = format!("../target/typeset-unit-{}.pdf", name);
         let file = BufWriter::new(File::create(path).unwrap());
         let exporter = PdfExporter::new();
diff --git a/src/parsing.rs b/src/parsing.rs
index 594077f7..5ee8b382 100644
--- a/src/parsing.rs
+++ b/src/parsing.rs
@@ -1,14 +1,13 @@
 //! Tokenization and parsing of source code into syntax trees.
 
 use std::collections::HashMap;
-use std::mem::swap;
 use std::str::CharIndices;
 
 use smallvec::SmallVec;
 use unicode_xid::UnicodeXID;
 
-use crate::syntax::*;
 use crate::func::{Function, Scope};
+use crate::syntax::*;
 
 
 /// Builds an iterator over the tokens of the source code.
@@ -27,7 +26,7 @@ pub struct Tokens<'s> {
 }
 
 /// The state the tokenizer is in.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
 enum TokensState {
     /// The base state if there is nothing special we are in.
     Body,
@@ -55,9 +54,9 @@ impl<'s> Tokens<'s> {
     }
 
     /// Switch to the given state.
-    fn switch(&mut self, mut state: TokensState) {
-        swap(&mut state, &mut self.state);
-        self.stack.push(state);
+    fn switch(&mut self, state: TokensState) {
+        self.stack.push(self.state);
+        self.state = state;
     }
 
     /// Go back to the top-of-stack state.
@@ -84,7 +83,7 @@ impl<'s> Iterator for Tokens<'s> {
     fn next(&mut self) -> Option<Token<'s>> {
         use TokensState as TS;
 
-        // Function maybe has a body
+        // Go to the body state if the function has a body or return to the top-of-stack state.
         if self.state == TS::MaybeBody {
             if self.chars.peek()?.1 == '[' {
                 self.state = TS::Body;
@@ -244,7 +243,7 @@ fn is_newline_char(character: char) -> bool {
     }
 }
 
-/// A index + char iterator with double lookahead.
+/// A (index, char) iterator with double lookahead.
 #[derive(Debug, Clone)]
 struct PeekableChars<'s> {
     offset: usize,
@@ -324,6 +323,8 @@ impl Iterator for PeekableChars<'_> {
     }
 }
 
+//------------------------------------------------------------------------------------------------//
+
 /// Parses source code into a syntax tree given a context.
 #[inline]
 pub fn parse(src: &str, ctx: &ParseContext) -> ParseResult<SyntaxTree> {
@@ -338,6 +339,7 @@ pub struct ParseContext<'a> {
 }
 
 /// Transforms token streams to syntax trees.
+#[derive(Debug)]
 struct Parser<'s> {
     src: &'s str,
     tokens: PeekableTokens<'s>,
@@ -358,7 +360,7 @@ enum ParserState {
 }
 
 impl<'s> Parser<'s> {
-    /// Create a new parser from a stream of tokens and a scope of functions.
+    /// Create a new parser from the source and the context.
     fn new(src: &'s str, ctx: &'s ParseContext) -> Parser<'s> {
         Parser {
             src,
@@ -380,7 +382,7 @@ impl<'s> Parser<'s> {
         Ok(self.tree)
     }
 
-    /// Parse part of the body.
+    /// Parse the next part of the body.
     fn parse_body_part(&mut self) -> ParseResult<()> {
         if let Some(token) = self.tokens.peek() {
             match token {
@@ -398,8 +400,8 @@ impl<'s> Parser<'s> {
 
                 Token::Colon | Token::Equals => panic!("bad token for body: {:?}", token),
 
-                // The rest is handled elsewhere or should not happen, because Tokens does
-                // not yield colons or equals in the body, but their text equivalents instead.
+                // The rest is handled elsewhere or should not happen, because `Tokens` does not
+                // yield colons or equals in the body, but their text equivalents instead.
                 _ => panic!("unexpected token: {:?}", token),
             }
         }
@@ -526,6 +528,7 @@ impl<'s> Parser<'s> {
                 }
             }
         }
+
         Ok(())
     }
 
