import java.util.HashMap;
import java.util.Map;

public class Gradebook {
	/**
	 * Course -> Student -> Score.
	 */
	private final Map<String, Map<Student, Double>> grades;
	
	/**
	 * Gradebook constructor.
	 */
	public Gradebook() {
		this.grades = new HashMap<>(10);
	}
	
	/**
	 * Gets the average score for a given course. If the course can not be
	 * found, -1 is returned.
	 *
	 * @param course the course
	 * @return the average score, -1 if the course does not exist
	 */
	public double getAverageScore(String course) {
		double result = 0.0;
		
		Map<Student, Double> courseGrades = this.grades.get(course);
		if (courseGrades == null) {
			return -1.0;
		}
		
		int count = courseGrades.values().size();
		for (double score : courseGrades.values()) {
			result += score;
		}
		
		return (count == 0) ? 0.0 : (result / (double) count);
	}
	
	/**
	 * Gets the average score for a given student (across all courses). If the
	 * student can not be found in any course, -1 is returned.
	 *
	 * @param student the student
	 * @return the average score, -1 if the student isn't found in any course
	 */
	public double getAverageScore(Student student) {
		double result = 0.0;
		int count = 0;
		
		for (Map<Student, Double> studentGrades : this.grades.values()) {
			Double studentScore = studentGrades.get(student);
			if (studentScore != null) {
				result += studentScore;
				count += 1;
			}
		}
		
		return (count == 0) ? -1.0 : (result / (double) count);
	}
	
	/**
	 * Gets the score for a given course, for a given student. If the course or
	 * the student does not exist, -1 is returned.
	 *
	 * @param course  the course name
	 * @param student the student
	 * @return the score, -1 if not found
	 */
	public double getScore(String course, Student student) {
		Map<Student, Double> courseGrades = this.grades.get(course);
		if (courseGrades != null) {
			Double studentGrade = courseGrades.get(student);
			if (studentGrade != null) {
				return studentGrade;
			}
		}
		return -1.0;
	}
	
	/**
	 * Sets the score for a given student, for a given course. If the course or
	 * the student would not exist, they are created. If the student already
	 * has a score for this course, the previous score will be discarded and
	 * overwritten.
	 *
	 * @param course  the course name
	 * @param student the student
	 * @param score   the score to set
	 */
	public void setScore(String course, Student student, double score) {
		Map<Student, Double> scores = this.grades.get(course);
		if (scores == null) {
			scores = new HashMap<>(10);
		}
		
		scores.put(student, score);
		
		this.grades.put(course, scores);
	}
}