function camelToSnake(key: string): string {
  return key.replace(/[A-Z]/g, (letter) => `_${letter.toLowerCase()}`);
}

function snakeToCamel(key: string): string {
  return key.replace(/_([a-z0-9])/g, (_, letter) => letter.toUpperCase());
}

export function toSnakeCase<T extends Record<string, unknown>>(obj: T): Record<string, unknown> {
  const result: Record<string, unknown> = {};
  for (const [key, value] of Object.entries(obj)) {
    result[camelToSnake(key)] = value;
  }
  return result;
}

export function toCamelCase<T extends Record<string, unknown>>(obj: T): Record<string, unknown> {
  const result: Record<string, unknown> = {};
  for (const [key, value] of Object.entries(obj)) {
    result[snakeToCamel(key)] = value;
  }
  return result;
}

// Keeps only the allowed keys from an object — used to drop unexpected/protected
// fields from request bodies before they reach the database.
export function pick(
  obj: Record<string, unknown>,
  allowed: readonly string[]
): Record<string, unknown> {
  const allowedSet = new Set(allowed);
  const result: Record<string, unknown> = {};
  for (const [key, value] of Object.entries(obj)) {
    if (allowedSet.has(key)) result[key] = value;
  }
  return result;
}