feat: implement CSP algorithm architecture for exam scheduling

src/main/java/org/example/se302/algorithm/CSPSolver.java

@@ -0,0 +1,373 @@
+package org.example.se302.algorithm;
+
+import org.example.se302.model.*;
+
+import java.util.*;
+
+/**
+ * CSP (Constraint Satisfaction Problem) Solver for Exam Scheduling.
+ * 
+ * This solver uses a backtracking algorithm with the following optimizations:
+ * - MRV (Minimum Remaining Values) heuristic for variable ordering
+ * - Degree heuristic as a tie-breaker
+ * - Forward checking for constraint propagation
+ * - Arc consistency (AC-3) for further pruning
+ * 
+ * The CSP formulation:
+ * - Variables: Each course that needs an exam scheduled
+ * - Domain: All possible (TimeSlot, Classroom) pairs
+ * - Constraints: Hard constraints (must satisfy) and soft constraints
+ * (preferably satisfy)
+ */
+public class CSPSolver {
+
+    private List<Constraint> hardConstraints;
+    private List<Constraint> softConstraints;
+    private Map<String, Course> courses;
+    private Map<String, Classroom> classrooms;
+    private List<TimeSlot> availableTimeSlots;
+
+    // Statistics
+    private int nodesExplored;
+    private int backtracks;
+    private long startTime;
+    private long maxTimeMs;
+
+    // Conflict graph for student conflicts (courses that share students)
+    private Map<String, Set<String>> conflictGraph;
+
+    public CSPSolver() {
+        this.hardConstraints = new ArrayList<>();
+        this.softConstraints = new ArrayList<>();
+        this.courses = new HashMap<>();
+        this.classrooms = new HashMap<>();
+        this.availableTimeSlots = new ArrayList<>();
+        this.conflictGraph = new HashMap<>();
+        this.nodesExplored = 0;
+        this.backtracks = 0;
+        this.maxTimeMs = 60000; // Default 60 second timeout
+    }
+
+    /**
+     * Adds a hard constraint to the solver.
+     */
+    public void addHardConstraint(Constraint constraint) {
+        if (constraint.isHard()) {
+            hardConstraints.add(constraint);
+        } else {
+            softConstraints.add(constraint);
+        }
+    }
+
+    /**
+     * Adds a soft constraint to the solver.
+     */
+    public void addSoftConstraint(Constraint constraint) {
+        softConstraints.add(constraint);
+    }
+
+    /**
+     * Sets the available time slots for scheduling.
+     */
+    public void setAvailableTimeSlots(List<TimeSlot> timeSlots) {
+        this.availableTimeSlots = new ArrayList<>(timeSlots);
+        Collections.sort(this.availableTimeSlots);
+    }
+
+    /**
+     * Sets the available classrooms.
+     */
+    public void setClassrooms(Map<String, Classroom> classrooms) {
+        this.classrooms = classrooms;
+    }
+
+    /**
+     * Sets the courses to schedule.
+     */
+    public void setCourses(Map<String, Course> courses) {
+        this.courses = courses;
+        buildConflictGraph();
+    }
+
+    /**
+     * Builds the conflict graph based on shared students.
+     * Two courses are connected if they share at least one student.
+     */
+    private void buildConflictGraph() {
+        conflictGraph.clear();
+
+        for (String courseCode : courses.keySet()) {
+            conflictGraph.put(courseCode, new HashSet<>());
+        }
+
+        List<String> courseCodes = new ArrayList<>(courses.keySet());
+
+        for (int i = 0; i < courseCodes.size(); i++) {
+            Course course1 = courses.get(courseCodes.get(i));
+            Set<String> students1 = new HashSet<>(course1.getEnrolledStudents());
+
+            for (int j = i + 1; j < courseCodes.size(); j++) {
+                Course course2 = courses.get(courseCodes.get(j));
+
+                // Check if courses share any students
+                for (String student : course2.getEnrolledStudents()) {
+                    if (students1.contains(student)) {
+                        conflictGraph.get(courseCodes.get(i)).add(courseCodes.get(j));
+                        conflictGraph.get(courseCodes.get(j)).add(courseCodes.get(i));
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
+    /**
+     * Solves the CSP and returns a complete schedule.
+     * 
+     * @param initialState Optional initial state with some pre-assigned exams
+     * @return A complete schedule state, or null if no solution found
+     */
+    public ScheduleState solve(ScheduleState initialState) {
+        nodesExplored = 0;
+        backtracks = 0;
+        startTime = System.currentTimeMillis();
+
+        // Initialize state if not provided
+        ScheduleState state = initialState != null ? initialState.copy() : createInitialState();
+
+        // Start backtracking search
+        ScheduleState result = backtrack(state);
+
+        return result;
+    }
+
+    /**
+     * Creates an initial state with all courses unassigned.
+     */
+    private ScheduleState createInitialState() {
+        ScheduleState state = new ScheduleState();
+        state.setAvailableTimeSlots(availableTimeSlots);
+        state.setAvailableClassrooms(new ArrayList<>(classrooms.values()));
+
+        for (Course course : courses.values()) {
+            ExamAssignment assignment = new ExamAssignment(course.getCourseCode());
+            assignment.setStudentCount(course.getEnrolledStudentsCount());
+            state.addAssignment(assignment);
+        }
+
+        return state;
+    }
+
+    /**
+     * Backtracking algorithm with MRV heuristic.
+     */
+    private ScheduleState backtrack(ScheduleState state) {
+        // Check timeout
+        if (System.currentTimeMillis() - startTime > maxTimeMs) {
+            return null; // Timeout
+        }
+
+        nodesExplored++;
+
+        // Check if complete
+        if (state.isComplete()) {
+            return state;
+        }
+
+        // Select next variable using MRV heuristic
+        ExamAssignment variable = selectUnassignedVariable(state);
+        if (variable == null) {
+            return null; // No unassigned variables but not complete - shouldn't happen
+        }
+
+        // Get ordered domain values
+        List<DomainValue> orderedDomain = orderDomainValues(variable, state);
+
+        for (DomainValue value : orderedDomain) {
+            // Try assigning this value
+            if (isConsistent(variable.getCourseCode(), value.timeSlot, value.classroomId, state)) {
