/**
 * Short-code generation.
 *
 * Strategy (mirrors what the Go backend will do):
 *  - "random"   → base62 encoding of a monotonically increasing counter. This
 *                 yields the SHORTEST possible string that is still globally
 *                 unique (no collision checks, no wasted length). A counter of
 *                 N needs ceil(log62(N)) chars: 62^4 ≈ 14.7M, 62^5 ≈ 916M.
 *                 To avoid leaking sequence/volume, the counter is passed
 *                 through a reversible bit-scramble before encoding.
 *  - "memorable" → 3 short, easy words joined by hyphens.
 *  - "custom"    → user supplied (validated, authenticated only).
 */

const BASE62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

export function encodeBase62(num: number): string {
  if (num === 0) return BASE62[0];
  let n = num;
  let out = "";
  while (n > 0) {
    out = BASE62[n % 62] + out;
    n = Math.floor(n / 62);
  }
  return out;
}

/**
 * Reversible scramble so sequential counter values don't produce sequential
 * codes. Knuth multiplicative hash over a 31-bit space with a coprime
 * multiplier — keeps codes short while looking random.
 */
function scramble(id: number): number {
  const PRIME = 2654435761; // 2^32 golden-ratio prime
  const MOD = 0x7fffffff; // 2^31 - 1
  return ((id + 1) * PRIME) % MOD;
}

export function codeFromCounter(counter: number): string {
  // Bump the base so even the first link gets a pleasant 4-char code.
  return encodeBase62(scramble(counter + 100_000));
}

const CUSTOM_RE = /^[a-zA-Z0-9_-]{3,32}$/;

export function validateCustomAlias(alias: string): string | null {
  if (!alias) return "Pick an alias for your link.";
  if (alias.length < 3) return "At least 3 characters.";
  if (alias.length > 32) return "Keep it under 32 characters.";
  if (!CUSTOM_RE.test(alias))
    return "Only letters, numbers, hyphens and underscores.";
  return null;
}

const URL_RE = /^https?:\/\/[^\s.]+\.[^\s]{2,}$/i;

export function normalizeUrl(input: string): string {
  const trimmed = input.trim();
  if (!trimmed) return "";
  if (/^https?:\/\//i.test(trimmed)) return trimmed;
  return `https://${trimmed}`;
}

export function isValidUrl(input: string): boolean {
  return URL_RE.test(normalizeUrl(input));
}