/**
 * Pick the text color for a sidebar badge label drawn on a theme accent.
 * Keep this logic in sync with Magic Context so both sidebars make the same
 * contrast decision for shared themes.
 */

type Color = { r: number; g: number; b: number; a?: number };

const MIN_OPAQUE_ALPHA = 0.5;
const MIN_CHANNEL_DISTANCE = 0.06;
const LIGHT_ACCENT_LUMINANCE = 0.5;

function srgbChannelToLinear(c: number): number {
  return c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4;
}

function relativeLuminance(bg: Color): number {
  return (
    0.2126 * srgbChannelToLinear(bg.r) +
    0.7152 * srgbChannelToLinear(bg.g) +
    0.0722 * srgbChannelToLinear(bg.b)
  );
}

function nearlyEqual(a: Color, b: Color): boolean {
  return (
    Math.abs(a.r - b.r) < MIN_CHANNEL_DISTANCE &&
    Math.abs(a.g - b.g) < MIN_CHANNEL_DISTANCE &&
    Math.abs(a.b - b.b) < MIN_CHANNEL_DISTANCE
  );
}

export function readableTextColorOn(bg: Color): string {
  return relativeLuminance(bg) < LIGHT_ACCENT_LUMINANCE ? "#ffffff" : "#000000";
}

export function badgeTextColor<T extends Color>(accent: T, background: T): T | string {
  const alpha = background.a ?? 1;
  if (alpha >= MIN_OPAQUE_ALPHA && !nearlyEqual(accent, background)) {
    return background;
  }
  return readableTextColorOn(accent);
}