import java.util.*;
import java.text.SimpleDateFormat;

public class Repository {

    private String name;
    private LinkedList<Commit> commits;

    public Repository(String name) {
        if (name.isEmpty() || name == null) {
            throw new IllegalArgumentException("Repository must have a name!");
        }
        this.name = name;
        this.commits = new LinkedList<>();
    }

    public String getRepoHead() {
        return commits.peek().id;
    }

    public int getRepoSize() {
        return commits.size();
    }

    public String toString() {
        return name + " -  Current head: " + commits.peek().toString();
    }

    public boolean contains(String targetId) {
        for (Commit commit : commits) {
            if (commit.id == targetId) {
                return true;
            }
        }
        return false;
    }

    public String getHistory(int n) {
        String output = "";
        for (int i = 0; i < commits.size() - 1; i++) {
            output += commits.get(i).toString() + "\n";
        }
        return output;
    }

    public String commit(String message) {
        commits.add(new Commit(message));
        return commits.peek().id;
    }

    public boolean drop(String targetId) {
        for (int n = 0; n < commits.size() - 1; n ++) {
            if (commits.get(n).id == targetId) {
                commits.remove(n);
                return true;
            }
        }
        return false;
    }

    public void synchronize(Repository other) {
        
    }
    /**
     * DO NOT MODIFY
     * A class that represents a single commit in the repository.
     * Commits are characterized by an identifier, a commit message,
     * and the time that the commit was made. A commit also stores
     * a reference to the immediately previous commit if it exists.
     *
     * Staff Note: You may notice that the comments in this 
     * class openly mention the fields of the class. This is fine 
     * because the fields of the Commit class are public. In general, 
     * be careful about revealing implementation details!
     */
    public class Commit {

        private static int currentCommitID;

        /**
         * The time, in milliseconds, at which this commit was created.
         */
        public final long timeStamp;

        /**
         * A unique identifier for this commit.
         */
        public final String id;

        /**
         * A message describing the changes made in this commit.
         */
        public final String message;

        /**
         * A reference to the previous commit, if it exists. Otherwise, null.
         */
        public Commit past;

        /**
         * Constructs a commit object. The unique identifier and timestamp
         * are automatically generated.
         * @param message A message describing the changes made in this commit.
         * @param past A reference to the commit made immediately before this
         *             commit.
         */
        public Commit(String message, Commit past) {
            this.id = "" + currentCommitID++;
            this.message = message;
            this.timeStamp = System.currentTimeMillis();
            this.past = past;
        }

        /**
         * Constructs a commit object with no previous commit. The unique
         * identifier and timestamp are automatically generated.
         * @param message A message describing the changes made in this commit.
         */
        public Commit(String message) {
            this(message, null);
        }

        /**
         * Returns a string representation of this commit. The string
         * representation consists of this commit's unique identifier,
         * timestamp, and message, in the following form:
         *      "[identifier] at [timestamp]: [message]"
         * @return The string representation of this collection.
         */
        @Override
        public String toString() {
            SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd 'at' HH:mm:ss z");
            Date date = new Date(timeStamp);

            return id + " at " + formatter.format(date) + ": " + message;
        }

        /**
        * Resets the IDs of the commit nodes such that they reset to 0.
        * Primarily for testing purposes.
        */
        public static void resetIds() {
            Commit.currentCommitID = 0;
        }
    }
}