Nice! I don't mind spoilers, though I've only looked at the 3 days I've done so far. It was interesting to see the similarities and differences between our implementations. For day 3, I was converting each character to a number, then summing the collection; if the resulting number was >= half the size of the collection, the majority were 1s. This worked for part 1, and for the oxygen part of part 2, but it bombed on the CO2 part; at one of the steps, there are two entries left, and both had a "1" in that position (ed: which filtered for "0", so I had no options left). So, I ended up reworking everything to count instead of sum. Darn shame - I thought that summing was a pretty elegant way to solve it. Elegant, but wrong... LOL