The Weaver's Warp Card

The mill above bitwick stores every pattern on a punched warp card, and the cards that come back from the villages are written any old way. The weaver wants each one rewritten in the mill's own hand.

card is a string. Read it left to right as a sequence of blocks. A block is an optional count — one or more decimal digits, leading zeros allowed — followed by a unit. A unit is either a single lowercase letter, or one or more lowercase letters enclosed in square brackets. A block with no count has a count of 1. A block contributes its unit's letters repeated *count* times; a count of 0 contributes nothing. The card's thread is the contribution of every block, joined in order. An empty card has an empty thread.

A card is malformed if it does not split exactly into such blocks. That includes: any character other than a–z, 0–9, [, ]; a run of digits not immediately followed by a unit; a [ with no later ]; a ] with no open [; an empty pair of brackets; and anything other than lowercase letters between brackets.

Write canonicalCard(card). If the card is malformed, return the null value of your language (null in TypeScript, None in Python). Otherwise return a string: the mill's canonical writing of the thread, built like this.

• Cut the thread into maximal runs of one repeated letter.
• Cut each run of length L into pieces: if L is 9 or less it is a single piece of length L; otherwise take pieces of 9 from the left until 9 or fewer letters remain, and that remainder is the last piece.
• Write a piece of length 1 or 2 as that many copies of the letter. Write a piece of length 3 to 9 as the single digit, then the letter.
• Concatenate the pieces of every run, in order.

Every count in card has value at most 999, and the thread is at most a few thousand letters.