Solving day 16

[truggeri]

Summary

Use a map to store all empty spaces (thus assuming the puzzle will be mostly walls). There are a few "performance" improvements here such as,

• Only store a solution if the cost is less than other solutions
• Save the cost it takes to get to a square, only continue with a thread if the cost is less than what another solution took to get to that square
• If the cost of a thread exceeds the current best solution, then don't bother continuing