Continuing The Advent Of Code In T-SQL

Kevin Feasel

2018-12-06

T-SQL

Wayne Sheffield has a few more posts in the Advent of Code series.  His latest edition:

In Day 5, we find ourselves working with the polymers of the new Santa suit. A polymer (the input file), consists of units, represented by upper and lower case letters. Adjacent units of the same letter, but of different polarity (case), cancel each other out. This may lead to other units that can then cancel each other out. The goal is to reduce the polymer to as small as possible, and report back the reduced size.

Tasks:

  1. Perform a case-sensitive search/replace for each letter of the alphabet. The search is for a pair of the same letter, where one is upper case, and the other is lower case.
  2. Recursively perform this operation until the string can no longer be reduced.

In my opinion, the key part to this is that the operation needs to be performed recursively. I can think of only two ways to recursively perform an operation in SQL Server:

  1. A recursive common table expression (cte).
  2. Using a WHILE loop.

I don’t like using either of these mechanisms in SQL Server – they both perform operations in a “Row-By-Agonizing-Row” method, instead of a more set-based approach. However, set-based recursion usually performs extremely poorly. So, I’m going to use a while loop.

The recursion requirement does limit things a bit; otherwise I could see putting something together with the LEAD() window function.

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