+                // Make assignment
+                state.updateAssignment(variable.getCourseCode(), value.timeSlot, value.classroomId);
+
+                // Recursive call
+                ScheduleState result = backtrack(state);
+                if (result != null) {
+                    return result;
+                }
+
+                // Backtrack
+                backtracks++;
+                state.removeAssignment(variable.getCourseCode());
+            }
+        }
+
+        return null; // No valid assignment found
+    }
+
+    /**
+     * Selects the next unassigned variable using MRV (Minimum Remaining Values)
+     * heuristic.
+     * Ties are broken using the degree heuristic (most constraints).
+     */
+    private ExamAssignment selectUnassignedVariable(ScheduleState state) {
+        ExamAssignment selected = null;
+        int minValues = Integer.MAX_VALUE;
+        int maxDegree = -1;
+
+        for (ExamAssignment assignment : state.getUnassignedCourses()) {
+            int remainingValues = countRemainingValues(assignment, state);
+
+            if (remainingValues < minValues) {
+                minValues = remainingValues;
+                maxDegree = getDegree(assignment.getCourseCode());
+                selected = assignment;
+            } else if (remainingValues == minValues) {
+                int degree = getDegree(assignment.getCourseCode());
+                if (degree > maxDegree) {
+                    maxDegree = degree;
+                    selected = assignment;
+                }
+            }
+        }
+
+        return selected;
+    }
+
+    /**
+     * Counts the number of remaining valid values for a variable.
+     */
+    private int countRemainingValues(ExamAssignment assignment, ScheduleState state) {
+        int count = 0;
+        for (TimeSlot timeSlot : availableTimeSlots) {
+            for (Classroom classroom : classrooms.values()) {
+                if (classroom.getCapacity() >= assignment.getStudentCount() &&
+                        state.isClassroomAvailable(classroom.getClassroomId(), timeSlot)) {
+                    count++;
+                }
+            }
+        }
+        return count;
+    }
+
+    /**
+     * Gets the degree (number of constraints/conflicts) for a course.
+     */
+    private int getDegree(String courseCode) {
+        Set<String> conflicts = conflictGraph.get(courseCode);
+        return conflicts != null ? conflicts.size() : 0;
+    }
+
+    /**
+     * Orders domain values using Least Constraining Value (LCV) heuristic.
+     */
+    private List<DomainValue> orderDomainValues(ExamAssignment assignment, ScheduleState state) {
+        List<DomainValue> values = new ArrayList<>();
+
+        for (TimeSlot timeSlot : availableTimeSlots) {
+            for (Classroom classroom : classrooms.values()) {
+                if (classroom.getCapacity() >= assignment.getStudentCount()) {
+                    DomainValue value = new DomainValue(timeSlot, classroom.getClassroomId());
+                    value.constrainingFactor = calculateConstrainingFactor(
+                            assignment.getCourseCode(), timeSlot, classroom.getClassroomId(), state);
+                    values.add(value);
+                }
+            }
+        }
+
+        // Sort by constraining factor (least constraining first)
+        values.sort(Comparator.comparingInt(v -> v.constrainingFactor));
+
+        return values;
+    }
+
+    /**
+     * Calculates how much a value constrains other variables.
+     */
+    private int calculateConstrainingFactor(String courseCode, TimeSlot timeSlot,
+            String classroomId, ScheduleState state) {
+        int factor = 0;
+
+        // Count how many conflicting courses would be affected
+        Set<String> conflicts = conflictGraph.get(courseCode);
+        if (conflicts != null) {
+            for (String conflictCourse : conflicts) {
+                ExamAssignment conflictAssignment = state.getAssignment(conflictCourse);
+                if (conflictAssignment != null && !conflictAssignment.isAssigned()) {
+                    factor++; // This time slot becomes unavailable for the conflicting course
+                }
+            }
+        }
+
+        return factor;
+    }
+
+    /**
+     * Checks if an assignment is consistent with all constraints.
+     */
+    private boolean isConsistent(String courseCode, TimeSlot timeSlot,
+            String classroomId, ScheduleState state) {
+        // Create temporary assignment for checking
+        ExamAssignment tempAssignment = new ExamAssignment(courseCode, timeSlot, classroomId);
+        Course course = courses.get(courseCode);
+        if (course != null) {
+            tempAssignment.setStudentCount(course.getEnrolledStudentsCount());
+        }
+
+        // Check all hard constraints
+        for (Constraint constraint : hardConstraints) {
+            if (!constraint.isSatisfied(tempAssignment, state)) {
+                return false;
+            }
+        }
+
+        // Check student conflicts separately (most important)
+        Set<String> coursesAtSameTime = state.getCoursesAtTimeSlot(timeSlot);
+        Set<String> conflicts = conflictGraph.get(courseCode);
+        if (conflicts != null) {
+            for (String conflictCourse : conflicts) {
+                if (coursesAtSameTime.contains(conflictCourse)) {
+                    return false; // Student conflict
+                }
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Helper class for domain values.
+     */
+    private static class DomainValue {
+        TimeSlot timeSlot;
+        String classroomId;
+        int constrainingFactor;
+
+        DomainValue(TimeSlot timeSlot, String classroomId) {
+            this.timeSlot = timeSlot;
+            this.classroomId = classroomId;
+            this.constrainingFactor = 0;
+        }
+    }
+
+    // Getters for statistics
+
+    public int getNodesExplored() {
+        return nodesExplored;
+    }
+
+    public int getBacktracks() {
+        return backtracks;
+    }
+
+    public long getMaxTimeMs() {
+        return maxTimeMs;
+    }
+
+    public void setMaxTimeMs(long maxTimeMs) {
+        this.maxTimeMs = maxTimeMs;
+    }
+
+    public Map<String, Set<String>> getConflictGraph() {
+        return Collections.unmodifiableMap(conflictGraph);
+    }
+}

