### Defining Text Effect Structure in Rust

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This Rust `struct` defines the basic internal representation for a text effect. It includes `start` (character index), `colour` (sRGB color), and `flags` (effect properties). This structure is fundamental for applying various text effects and managing their state within the `kas-text` library.

```Rust
struct Effect {
    start: u32,
    colour: Srgb,
    flags: EffectFlags,
}
```

--------------------------------

### Defining Generic EffectToken Structure (Rust)

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

The `EffectToken` struct represents a text effect, parameterized by a generic type `X` for auxiliary data. It contains `start` (the starting index of the effect), `flags` (effect-specific flags), and `aux` (additional data specific to the effect type). This structure is generated by the `effect_tokens` method.

```Rust
struct EffectToken<X> {
    start: u32,
    flags: EffectFlags,
    aux: X,
}
```

--------------------------------

### Defining FontToken Structure (Rust)

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

The `FontToken` struct represents properties associated with a font segment. It includes `start` (the starting index), `face` (the font face ID), `dpem` (dots per em), and an optional `tag` for consumption by the draw-stage resolver. This structure is yielded by the `font_tokens` iterator.

```Rust
struct FontToken {
    start: u32,
    face: FaceId,
    dpem: f32,
    // And maybe this, for consumption by draw-stage resolver:
    tag: u32,
}
```

--------------------------------

### Managing Text Glyphs and Effects in Rust

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This Rust `impl` block defines methods for the `Text` struct to retrieve positioned glyphs, with or without applying text effects. `glyphs` provides raw glyph positioning, `glyphs_and_effects` returns glyphs along with associated effects, and `glyphs_with_effects` allows applying additional effects dynamically. These methods are crucial for efficient text rendering and effect combination.

```Rust
impl Text {
    /// Returns a Vec of positioned glyphs, ignoring all effects
    pub fn glyphs<F: FnMut(FontId, f32, f32, Glyph)>(&self, mut f: F) {
        ..
    }
    
    /// Returns a Vec of positioned glyphs and a Vec of effects
    pub fn glyphs_and_effects<F: Fn(...) -> G>(&self) -> (Vec<G>, Vec<Drawable>) {
        if self.text_effects.is_empty() {
            (self.glyphs(f), vec![])
        } else {
            self.glyphs_with_effects_impl(&self.text_effects, f)
        }
    }
    
    /// Same as `glyphs_and_effects` but applies extra text-effects on-the-fly
    pub fn glyphs_with_effects<U, F>(&self, effects: &[Effect<U>], mut f: F) -> Vec<Drawable>
    where
        U: Copy + Default,
        F: FnMut(FontId, f32, f32, Glyph, U),
    {
        if !self.text_effects.is_empty() {
            effects = combine(&self.text_effects, effects);
        }
        self.glyphs_with_effects_impl(effects, f)
    }
}
```

--------------------------------

### Defining the KAS GUI Theme Trait in Rust

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This Rust trait defines the interface for a KAS GUI theme, responsible for providing font selection, font sizing, and auxiliary data (like colors) for glyphs. It includes methods for selecting fonts based on a `FontSelector` and auxiliary data based on an `AuxSelector`, as well as providing default values.

```Rust
/// Trait object provided by downstream; may include default impl
// TODO: separate ThemeFont and ThemeAux?
pub trait Theme {
    /// Auxilliary data associated with glyphs; typically used for colour
    type Aux: Clone;

    /// Allow downcast as an extension mechanism
    fn as_any(&self) -> &Any;
    
    fn default_font(&self) -> FontId;
    fn select_font(&self, selector: &FontSelector) -> FontId;
    
    /// Provides default font size as `(dpp, pt_size)`
    fn font_size(&self) -> (f32, f32);
    
    /// Provides default instance of `Aux` type
    fn default_aux(&self) -> Aux;
    /// Provides `Aux` instance
    fn select_aux(&self, selector: AuxSelector) -> Aux;
}
```

--------------------------------

### Generating Raw Effect Tokens (Rust)

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This is an overloaded version of the `effect_tokens` method, also run during drawing. It takes `storage` (initialized by `font_tokens`) and `state` as input, returning a vector of raw `Self::EffectToken` items without an explicit translation closure. This variant is suitable when direct `EffectToken` generation is sufficient.

```Rust
fn effect_tokens<'a, Aux>(&'a self,
    storage: &mut Self::Storage,
    state: &DrawState,
) -> Vec<Self::EffectToken>;
```

--------------------------------

### Defining Core Text Structures and Traits in Rust

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This snippet defines the core `Text` struct, parameterized by `TextRep`, which encapsulates the text content and derived properties. It also introduces `TextDerived` for environmental and derived data, and two traits, `TextDerivedApi` and `TextApi`, to provide read-only and update functionalities respectively, promoting a clear separation of concerns for text manipulation and access.

```rust
struct Text<T: TextRep> {
    text: T,
    derived: TextDerived,
}

struct TextDerived {
    // env and all derived
}

trait TextDerivedApi {
    // all read-env and read-derived fns
}

trait TextApi: TextDerivedApi {
    // update env, prepare
    // read text (fns independent of T)
    // access as &TextDerived
}
```

--------------------------------

### Generating Effect Tokens with Translation (Rust)

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This method provides an iterator yielding `EffectToken` items, primarily executed during the drawing phase. It utilizes the `storage` which must have been previously initialized by `font_tokens`. The `translate` closure allows for custom transformation of `Self::EffectToken` into `EffectToken<Aux>` and can force generation of additional tokens at a given index.

```Rust
fn effect_tokens<'a, Aux>(&'a self,
    storage: &mut Self::Storage,
    state: &DrawState,
    translate: &mut FnMut(u32, Self::EffectToken) -> (u32, EffectToken<Aux>),
) -> Vec<EffectToken<Aux>>;
```

--------------------------------

### Updating Storage and Yielding Font Tokens (Rust)

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This function updates the provided `storage` and returns an iterator yielding `FontToken` items. It is specifically used during the run-breaking phase to initialize `storage` for subsequent use by `effect_tokens`. Key parameters include `dpp` (dots per pixel) and `pt_size` (point size) which influence font metrics.

```Rust
fn font_tokens<'a>(&'a self, storage: &mut Self::Storage, dpp: f32, pt_size: f32)
    -> Self::FontTokenIter<'a>;
```

--------------------------------

### Defining KAS GUI AuxSelector and FormattableText Traits in Rust

Source: https://github.com/kas-gui/kas-text/blob/master/design/text-effects.md

This Rust code defines the `AuxSelector` enum, used to specify different types of auxiliary data (e.g., for selected text), and the `FormattableText` trait. The `FormattableText` trait is implemented by rich-text parsers to provide text content, font tokens, and effect tokens, enabling the rendering engine to process and display formatted text.

```Rust
#[non_exhaustive]
pub enum AuxSelector {
    Default,
    Selected, // selected text
}

/// Trait impls provided by rich-text parsers, both provided and downstream impls
pub trait FormattableText: std::fmt::Debug {
    type FontTokenIter<'a>: Iterator<Item = FontToken>;
    /// Storage available
    type Storage: Clone + Default + Debug;
    /// State used to generate effect tokens
    type DrawState: ?Sized;
    /// Effect tokens as generated by this object
    type EffectToken;

    fn clone_boxed(&self) -> Box<dyn FormattableText>;
    fn str_len(&self) -> usize;
    fn as_str(&self) -> &str;
}
```

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