feat: allow darkCurve to accept [normal, hc] pair

.changeset/swift-bears-dance.md

@@ -0,0 +1,5 @@
+---
+'@tenphi/glaze': patch
+---
+
+`darkCurve` now accepts a `[normal, highContrast]` pair for separate HC tuning.

README.md

@@ -647,7 +647,7 @@ const t = (100 - lightness) / 100;
 const invertedL = lo + (hi - lo) * t / (t + darkCurve * (1 - t)); // darkCurve default: 0.5
 ```
 
-The `darkCurve` parameter (default `0.5`, range 0–1) controls how much the dark-mode inversion expands lightness deltas. Lower values produce stronger expansion; `1` gives linear (legacy) behavior. Unlike a power curve, the Möbius transformation provides **proportional expansion** — small and large deltas are scaled by similar ratios, preserving the visual hierarchy of the light theme.
+The `darkCurve` parameter (default `0.5`, range 0–1) controls how much the dark-mode inversion expands lightness deltas. Lower values produce stronger expansion; `1` gives linear (legacy) behavior. Accepts a `[normal, highContrast]` pair for separate HC tuning (e.g. `darkCurve: [0.5, 0.3]`); a single number applies to both. Unlike a power curve, the Möbius transformation provides **proportional expansion** — small and large deltas are scaled by similar ratios, preserving the visual hierarchy of the light theme.
 
 **`fixed`** — mapped without inversion (not affected by `darkCurve`):
 
@@ -661,7 +661,7 @@ const mappedL = (lightness * (hi - lo)) / 100 + lo;
 | accent-fill (L=52) | 52 | 66.9 | 53.4 | 56.6 |
 | accent-text (L=100) | 100 | 15 | 15 | 95 |
 
-In high-contrast variants, the `darkLightness` window is bypassed. Auto uses the same Möbius curve over the full [0, 100] range. Fixed uses identity (`L`). This allows HC colors to reach the full 0–100 range.
+In high-contrast variants, the `darkLightness` window is bypassed — auto uses the Möbius curve over the full [0, 100] range, and fixed uses identity (`L`). To use a different curve shape for HC, pass a `[normal, hc]` pair to `darkCurve` (e.g. `darkCurve: [0.5, 0.3]`).
 
 ### Saturation
 
@@ -914,7 +914,7 @@ glaze.configure({
   lightLightness: [10, 100],   // Light scheme lightness window [lo, hi] (bypassed in HC)
   darkLightness: [15, 95],     // Dark scheme lightness window [lo, hi] (bypassed in HC)
   darkDesaturation: 0.1,       // Saturation reduction in dark scheme (0–1)
-  darkCurve: 0.5,              // Möbius beta for dark auto-inversion (0–1, lower = more expansion)
+  darkCurve: 0.5,              // Möbius beta for dark auto-inversion (0–1); or [normal, hc] pair
   states: {
     dark: '@dark',             // State alias for dark mode tokens
     highContrast: '@high-contrast',

src/glaze.test.ts

@@ -471,6 +471,47 @@ describe('glaze', () => {
       // l_d = 100 * 0.05825 ≈ 5.83
       expect(surface.darkContrast.l).toBeCloseTo(0.0583, 2);
     });
+
+    it('accepts [normal, hc] pair for separate HC curve', () => {
+      glaze.configure({ darkCurve: [0.5, 0.3] });
+      const theme = glaze(0, 0);
+      theme.colors({
+        surface: { lightness: 97 },
+      });
+
+      const resolved = theme.resolve();
+      const surface = resolved.get('surface')!;
+
+      // Normal dark: beta=0.5, lightL=97.3, t=2.7/90=0.03
+      // Möbius(0.03, 0.5) = 0.03/0.515 ≈ 0.05825, l_d = 15+80*0.05825 ≈ 19.66
+      expect(surface.dark.l).toBeCloseTo(0.1966, 2);
+
+      // HC dark: beta=0.3, t=(100-97)/100=0.03
+      // Möbius(0.03, 0.3) = 0.03/(0.03+0.3*0.97) = 0.03/0.321 ≈ 0.09346
+      // l_d = 100*0.09346 ≈ 9.35
+      expect(surface.darkContrast.l).toBeCloseTo(0.0935, 2);
+
+      glaze.resetConfig();
+    });
+
+    it('single darkCurve number applies to both normal and HC', () => {
+      glaze.configure({ darkCurve: 0.5 });
+      const theme = glaze(0, 0);
+      theme.colors({
+        surface: { lightness: 97 },
+      });
+
+      const resolved = theme.resolve();
+      const surface = resolved.get('surface')!;
+
+      // Both use beta=0.5
+      // Normal dark: lightL=97.3, t=0.03, Möbius ≈ 0.05825, l_d = 15+80*0.05825 ≈ 19.66
+      expect(surface.dark.l).toBeCloseTo(0.1966, 2);
+      // HC dark: t=0.03, Möbius ≈ 0.05825, l_d = 100*0.05825 ≈ 5.83
+      expect(surface.darkContrast.l).toBeCloseTo(0.0583, 2);
+
+      glaze.resetConfig();
+    });
   });
 
   describe('dark scheme', () => {

src/glaze.ts

@@ -391,7 +391,9 @@ function mapLightnessDark(
 ): number {
   if (mode === 'static') return l;
 
-  const beta = globalConfig.darkCurve;
+  const beta = isHighContrast
+    ? pairHC(globalConfig.darkCurve)
+    : pairNormal(globalConfig.darkCurve);
   const [darkLo, darkHi] = lightnessWindow(isHighContrast, 'dark');
 
   if (mode === 'fixed') {
@@ -405,7 +407,9 @@ function mapLightnessDark(
 }
 
 function lightMappedToDark(lightL: number, isHighContrast: boolean): number {
-  const beta = globalConfig.darkCurve;
+  const beta = isHighContrast
+    ? pairHC(globalConfig.darkCurve)
+    : pairNormal(globalConfig.darkCurve);
   const [lightLo, lightHi] = lightnessWindow(isHighContrast, 'light');
   const [darkLo, darkHi] = lightnessWindow(isHighContrast, 'dark');
   const clamped = clamp(lightL, lightLo, lightHi);

src/types.ts

@@ -204,11 +204,12 @@ export interface GlazeConfig {
   /** Saturation reduction factor for dark scheme (0–1). Default: 0.1. */
   darkDesaturation?: number;
   /**
-   * Power-curve exponent for dark auto-inversion (0–1).
+   * Möbius beta for dark auto-inversion (0–1).
    * Lower values expand subtle near-white distinctions in dark mode.
    * Set to 1 for linear (legacy) behavior. Default: 0.5.
+   * Accepts [normal, highContrast] pair for separate HC tuning.
    */
-  darkCurve?: number;
+  darkCurve?: HCPair<number>;
   /** State alias names for token export. */
   states?: {
     dark?: string;
@@ -224,7 +225,7 @@ export interface GlazeConfigResolved {
   lightLightness: [number, number];
   darkLightness: [number, number];
   darkDesaturation: number;
-  darkCurve: number;
+  darkCurve: HCPair<number>;
   states: {
     dark: string;
     highContrast: string;