Source: Day 12: Christmas Tree Farm
Full solution for today (spoilers!).
Part 1
Solve the knapsack problem.
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But really, you are given a set of tiles (which all happen to be some subset of a 3x3) and a set of constraints–a MxN grid and how many of each tile to place. Count how many constraints are possible.
Tiles may be rotated and/or flipped.