AdventOfCode/src/Problem2.java

package src;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class Problem2 {

    public static void main(String[] args) {
        String filePath = "inputs/inputProblem2.txt";

        try {
            String input = readFileAsString(filePath);
            List<long[]> ranges = parseRanges(input);
            long tally = solve(ranges);
            System.out.println("Sum of invalid IDs: " + tally);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // Solve by generating only repeated IDs
    public static long solve(List<long[]> ranges) {
        long sum = 0;

        for (long[] range : ranges) {
            long start = range[0];
            long end = range[1];

            for (long num = start; num <= end; num++) {
                String numStr = String.valueOf(num);
                int len = numStr.length();

                if (len % 2 != 0) continue;

                boolean repeated = false;
                for (int prefixLen = 1; prefixLen <= len / 2; prefixLen++) {
                    if (len % prefixLen != 0) continue; // must divide evenly

                    String prefix = numStr.substring(0, prefixLen);
                    String repeatedNum = prefix + prefix;
                    if (repeatedNum.equals(numStr)) {
                        repeated = true;
                        break;
                    }
                }

                if (repeated) sum += num;
            }
        }

        return sum;
    }

    public static String readFileAsString(String filePath) throws IOException {
        StringBuilder sb = new StringBuilder();
        try (BufferedReader reader = new BufferedReader(new FileReader(filePath))) {
            String line;
            while ((line = reader.readLine()) != null) {
                sb.append(line.trim());
            }
        }
        return sb.toString();
    }

    public static List<long[]> parseRanges(String input) {
        List<long[]> result = new ArrayList<>();
        String[] ranges = input.split(",");

        for (String range : ranges) {
            String[] parts = range.trim().split("-");
            if (parts.length != 2) {
                throw new IllegalArgumentException("Invalid range: " + range);
            }
            long start = Long.parseLong(parts[0]);
            long end = Long.parseLong(parts[1]);
            result.add(new long[]{start, end});
        }

        return result;
    }
}