/**
 * Deterministic avatar background color per user/topic ID. Algorithm (Java-style String#hashCode,
 * rolling `hash = hash*31 + charCode`) ported from the old app's `letterTileColorId` -- but the old
 * app's palette had 16 colors (`% 16`) where this design's token palette (index.css's --color-avatar-1
 * through -11) has 11, so the modulus is adjusted to match; the hash function itself is unchanged.
 */
function stringToColorHash(value: string): number {
  let hash = 0;
  for (let i = 0; i < value.length; i++) {
    hash = (hash << 5) - hash + value.charCodeAt(i);
    hash = hash & hash;
  }
  return hash;
}

const AVATAR_COLOR_COUNT = 11;

/** 1-based index matching the --color-avatar-1..11 tokens in index.css. */
export function avatarColorIndex(id: string): number {
  return (Math.abs(stringToColorHash(id)) % AVATAR_COLOR_COUNT) + 1;
}

/**
 * Written out as literal strings, NOT template-generated (`bg-avatar-${i}`) -- Tailwind's build
 * finds utility classes by statically scanning source text for complete class-name strings, so a
 * dynamically-assembled name is invisible to it and never gets its CSS generated at all. That exact
 * bug shipped here once already: every avatar rendered with a fully transparent background instead
 * of its color, discovered by checking computed style, not by reading the class name off the DOM
 * (which looked correct).
 */
const AVATAR_BG_CLASSES = [
  'bg-avatar-1',
  'bg-avatar-2',
  'bg-avatar-3',
  'bg-avatar-4',
  'bg-avatar-5',
  'bg-avatar-6',
  'bg-avatar-7',
  'bg-avatar-8',
  'bg-avatar-9',
  'bg-avatar-10',
  'bg-avatar-11',
];

export function avatarBgClass(id: string): string {
  return AVATAR_BG_CLASSES[avatarColorIndex(id) - 1];
}

/** First character of a display name, matching the old app's letter-tile convention. */
export function avatarLetter(name: string | undefined): string {
  return (name || '').trim().slice(0, 1) || '?';
}