/**
 * Contrast utilities for the design-system contrast checker section.
 *
 * Implements the WCAG 2.1 relative-luminance + contrast-ratio formulas.
 * No external dependencies.
 */

export interface ContrastResult {
  ratio: number;
  passesAA: boolean;
  passesAALarge: boolean;
  passesAAA: boolean;
}

function srgbChannelToLinear(c: number): number {
  const v = c / 255;
  return v <= 0.03928 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, 2.4);
}

function relativeLuminance(hex: string): number {
  const parsed = parseHex(hex);
  if (!parsed) return 0;
  const [r, g, b] = parsed;
  const rl = srgbChannelToLinear(r);
  const gl = srgbChannelToLinear(g);
  const bl = srgbChannelToLinear(b);
  return 0.2126 * rl + 0.7152 * gl + 0.0722 * bl;
}

function parseHex(hex: string): [number, number, number] | null {
  const clean = hex.trim().replace(/^#/, '');
  if (clean.length === 3) {
    const r = parseInt(clean[0]! + clean[0]!, 16);
    const g = parseInt(clean[1]! + clean[1]!, 16);
    const b = parseInt(clean[2]! + clean[2]!, 16);
    return [r, g, b];
  }
  if (clean.length === 6) {
    const r = parseInt(clean.slice(0, 2), 16);
    const g = parseInt(clean.slice(2, 4), 16);
    const b = parseInt(clean.slice(4, 6), 16);
    return [r, g, b];
  }
  return null;
}

/**
 * Compute the WCAG 2.1 contrast ratio between two hex colors.
 * Returns a number in [1, 21].
 */
export function contrastRatio(fgHex: string, bgHex: string): number {
  const l1 = relativeLuminance(fgHex);
  const l2 = relativeLuminance(bgHex);
  const lighter = Math.max(l1, l2);
  const darker = Math.min(l1, l2);
  return (lighter + 0.05) / (darker + 0.05);
}

/**
 * Compute the full contrast result for a foreground/background pair.
 */
export function checkContrast(fgHex: string, bgHex: string): ContrastResult {
  const ratio = contrastRatio(fgHex, bgHex);
  return {
    ratio,
    passesAA: ratio >= 4.5,
    passesAALarge: ratio >= 3,
    passesAAA: ratio >= 7,
  };
}

/**
 * Read a CSS custom-property value (resolved) from `:root` at runtime.
 * Falls back to an empty string in non-DOM contexts.
 */
export function readToken(name: string): string {
  if (typeof document === 'undefined') return '';
  const value = getComputedStyle(document.documentElement).getPropertyValue(name).trim();
  return value;
}