AoC2017_19.java

import static com.github.pareronia.aoc.Utils.toAString;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.List;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Stream;

import com.github.pareronia.aoc.CharGrid;
import com.github.pareronia.aoc.Grid.Cell;
import com.github.pareronia.aocd.Aocd;
import com.github.pareronia.aocd.Puzzle;

public final class AoC2017_19 extends AoCBase {
    
    private static final char CROSSING = '+';
    private static final char HORIZONTAL = '-';
    private static final char VERTICAL = '|';
    private static final char EMPTY = ' ';

    private final CharGrid grid;
    
    private AoC2017_19(final List<String> inputs, final boolean debug) {
        super(debug);
        this.grid = new CharGrid(inputs);
    }

    public static AoC2017_19 create(final List<String> input) {
        return new AoC2017_19(input, false);
    }

    public static AoC2017_19 createDebug(final List<String> input) {
        return new AoC2017_19(input, true);
    }
    
    private Stream<Cell> getPath() {
        final Deque<Cell> seen = new ArrayDeque<>();
        final Cell start = Cell.at(0, this.grid.getRowAsString(0).indexOf(VERTICAL));
        seen.addLast(start);
        log(start);
        Stream<Cell> stream = this.grid.getCellsS(start);
        while (true) {
            stream
                .peek(c -> log(c))
                .takeWhile(c -> this.grid.getValue(c) != EMPTY)
                .forEach(seen::addLast);
            final Cell last = seen.pollLast();
            final Cell prev = seen.peekLast();
            seen.addLast(last);
            final Optional<Cell> _next = this.grid.getCapitalNeighbours(last)
                    .filter(c -> !c.equals(prev))
                    .filter(c -> this.grid.getValue(c) != EMPTY)
                    .findFirst();
            if (_next.isEmpty()) {
                break;
            }
            final Cell next = _next.get();
            seen.addLast(next);
            log(next);
            if (next.getCol() < last.getCol()) {
                stream = this.grid.getCellsW(next);
            } else if (next.getCol() > last.getCol()) {
                stream = this.grid.getCellsE(next);
            } else if (next.getRow() < last.getRow()) {
                stream = this.grid.getCellsN(next);
            } else {
                stream = this.grid.getCellsS(next);
            }
        }
        return seen.stream();
    }
    
    @Override
    public String solvePart1() {
        return getPath()
            .map(this.grid::getValue)
            .filter(ch -> !Set.of(CROSSING, VERTICAL, HORIZONTAL).contains(ch))
            .collect(toAString());
    }
    
    @Override
    public Integer solvePart2() {
        return (int) getPath().count();
    }

    public static void main(final String[] args) throws Exception {
        assert AoC2017_19.createDebug(TEST).solvePart1().equals("ABCDEF");
        assert AoC2017_19.createDebug(TEST).solvePart2().equals(38);

        final Puzzle puzzle = Aocd.puzzle(2017, 19);
        puzzle.check(
            () -> lap("Part 1", () -> AoC2017_19.create(puzzle.getInputData()).solvePart1()),
            () -> lap("Part 2", () -> AoC2017_19.create(puzzle.getInputData()).solvePart2())
        );
    }
    
    private static final List<String> TEST = splitLines(
            "     |          \r\n" +
            "     |  +--+    \r\n" +
            "     A  |  C    \r\n" +
            " F---|----E|--+ \r\n" +
            "     |  |  |  D \r\n" +
            "     +B-+  +--+ "
    );
}