The Beacon-Codes of Ostrel Point

Along the black cliffs of Ostrel Point, the Keepers' Guild flashes coded warnings from tower to tower whenever raider sails are sighted. A malformed code can send a whole fleet onto the rocks. The Guild posts a bounty for a checker that inspects each code before it is lit and names exactly what is wrong with it.

Write checkBeacon(code), where code is a string. Return a string: the label of the first rule below that code breaks, checking the rules in the order listed. If it breaks none of them, return "valid".

1. "empty": the code is the empty string. 2. "length": the code is shorter than 4 characters or longer than 12 characters. 3. "symbol": the code contains any character that is not an uppercase ASCII letter A-Z or an ASCII digit 0-9. Lowercase letters, spaces and punctuation all break this rule. 4. "start": the first character of the code is not a letter. 5. "digit": the code contains no digit at all. 6. "repeat": the same character appears three or more times consecutively, in adjacent positions.

A code may break several rules at once. Only the earliest broken rule in this list is reported.