Solutions written in Elixir as Livebook notebooks.
Part 1 solved with Enum.reduce/3 with pattern-matching. 2nd part solved with Enum.reduce_while/3, pattern-matching, and guards.
Part 1 solved with Enum.reduce/3, Enum.min/1, and simple arithmetics. 2nd part solved in a very similar way, just another set of calculations.
Part 1 solved MapSet and Enum.reduce/3; in the accumulator we store visited coordinates (houses) and current one, every step we update the current coordinate according too rules.
2nd part solved in a very similar way, but we just have to track coordinates of both Santa and Robo, and use two clauses in the reduce cycle; to simplify the code, we extracted new coordinate calculation to a module.
Part 1 solved with pattern-matching on the first 20 bit of bitstring that get returned by :crypto.hash(:md5, "..."), plus a bit of recursion. 2nd part solved the same way (though initial solution got extended to check more bits in the head of a hash).
Both parts solved with Regex-es. Important to note that we rely on regex back-references to find pairs and repeatable groups. For example, /(hello) \1/ =~ "hello hello"; here \1 refers to the first declared capture group (hello).
Both parts solved with quite straightforward Enum.reduce and Map.update(map, key, default, fun) approach. Maybe not very performant (part 1 get calculated in 26-27 seconds, part 2 in 33-34 seconds), but works. We do not initialize light grid, because we leverage default argument of Map.update/4 function. Input parsed with Regex.run/2 and pattern matching on matched groups.