ILP Part 106 — Golden waste

~ afish

This is the one hundred and sixth part of the ILP series. For your convenience you can find other parts in the table of contents in Part 1 – Boolean algebra

Let’s solve Golden Waste (Złoty Odpad) riddle. We have a board with golden coins on it. One of the coins is a starting point and is numbered 1. We need to go along the blue lines and collect all the coins with the following rules:

  • we must collect a coin when we get to it
  • when collecting a coin, we can either continue straight or turn left or right; we can’t turn around (do U turn)

Let’s solve that with ILP. The idea is as follows:

  • We want to number coins from one to n (number of coins); we represent this number with a bitset (to make things faster)
  • each coin has two bitsets indicating which direction we entered this coin (up, right, bottom, left) and which direction we left
  • for each coin, we analyze all four directions and make sure that all coins on the path in that direction have either lower number (so they were collected earlier) or we entered the first coin with higher number from the correct direction

Let’s see the code:

First, we define our coin class (lines 1-5). Next, we define a starting board (lines 7-16). We also count the number of coins (18) and then initialize variables (20-24).

Next, we iterate over all fields (26) and look for coins (28). We hardcode the starting point (31-34). Next, we make sure that the bitset indicating the number has exactly one value (36-37), that we can’t exit the coin to go back (to the U-turn) (39-42), and that there is exactly one input direction and output direction (44-48).

Now, we encode the main part. We first store a variable indicating whether this is the last coin to collect (53). We define a helper variable that will decode number bitset into a single integer so we can compare them easily (55-57). Finally, we encode the main function that builds the path.

The idea is: we start from some coin localThis and we check other fields along some straight line. When we spot another coin, we calculate if it has lower number (and was collected earlier) (63), or is exactly next to be collected (68-71), and that we entered from the right direction (73). We then enforce this with material implication (75-84). Finally, we return the flag indicating whether we should continue looking along this path (lines 86, 89).

We then use this function in four directions: (94-99, 101-106, 108-113, 115-120). Finally, we just make sure that all coins have different numbers (125-128) and solve the problem. Output: