vectorcraft/crates/text/src/features.rs

//! OpenType feature selection (the OpenType panel), resolved per character style.

use harfrust::{Feature, Tag};
use vectorcraft_doc::CharStyle;

/// Tracking (in 1/1000 em) outside which standard ligatures are dropped automatically: tighter
/// than the first, looser than the second.
///
/// Letterspaced type should not ligate: an `fi` glyph keeps its letters tight while everything
/// around it opens up (or stays open while it closes up), which reads as a blot. Small tracking
/// adjustments made to fit a line are not letterspacing and keep the ligatures. The limits are
/// about −8% and +22% of a typical word space (250/1000 em), the thresholds Adobe's composers are
/// described as using (John Hudson, www-style, 2012-07): condensing shows the fixed ligature
/// sooner than expanding. An explicit `liga` / `clig` in the character's own features always wins.
pub const LIGATURE_TRACKING_LIMITS: (f64, f64) = (-20.0, 55.0);

/// OpenType features applied during shaping. Kerning, `case` and ligature suppression are driven
/// by the character style (`kerning`, `all_caps`, `tracking`); the rest are layout-wide options.
/// Standard ligatures are dropped when tracking is outside [`LIGATURE_TRACKING_LIMITS`], unless
/// the character's features turn `liga` (or `clig`) on explicitly.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct OtFeatures {
    /// Vertical glyph alternates (`vert`), set by the writing direction.
    pub vertical: bool,
    /// Standard ligatures (`liga`, `clig`). Suppressed automatically when tracking is outside
    /// [`LIGATURE_TRACKING_LIMITS`], unless the character turns them on explicitly.
    pub ligatures: bool,
    /// Contextual alternates (`calt`).
    pub contextual: bool,
    /// Discretionary ligatures (`dlig`).
    pub discretionary_ligatures: bool,
    /// Small capitals (`smcp`).
    pub small_caps: bool,
    /// Diagonal fractions (`frac`).
    pub fractions: bool,
    /// Oldstyle figures (`onum`).
    pub oldstyle_figures: bool,
    /// Tabular figures (`tnum`).
    pub tabular_figures: bool,
    /// Ordinals (`ordn`).
    pub ordinals: bool,
    /// Swashes (`swsh`).
    pub swash: bool,
    /// The features of [`EXTRA_FEATURES`] turned on, one bit each (off by default): stylistic
    /// sets, alternates, titling, proportional and lining figures, superscript and subscript.
    pub extra: u32,
}

/// The most entries of a style's feature list read: every segment of its text reads the list again.
const MAX_STYLE_FEATURES: usize = 256;

/// Features beyond the named fields, off unless asked for, each bit `i` of [`OtFeatures::extra`]:
/// stylistic sets `ss01`–`ss20`, stylistic alternates `salt`, titling `titl`, proportional `pnum`
/// and lining `lnum` figures, superscript `sups` and subscript `subs`.
pub const EXTRA_FEATURES: [&str; 26] = [
    "ss01", "ss02", "ss03", "ss04", "ss05", "ss06", "ss07", "ss08", "ss09", "ss10", "ss11", "ss12", "ss13", "ss14", "ss15", "ss16", "ss17", "ss18",
    "ss19", "ss20", "salt", "titl", "pnum", "lnum", "sups", "subs",
];

impl Default for OtFeatures {
    fn default() -> Self {
        Self {
            vertical: false,
            ligatures: true,
            contextual: true,
            discretionary_ligatures: false,
            small_caps: false,
            fractions: false,
            oldstyle_figures: false,
            tabular_figures: false,
            ordinals: false,
            swash: false,
            extra: 0,
        }
    }
}

fn f(tag: &[u8; 4], on: bool) -> Feature {
    Feature::new(Tag::new(tag), on as u32, ..)
}

impl OtFeatures {
    /// Parse a feature list like `["dlig", "smcp", "-liga"]` (unknown tags are ignored).
    pub fn from_tags<'a>(tags: impl IntoIterator<Item = &'a str>) -> Self {
        Self::default().with_tags(tags)
    }

