CIDR-v1.java

 public List<String> ipToCIDR(String ip, long n) {
        List<String> result = new ArrayList<>();
        long ipNum = 0;
        String[] tokens = ip.split("\\.");
        for (int i = 0; i < 4; i++) {
            ipNum = ipNum * 256 + (long) Integer.parseInt(tokens[i]);
        }
        if (n + ipNum > 4294967296L) {
            return result;
        } else if (n == 4294967296L) {
            result.add("0.0.0.0/0");
            return result;
        }
        long cnt = 0;
        while(cnt < n) {
            // get rightmost set bit
            long x = ipNum & -ipNum;
            if (x == 0) {
                x = 1;
                while((x << 1) <= n) {
                    x <<= 1;
                }
            } else {
                while(cnt + x > n) {
                    x >>= 1;
                }
            }
            cnt += x;
            result.add(toCIDR(ipNum, x));
            ipNum += x;
        }
        return result;
    }
    public String toCIDR(long num, long x) {
        int cnt = 0;
        while(x > 0) {
            x >>= 1;
            cnt += 1;
        }
        int[] ip = new int[4];
        for (int i = 3; i >= 0; i--) {
            ip[i] = (int) num & 255;
            num >>= 8;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 4; i++) {
            sb.append(ip[i]).append(".");
        }
        sb.setLength(sb.length() - 1);
        int preBit = 32 - (cnt - 1);
        sb.append("/").append(preBit);
        return sb.toString();
    }

CIDR.java

import org.junit.Test;

import java.io.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class CIDR {

    class InputValues {
        String ip;
        long n;
        public InputValues(String ip, long n) {
            this.ip = ip;
            this.n = n;
        }
        @Override
        public String toString() {
            return ip + "\t" + n;
        }
    }

    @Test
    public void test() {
        System.out.println(0 & -0);
    }
    
    public void generateOutput() throws IOException {
        File[] files = readFolder("src/input/CIDR");
        for (int i = 0; i < files.length; i++) {
            File file = files[i];
            CIDR.InputValues test = readFile(file);
            List<String> res = ipToCIDR(test.ip, test.n);
            String outfileName = "src/output/CIDR/" + file.getName().replace(".in", ".out");
            File outfile = new File(outfileName);
            outfile.getParentFile().mkdirs();
            outfile.createNewFile();
            PrintWriter printWriter = new PrintWriter(new FileWriter(outfileName));
            for (String cidr : res) {
                printWriter.print(cidr + " ");
            }
            printWriter.close();
        }
    }

    public List<String> ipToCIDR(String ip, long n) {
        List<String> result = new ArrayList<>();
        long ipNum = 0;
        String[] tokens = ip.split("\\.");
        for (int i = 0; i < 4; i++) {
            ipNum = ipNum * 256 + (long) Integer.parseInt(tokens[i]);
        }
        if (n + ipNum > 4294967296L) {
            return result;
        } else if (n == 4294967296L) {
            result.add("0.0.0.0/0");
            return result;
        }
        long cnt = 0;
        while(cnt < n) {
            // get rightmost set bit
            long x = ipNum & -ipNum;
            if (x == 0) {
                x = 1;
                while((x << 1) <= n) {
                    x <<= 1;
                }
            } else {
                while(cnt + x > n) {
                    x >>= 1;
                }
            }
            cnt += x;
            result.add(toCIDR(ipNum, x));
            ipNum += x;
        }
        return result;
    }
    public String toCIDR(long num, long x) {
        int cnt = 0;
        while(x > 0) {
            x >>= 1;
            cnt += 1;
        }
        int[] ip = new int[4];
        for (int i = 3; i >= 0; i--) {
            ip[i] = (int) num & 255;
            num >>= 8;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 4; i++) {
            sb.append(ip[i]).append(".");
        }
        sb.setLength(sb.length() - 1);
        int preBit = 32 - (cnt - 1);
        sb.append("/").append(preBit);
        return sb.toString();
    }

    private int getInt(String name) {
        return Integer.parseInt(name.replaceAll("\\D", ""));
    }

    private File[] readFolder(String path) {
        File folder = new File(path);
        File[] files = folder.listFiles();
        Arrays.sort(files, (a, b) -> getInt(a.getName()) - getInt(b.getName()));
        return files;
    }

    private CIDR.InputValues readFile(File file) {
        try {
            // assume the input file is always valid
            Scanner sc = new Scanner(file);
            String ip = sc.nextLine().trim();
            long n = Long.parseLong(sc.nextLine().trim());
            return new CIDR.InputValues(ip, n);
        } catch (FileNotFoundException e) {
            System.out.println(file.getName() + " not exist");
        }
        return null;
    }
}