Day5Part1.java

package uk.oczadly.karl.aoc20.solution.day5;

import uk.oczadly.karl.aoc20.input.IllegalInputException;
import uk.oczadly.karl.aoc20.input.PuzzleInput;
import uk.oczadly.karl.aoc20.PuzzleSolution;

/**
 * @author Karl Oczadly
 */
public class Day5Part1 extends PuzzleSolution {
    
    public Day5Part1() { super(5, 1); } // Initializes the day and part number
    
    @Override
    public Object solve(PuzzleInput input) {
        return input.asStream()
                .map(Day5Part1::parseSeat)
                .mapToInt(Seat::getId)
                .max().getAsInt();
    }
    
    
   /** Parses a Seat object (row and column) from a string input */
    private static Seat parseSeat(String str) {
        return new Seat(
                calcBinaryNumber(str.substring(0, 7)),
                calcBinaryNumber(str.substring(7, 10)));
    }
    
    /** Calculates the binary number from a sequence of steps. */
    private static int calcBinaryNumber(String str) {
        int lower = 0, upper = (1 << str.length()) - 1;
        for (char c : str.toCharArray()) {
            int range = upper - lower + 1;
            switch (c) {
                case 'F': case 'L':
                    upper -= range / 2; break;
                case 'B': case 'R':
                    lower += range / 2; break;
                default: throw new IllegalInputException("Unknown movement char.");
            }
        }
        if (lower != upper) throw new AssertionError();
        return lower;
    }
    
    static class Seat {
        final int row, column;
    
        public Seat(int row, int column) {
            this.row = row;
            this.column = column;
        }
    
        public int getId() {
            return row * 8 + column;
        }
    }
    
}