    /// The tags this set differs from the defaults by (what `CharStyle::features` stores).
    pub fn to_tags(&self) -> Vec<String> {
        let d = Self::default();
        let mut v = vec![];
        for (on, def, tag) in [
            (self.ligatures, d.ligatures, "liga"),
            (self.contextual, d.contextual, "calt"),
            (self.discretionary_ligatures, d.discretionary_ligatures, "dlig"),
            (self.small_caps, d.small_caps, "smcp"),
            (self.fractions, d.fractions, "frac"),
            (self.oldstyle_figures, d.oldstyle_figures, "onum"),
            (self.tabular_figures, d.tabular_figures, "tnum"),
            (self.ordinals, d.ordinals, "ordn"),
            (self.swash, d.swash, "swsh"),
        ] {
            if on != def {
                v.push(if on { tag.to_string() } else { format!("-{tag}") });
            }
        }
        v.extend(EXTRA_FEATURES.iter().enumerate().filter(|(i, _)| self.extra & (1 << i) != 0).map(|(_, t)| t.to_string()));
        v
    }

    /// Is `tag` (optionally prefixed with `-` or `+`) one this set understands?
    pub fn known_tag(tag: &str) -> bool {
        let t = tag.trim_start_matches(['-', '+']);
        matches!(t, "liga" | "calt" | "dlig" | "smcp" | "frac" | "onum" | "tnum" | "ordn" | "swsh") || EXTRA_FEATURES.contains(&t)
    }

    /// This set with `tags` applied on top.
    pub fn with_tags<'a>(mut self, tags: impl IntoIterator<Item = &'a str>) -> Self {
        let o = &mut self;
        for t in tags {
            let (on, t) = match t.strip_prefix('-') {
                Some(r) => (false, r),
                None => (true, t.strip_prefix('+').unwrap_or(t)),
            };
            match t {
                "liga" => o.ligatures = on,
                "calt" => o.contextual = on,
                "dlig" => o.discretionary_ligatures = on,
                "smcp" => o.small_caps = on,
                "frac" => o.fractions = on,
                "onum" => o.oldstyle_figures = on,
                "tnum" => o.tabular_figures = on,
                "ordn" => o.ordinals = on,
                "swsh" => o.swash = on,
                _ => {
                    if let Some(i) = EXTRA_FEATURES.iter().position(|e| *e == t) {
                        if on {
                            o.extra |= 1 << i;
                        } else {
                            o.extra &= !(1 << i);
                        }
                    }
                }
            }
        }
        self
    }

    /// The harfrust features for text in style `st`.
    pub(crate) fn resolve(&self, st: &CharStyle) -> Vec<Feature> {
        // The character's own OpenType settings override the layout-wide ones.
        // (A style lists a few features; one with thousands, in a document, is read for the first.)
        let listed = st.features.get(..MAX_STYLE_FEATURES).unwrap_or(&st.features);
        let own = self.with_tags(listed.iter().map(String::as_str));
        let s = &own;
        let mut v = Vec::with_capacity(8);
        if st.kerning.is_some() {
            v.push(f(b"kern", false));
        }
        let liga = s.ligatures && (!ligatures_suppressed_by(st.tracking) || explicit_ligatures(listed));
        if !liga {
            v.push(f(b"liga", false));
            v.push(f(b"clig", false));
        }
        if !s.contextual {
            v.push(f(b"calt", false));
        }
        if st.all_caps {
            v.push(f(b"case", true));
        }
        for (on, tag) in [
            (s.discretionary_ligatures, b"dlig"),
            (s.small_caps, b"smcp"),
            (s.fractions, b"frac"),
            (s.oldstyle_figures, b"onum"),
            (s.tabular_figures, b"tnum"),
            (s.ordinals, b"ordn"),
            (s.swash, b"swsh"),
        ] {
            if on {
                v.push(f(tag, true));
            }
        }
        for (i, tag) in EXTRA_FEATURES.iter().enumerate() {
            if let (true, Ok(t)) = (s.extra & (1 << i) != 0, <[u8; 4]>::try_from(tag.as_bytes())) {
                v.push(f(&t, true));
            }
        }
        if self.vertical {
            // Not `vrt2`: its glyphs are already turned on their side, and the layout turns Latin
            // and digits itself (they would lie upside down, and tate-chu-yoko break).
            v.push(f(b"vert", true));
        }
        if self.proportional(st) {
            v.push(f(b"palt", true));
            // Kerning for the glyphs `palt` makes proportional (OpenType `apkn`), with automatic
            // kerning only.
            v.push(f(b"apkn", st.kerning.is_none()));
        }
        v
    }

    /// Does text in style `st` take proportional widths (`palt`)? Proportional Metrics in
    /// horizontal type; vertical type takes proportional heights instead
    /// ([`Self::proportional_vertical`]).
    pub(crate) fn proportional(&self, st: &CharStyle) -> bool {
        st.proportional_metrics && !self.vertical
    }