@@ -564,7 +567,7 @@ impl<'s> Parser<'s> {
     }
 }
 
-/// Find the index of the first unbalanced (unescaped) closing bracket.
+/// Find the index of the first unbalanced and unescaped closing bracket.
 fn find_closing_bracket(src: &str) -> Option<usize> {
     let mut parens = 0;
     let mut escaped = false;
@@ -584,8 +587,8 @@ fn find_closing_bracket(src: &str) -> Option<usize> {
     None
 }
 
-/// A peekable iterator for tokens which allows access to the original iterator
-/// inside this module (which is needed by the parser).
+/// A peekable iterator for tokens which allows access to the original iterator inside this module
+/// (which is needed by the parser).
 #[derive(Debug, Clone)]
 struct PeekableTokens<'s> {
     tokens: Tokens<'s>,
@@ -649,6 +652,8 @@ fn is_identifier(string: &str) -> bool {
     true
 }
 
+//------------------------------------------------------------------------------------------------//
+
 /// The error type for parsing.
 pub struct ParseError(String);
 
@@ -693,7 +698,7 @@ mod token_tests {
         test("\n", vec![N]);
     }
 
-    /// This test looks if LF- and CRLF-style newlines get both identified correctly
+    /// This test looks if LF- and CRLF-style newlines get both identified correctly.
     #[test]
     fn tokenize_whitespace_newlines() {
         test(" \t", vec![S]);
@@ -743,8 +748,8 @@ mod token_tests {
         ]);
     }
 
-    /// This test checks whether the colon and equals symbols get parsed correctly
-    /// depending on the context: Either in a function header or in a body.
+    /// This test checks whether the colon and equals symbols get parsed correctly depending on the
+    /// context: Either in a function header or in a body.
     #[test]
     fn tokenize_symbols_context() {
         test("[func: key=value][Answer: 7]",
@@ -801,8 +806,8 @@ mod parse_tests {
     use Node::{Space as S, Newline as N, Func as F};
     use funcs::*;
 
-    /// Two test functions, one which parses it's body as another syntax tree
-    /// and another one which does not expect a body.
+    /// Two test functions, one which parses it's body as another syntax tree and another one which
+    /// does not expect a body.
     mod funcs {
         use super::*;
 
@@ -871,8 +876,7 @@ mod parse_tests {
     #[allow(non_snake_case)]
     fn T(s: &str) -> Node { Node::Text(s.to_owned()) }
 
-    /// Shortcut macro to create a syntax tree.
-    /// Is `vec`-like and the elements are the nodes.
+    /// Shortcut macro to create a syntax tree. Is `vec`-like and the elements are the nodes.
     macro_rules! tree {
         ($($x:expr),*) => (
             SyntaxTree { nodes: vec![$($x),*] }
diff --git a/src/syntax.rs b/src/syntax.rs
index b3f1d04d..cfd41719 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -17,8 +17,8 @@ pub enum Token<'s> {
     RightBracket,
     /// A colon (`:`) indicating the beginning of function arguments.
     ///
-    /// If a colon occurs outside of a function header, it will be
-    /// tokenized as a [Word](Token::Word).
+    /// If a colon occurs outside of a function header, it will be tokenized as a
+    /// [Word](Token::Word).
     Colon,
     /// An equals (`=`) sign assigning a function argument a value.
     ///
@@ -34,9 +34,8 @@ pub enum Token<'s> {
     LineComment(&'s str),
     /// A block comment.
     BlockComment(&'s str),
-    /// A star followed by a slash unexpectedly ending a block comment
-    /// (the comment was not started before, otherwise a
-    /// [BlockComment](Token::BlockComment) would be returned).
+    /// A star followed by a slash unexpectedly ending a block comment (the comment was not started
+    /// before, otherwise a [BlockComment](Token::BlockComment) would be returned).
     StarSlash,
     /// Everything else is just text.
     Text(&'s str),
@@ -64,11 +63,11 @@ pub enum Node {
     Space,
     /// A line feed.
     Newline,
-    /// Indicates that italics were enabled/disabled.
+    /// Indicates that italics were enabled / disabled.
     ToggleItalics,
-    /// Indicates that boldface was enabled/disabled.
+    /// Indicates that boldface was enabled / disabled.
     ToggleBold,
-    /// Indicates that math mode was enabled/disabled.
+    /// Indicates that math mode was enabled / disabled.
     ToggleMath,
     /// Literal text.
     Text(String),