/**
 * WCAG 2.2 contrast ratio utilities for runtime color validation
 */

export function hexToRgb(hex: string): [number, number, number] {
  const clean = hex.replace('#', '');
  const full = clean.length === 3
    ? clean.split('').map(c => c + c).join('')
    : clean;
  const num = parseInt(full, 16);
  return [(num >> 16) & 255, (num >> 8) & 255, num & 255];
}

export function relativeLuminance([r, g, b]: [number, number, number]): number {
  const [rs, gs, bs] = [r, g, b].map(c => {
    const s = c / 255;
    return s <= 0.04045 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
  });
  return 0.2126 * rs + 0.7152 * gs + 0.0722 * bs;
}

export function contrastRatio(color1: string, color2: string): number {
  const l1 = relativeLuminance(hexToRgb(color1));
  const l2 = relativeLuminance(hexToRgb(color2));
  const lighter = Math.max(l1, l2);
  const darker = Math.min(l1, l2);
  return (lighter + 0.05) / (darker + 0.05);
}

/**
 * Adjusts foreground color to meet minimum contrast ratio against background.
 * Lightens or darkens the foreground as needed.
 */
export function ensureContrast(fg: string, bg: string, minRatio: number): string {
  if (contrastRatio(fg, bg) >= minRatio) return fg;

  const bgLum = relativeLuminance(hexToRgb(bg));
  const [r, g, b] = hexToRgb(fg);

  // Determine direction: lighten if bg is dark, darken if bg is light
  const lighten = bgLum < 0.5;

  for (let step = 1; step <= 50; step++) {
    const factor = step * 0.02;
    let nr: number, ng: number, nb: number;

    if (lighten) {
      nr = Math.min(255, Math.round(r + (255 - r) * factor));
      ng = Math.min(255, Math.round(g + (255 - g) * factor));
      nb = Math.min(255, Math.round(b + (255 - b) * factor));
    } else {
      nr = Math.max(0, Math.round(r * (1 - factor)));
      ng = Math.max(0, Math.round(g * (1 - factor)));
      nb = Math.max(0, Math.round(b * (1 - factor)));
    }

    const adjusted = '#' + [nr, ng, nb].map(c => c.toString(16).padStart(2, '0')).join('');
    if (contrastRatio(adjusted, bg) >= minRatio) return adjusted;
  }

  // Fallback: return white or black
  return lighten ? '#FFFFFF' : '#000000';
}

/**
 * Determines if text at given size/weight qualifies as "large text" per WCAG.
 * Large text = 18pt+ (24px+) or 14pt+ bold (18.66px+)
 */
export function isLargeText(sizePt: number, bold: boolean): boolean {
  return sizePt >= 18 || (bold && sizePt >= 14);
}