    /// Does text in style `st` take proportional heights (`vpal`)? Proportional Metrics in
    /// vertical type.
    pub(crate) fn proportional_vertical(&self, st: &CharStyle) -> bool {
        st.proportional_metrics && self.vertical
    }

    /// The harfrust features for vertical text in style `st` shaped top to bottom: [`Self::resolve`],
    /// with `vpal` when `proportional`, and vertical kerning (`vkrn`, and `vapk` for the glyphs
    /// `vpal` makes proportional) when `kerning` and the style's kerning is automatic.
    pub(crate) fn resolve_vertical(&self, st: &CharStyle, proportional: bool, kerning: bool) -> Vec<Feature> {
        let mut v = self.resolve(st);
        if proportional {
            v.push(f(b"vpal", true));
        }
        let kern = kerning && st.kerning.is_none();
        v.push(f(b"vkrn", kern));
        v.push(f(b"vapk", kern));
        v
    }

    /// The harfrust features for text in style `st` with full-width glyphs on their full widths:
    /// [`Self::resolve`] without `palt` (and its kerning, `apkn`).
    pub(crate) fn resolve_fixed_width(&self, st: &CharStyle) -> Vec<Feature> {
        let mut v = self.resolve(st);
        v.retain(|x| x.tag != Tag::new(b"palt") && x.tag != Tag::new(b"apkn"));
        v
    }
}

/// Does `tracking` (1/1000 em) count as letterspacing that drops standard ligatures?
pub fn ligatures_suppressed_by(tracking: f64) -> bool {
    // A non-finite value is not a real letterspacing request; keep the default.
    let (tight, loose) = LIGATURE_TRACKING_LIMITS;
    tracking.is_finite() && (tracking < tight || tracking > loose)
}

/// Do `features` turn standard ligatures on explicitly (`liga`, `+liga`, `clig`, `+clig`)? The last
/// ligature tag wins, matching [`OtFeatures::with_tags`]. Absence of a tag means "default", which
/// [`OtFeatures::to_tags`] never writes, so this is distinguishable from an explicit request.
pub fn explicit_ligatures<S: AsRef<str>>(features: &[S]) -> bool {
    features
        .iter()
        .rev()
        .find_map(|t| match t.as_ref() {
            "liga" | "+liga" | "clig" | "+clig" => Some(true),
            "-liga" | "-clig" => Some(false),
            _ => None,
        })
        .unwrap_or(false)
}

#[cfg(test)]
mod tests {
    use super::*;

    /// #1607: stylistic sets, alternates, titling, figure styles and superscript/subscript are
    /// known, round-trip through the stored tags and reach the shaper.
    #[test]
    fn extra_features_are_kept_and_shaped() {
        for t in ["ss01", "ss20", "-ss07", "salt", "titl", "pnum", "lnum", "sups", "subs"] {
            assert!(OtFeatures::known_tag(t), "{t}");
        }
        assert!(!OtFeatures::known_tag("ss21"));
        let o = OtFeatures::from_tags(["ss03", "pnum", "sups", "-liga"]);
        assert_eq!(o.to_tags(), vec!["-liga", "ss03", "pnum", "sups"]);
        assert_eq!(OtFeatures::from_tags(["ss03", "-ss03"]).to_tags(), Vec::<String>::new(), "turned off again");
        let st = CharStyle { features: vec!["ss05".into(), "titl".into()], ..CharStyle::default() };
        let tags: Vec<String> = OtFeatures::default().resolve(&st).iter().filter(|f| f.value == 1).map(|f| f.tag.to_string()).collect();
        assert!(tags.contains(&"ss05".to_string()) && tags.contains(&"titl".to_string()), "{tags:?}");
    }
    use crate::{FontDb, layout};
    use vectorcraft_doc::TextObject;
    use vectorcraft_geom::Point;

    fn glyphs(features: &[&str]) -> usize {
        let st = CharStyle { font_family: "Source Serif 4".into(), features: features.iter().map(|s| s.to_string()).collect(), ..Default::default() };
        layout(FontDb::global(), &TextObject::point(Point::ZERO, "fi", st)).glyphs.len()
    }

    #[test]
    fn per_character_features_drive_shaping() {
        let with = glyphs(&[]);
        let without = glyphs(&["-liga"]);
        assert!(with < without, "the default ligature merges f+i ({with} vs {without})");
        assert_eq!(OtFeatures::from_tags(["-liga", "dlig"]).to_tags(), ["-liga", "dlig"]);
        assert!(OtFeatures::default().to_tags().is_empty());
        assert!(OtFeatures::known_tag("-onum") && !OtFeatures::known_tag("zzzz"));
    }
}