// Unique ID generation utilities

import { randomBytes, randomUUID } from 'crypto';

const BASE62_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

// Largest multiple of 62 that fits in a byte (62 * 4 = 248). Bytes >= 248
// are rejected so every character is drawn with uniform probability
// (avoids modulo bias from `byte % 62`).
const REJECTION_THRESHOLD = Math.floor(256 / BASE62_CHARS.length) * BASE62_CHARS.length;

/**
 * Generate a random base62 string of the specified length.
 * Uses cryptographically secure random bytes with rejection sampling
 * to eliminate modulo bias.
 */
export function generateSessionId(length: number = 12): string {
  let result = '';

  while (result.length < length) {
    // Over-fetch a little to reduce the number of syscalls when rejecting.
    const bytes = randomBytes(length);
    for (let i = 0; i < bytes.length && result.length < length; i++) {
      const byte = bytes[i];
      if (byte >= REJECTION_THRESHOLD) {
        continue; // reject to avoid bias
      }
      result += BASE62_CHARS[byte % BASE62_CHARS.length];
    }
  }

  return result;
}

/**
 * Generate a UUID v4 for request IDs using the crypto module.
 */
export function generateRequestId(): string {
  return randomUUID();
}