const DARK_ACTION_FOREGROUND = "#18181B";
const LIGHT_ACTION_FOREGROUND = "#FFFFFF";
const MIN_TEXT_CONTRAST = 4.5;

function parseHexColor(color: string): [number, number, number] | null {
  const value = color.trim().replace(/^#/, "");
  const expanded =
    value.length === 3
      ? value
          .split("")
          .map((character) => character.repeat(2))
          .join("")
      : value.slice(0, 6);

  if (expanded.length !== 6 || !/^[0-9a-f]+$/i.test(expanded)) return null;

  return [
    Number.parseInt(expanded.slice(0, 2), 16),
    Number.parseInt(expanded.slice(2, 4), 16),
    Number.parseInt(expanded.slice(4, 6), 16),
  ];
}

function luminance(color: string): number | null {
  const rgb = parseHexColor(color);
  if (!rgb) return null;

  const channels = rgb.map((channel) => {
    const value = channel / 255;
    return value <= 0.04045 ? value / 12.92 : ((value + 0.055) / 1.055) ** 2.4;
  });

  return 0.2126 * channels[0]! + 0.7152 * channels[1]! + 0.0722 * channels[2]!;
}

function contrastRatio(foreground: string, background: string): number | null {
  const foregroundLuminance = luminance(foreground);
  const backgroundLuminance = luminance(background);
  if (foregroundLuminance == null || backgroundLuminance == null) return null;

  const lighter = Math.max(foregroundLuminance, backgroundLuminance);
  const darker = Math.min(foregroundLuminance, backgroundLuminance);
  return (lighter + 0.05) / (darker + 0.05);
}

/** Chooses a readable action label/icon color while preserving a valid requested color. */
export function resolveActionForeground(background: string, requested: string): string {
  const requestedContrast = contrastRatio(requested, background);
  if (requestedContrast == null || requestedContrast >= MIN_TEXT_CONTRAST) {
    return requested;
  }

  const darkContrast = contrastRatio(DARK_ACTION_FOREGROUND, background) ?? 0;
  const lightContrast = contrastRatio(LIGHT_ACTION_FOREGROUND, background) ?? 0;
  return darkContrast >= lightContrast ? DARK_ACTION_FOREGROUND : LIGHT_ACTION_FOREGROUND;
}