feat(theme): per-app accent: pm gives window manifest accent before app runs (display ACCENT, refused after BUFFER); theme::with_accent keeps hue, shifts lightness until contrast passes, else theme accent; tally pink

crates/theme/src/accent.rs

@@ -0,0 +1,42 @@
+//! An app's own accent (its manifest's `[appearance] accent`), for its
+//! primary buttons and active states. Its hue is kept; its lightness may
+//! be moved toward the theme's far end until everything drawn with it
+//! still passes contrast. One that cannot pass, is black or white, or
+//! could be taken for the destructive colour, gives way to the theme's.
+
+use crate::contrast::{check, luminance, ratio};
+use crate::{Rgba, Theme};
+
+/// How far toward white or black the accent may be moved, in tenths.
+const MOST: u8 = 6;
+/// Channels all this close to the destructive colour's read as it.
+const LIKE_DESTRUCTIVE: u8 = 40;
+
+fn mix(a: Rgba, b: Rgba, tenths: u8) -> Rgba {
+    let m = |x: u8, y: u8| ((x as u16 * (10 - tenths) as u16 + y as u16 * tenths as u16) / 10) as u8;
+    Rgba(m(a.0, b.0), m(a.1, b.1), m(a.2, b.2), 255)
+}
+
+fn near(a: Rgba, b: Rgba) -> bool {
+    a.0.abs_diff(b.0) < LIKE_DESTRUCTIVE && a.1.abs_diff(b.1) < LIKE_DESTRUCTIVE && a.2.abs_diff(b.2) < LIKE_DESTRUCTIVE
+}
+
+/// `t`, with `accent` in place of its own if it can be made to pass.
+pub fn with_accent(t: Theme, accent: Rgba) -> Theme {
+    let (black, white) = (Rgba::rgb(0), Rgba::rgb(0xffffff));
+    if accent == black || accent == white || near(accent, t.destructive) {
+        return t;
+    }
+    let toward = if luminance(t.background) < 0.5 { white } else { black };
+    for tenths in 0..=MOST {
+        let a = mix(accent, toward, tenths);
+        // Whichever of the theme's own colours reads best on it.
+        let on = [t.on_accent, t.text_primary, t.background]
+            .into_iter()
+            .fold(t.on_accent, |best, c| if ratio(c, a) > ratio(best, a) { c } else { best });
+        if let Ok(ok) = check(Theme { accent: a, on_accent: on, ..t }) {
+            return ok;
+        }
+    }
+    t
+}

crates/theme/src/contrast.rs

@@ -10,7 +10,7 @@ fn channel(c: u8) -> f32 {
     if c <= 0.040_45 { c / 12.92 } else { libm::powf((c + 0.055) / 1.055, 2.4) }
 }
 
-fn luminance(c: Rgba) -> f32 {
+pub(crate) fn luminance(c: Rgba) -> f32 {
     0.2126 * channel(c.0) + 0.7152 * channel(c.1) + 0.0722 * channel(c.2)
 }
 

crates/theme/src/lib.rs

@@ -8,6 +8,7 @@
 #![no_std]
 #![forbid(unsafe_code)]
 
+mod accent;
 mod contrast;
 mod parse;
 pub mod wire;
@@ -15,6 +16,7 @@ pub mod wire;
 #[cfg(test)]
 mod tests;
 
+pub use accent::with_accent;
 pub use contrast::{check, ratio};
 pub use parse::{Error, parse};
 

crates/theme/src/tests.rs

@@ -51,3 +51,23 @@ fn every_token_is_needed_and_checked() {
     assert!(matches!(parse(&DARK.replace("highlight     = 18", "glow = 18")), Err(Error::Toml(T::Unknown { .. }))));
     assert!(matches!(parse(&std::format!("{DARK}\n[glass]\n")), Err(Error::Toml(T::Duplicate { .. }))));
 }
+
+#[test]
+fn an_app_accent_keeps_its_hue_and_passes_in_either_theme() {
+    let pink = Rgba::rgb(0xe91e63);
+    for text in [DARK, LIGHT] {
+        let t = parse(text).unwrap();
+        let a = with_accent(t, pink);
+        assert_ne!(a.accent, t.accent, "{}: the app's, not the theme's", std::str::from_utf8(t.name()).unwrap());
+        assert!(a.accent.0 > a.accent.1 && a.accent.0 > a.accent.2 / 2, "still pink: {:?}", a.accent);
+        check(a).unwrap();
+    }
+}
+
+#[test]
+fn some_accents_give_way_to_the_theme() {
+    let t = parse(DARK).unwrap();
+    for c in [0x000000, 0xffffff, 0xee5555] {
+        assert_eq!(with_accent(t, Rgba::rgb(c)), t, "{c:06x}");
+    }
+}