The Tapping Captive of Wrenhold Keep

By commission of the Quiet Hand, bounty payable on delivery of a true reading.

A spy of the Quiet Hand lies chained in the lowest cell of Wrenhold Keep. Each night the spy knocks a message through the stone. A listener in the next cell writes down each group of knocks as a single number. The Hand needs someone who can turn those numbers back into the message.

Write decodeKnocks(knocks).

• knocks is an array of integers, each from 0 to 31 inclusive, in the order they were heard. It may be empty.
• Return the decoded message as a string.

The decoder is always in one of two alphabets, letters or figures. It starts in letters and begins with an empty message. Process the groups in order:

| Group | Effect | |---|---| | 1–26 | In letters: append the uppercase letter at that position in the alphabet (1 = A, 2 = B, …, 26 = Z). In figures: append the single digit equal to the group modulo 10 (so 1 → 1, 10 → 0, 26 → 6). | | 27 | Append a space, then switch to letters. | | 28 | Switch to figures. | | 29 | Switch to letters. | | 30 | Erase: remove the last character of the message. If the message is empty, nothing happens. The alphabet is unchanged. | | 31 | Repeat: apply again the most recent earlier group that was not a 31, with exactly the effect that group has in the current state. If there is no such earlier group, nothing happens. | | 0 | End: stop decoding. All later groups are ignored. |

Further rules:

• The most recent non-31 group counts for repeat even if it had no visible effect. Examples are an erase on an empty message, or a switch to the alphabet already in use.
• A 31 never becomes the group that a later 31 refers to. A run of several 31s therefore applies the same earlier group once for each 31.
• Switching to the alphabet already in use changes nothing.
• The decoder keeps no state other than the message, the current alphabet, and the most recent non-31 group.
• Return the message exactly as built. Keep any leading, trailing or consecutive spaces.