/**
 * Stable user-id → color mapping for visual grouping in lists (e.g.
 * colour every row a quoter claimed so the team can see at a glance
 * who owns what).
 *
 * The mapping is deterministic — the same user id always resolves to
 * the same colour across reloads and across users. We use a 32-bit
 * FNV-1a hash so the palette selection is well-distributed without
 * importing a hash library.
 *
 * The palette is hand-tuned for accessibility: every colour is
 * distinct at a glance AND pairs with a matching tinted background
 * (5–10 % opacity) so the row highlight reads well in both light and
 * dark mode.
 */

export interface UserColor {
  /** Foreground accent — left border, dot, chip text. */
  accent: string;
  /** Background tint — used for row highlight, ~6–10% opacity. */
  tint: string;
  /** Solid chip background — used for the user-name pill. */
  chip: string;
  /** Foreground that reads on `chip`. */
  chipFg: string;
}

// Curated palette of 12 visually distinct hues. Going past 12 starts to
// cycle back to colours too close to neighbours. If the team grows past
// that, append more entries rather than re-ordering — existing user
// colour assignments must stay stable.
const PALETTE: UserColor[] = [
  { accent: "#2563eb", tint: "rgba(37, 99, 235, 0.10)", chip: "rgba(37, 99, 235, 0.18)", chipFg: "#1d4ed8" }, // blue
  { accent: "#dc2626", tint: "rgba(220, 38, 38, 0.10)", chip: "rgba(220, 38, 38, 0.18)", chipFg: "#b91c1c" }, // red
  { accent: "#16a34a", tint: "rgba(22, 163, 74, 0.10)", chip: "rgba(22, 163, 74, 0.18)", chipFg: "#15803d" }, // green
  { accent: "#ea580c", tint: "rgba(234, 88, 12, 0.10)", chip: "rgba(234, 88, 12, 0.18)", chipFg: "#c2410c" }, // orange
  { accent: "#9333ea", tint: "rgba(147, 51, 234, 0.10)", chip: "rgba(147, 51, 234, 0.18)", chipFg: "#7e22ce" }, // purple
  { accent: "#0891b2", tint: "rgba(8, 145, 178, 0.10)", chip: "rgba(8, 145, 178, 0.18)", chipFg: "#0e7490" }, // cyan
  { accent: "#db2777", tint: "rgba(219, 39, 119, 0.10)", chip: "rgba(219, 39, 119, 0.18)", chipFg: "#be185d" }, // pink
  { accent: "#ca8a04", tint: "rgba(202, 138, 4, 0.10)", chip: "rgba(202, 138, 4, 0.18)", chipFg: "#a16207" }, // amber
  { accent: "#0d9488", tint: "rgba(13, 148, 136, 0.10)", chip: "rgba(13, 148, 136, 0.18)", chipFg: "#0f766e" }, // teal
  { accent: "#7c3aed", tint: "rgba(124, 58, 237, 0.10)", chip: "rgba(124, 58, 237, 0.18)", chipFg: "#6d28d9" }, // violet
  { accent: "#65a30d", tint: "rgba(101, 163, 13, 0.10)", chip: "rgba(101, 163, 13, 0.18)", chipFg: "#4d7c0f" }, // lime
  { accent: "#475569", tint: "rgba(71, 85, 105, 0.10)", chip: "rgba(71, 85, 105, 0.18)", chipFg: "#334155" }, // slate
];

/**
 * FNV-1a 32-bit hash. Returns an unsigned 32-bit int. Fast enough for
 * per-row colour assignment; no need for crypto strength.
 */
function fnv1a(s: string): number {
  let h = 0x811c9dc5;
  for (let i = 0; i < s.length; i++) {
    h ^= s.charCodeAt(i);
    // Multiply by FNV prime, keeping within 32 bits via `>>> 0`.
    h = (h + ((h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24))) >>> 0;
  }
  return h >>> 0;
}

/**
 * Resolve a user id (PB record id) to a stable colour. Returns `null`
 * for empty input so callers can short-circuit.
 */
export function userColor(userId: string | null | undefined): UserColor | null {
  if (!userId) return null;
  const idx = fnv1a(userId) % PALETTE.length;
  return PALETTE[idx];
}

/**
 * Best-effort display name for a user. Prefers `name` (the PB auth
 * collection's human-readable field), falls back to the local-part of
 * `email`, then to "User".
 */
export function userDisplayName(
  user: { name?: string; email?: string } | null | undefined
): string {
  if (!user) return "—";
  const name = (user.name || "").trim();
  if (name) return name;
  const email = (user.email || "").trim();
  if (!email) return "—";
  return email.split("@")[0] || email;
}