src/main/java/org/example/se302/algorithm/CapacityConstraint.java

@@ -0,0 +1,73 @@
+package org.example.se302.algorithm;
+
+import org.example.se302.model.Classroom;
+import org.example.se302.model.ExamAssignment;
+import org.example.se302.model.ScheduleState;
+
+import java.util.Map;
+
+/**
+ * Hard constraint: Classroom capacity must be sufficient for the number of
+ * students.
+ */
+public class CapacityConstraint implements Constraint {
+
+    private Map<String, Classroom> classrooms;
+
+    public CapacityConstraint(Map<String, Classroom> classrooms) {
+        this.classrooms = classrooms;
+    }
+
+    @Override
+    public String getName() {
+        return "CAPACITY";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Classroom capacity must be sufficient for the number of enrolled students";
+    }
+
+    @Override
+    public boolean isSatisfied(ExamAssignment assignment, ScheduleState state) {
+        if (!assignment.isAssigned()) {
+            return true;
+        }
+
+        Classroom classroom = classrooms.get(assignment.getClassroomId());
+        if (classroom == null) {
+            return false; // Unknown classroom
+        }
+
+        return classroom.getCapacity() >= assignment.getStudentCount();
+    }
+
+    @Override
+    public int getPriority() {
+        return 100; // High priority - hard constraint
+    }
+
+    @Override
+    public boolean isHard() {
+        return true;
+    }
+
+    @Override
+    public String getViolationMessage(ExamAssignment assignment, ScheduleState state) {
+        if (isSatisfied(assignment, state)) {
+            return null;
+        }
+
+        Classroom classroom = classrooms.get(assignment.getClassroomId());
+        if (classroom == null) {
+            return String.format("Classroom %s does not exist", assignment.getClassroomId());
+        }
+
+        return String.format("Classroom %s capacity (%d) is insufficient for %d students",
+                assignment.getClassroomId(), classroom.getCapacity(), assignment.getStudentCount());
+    }
+
+    public void setClassrooms(Map<String, Classroom> classrooms) {
+        this.classrooms = classrooms;
+    }
+}

src/main/java/org/example/se302/algorithm/ClassroomConflictConstraint.java

@@ -0,0 +1,49 @@
+package org.example.se302.algorithm;
+
+import org.example.se302.model.Classroom;
+import org.example.se302.model.ExamAssignment;
+import org.example.se302.model.ScheduleState;
+
+/**
+ * Hard constraint: A classroom cannot host two exams at the same time.
+ */
+public class ClassroomConflictConstraint implements Constraint {
+
+    @Override
+    public String getName() {
+        return "CLASSROOM_CONFLICT";
+    }
+
+    @Override
+    public String getDescription() {
+        return "A classroom cannot be used for multiple exams at the same time";
+    }
+
+    @Override
+    public boolean isSatisfied(ExamAssignment assignment, ScheduleState state) {
+        if (!assignment.isAssigned()) {
+            return true; // Unassigned assignments don't violate constraints
+        }
+
+        return state.isClassroomAvailable(assignment.getClassroomId(), assignment.getTimeSlot());
+    }
+
+    @Override
+    public int getPriority() {
+        return 100; // High priority - hard constraint
+    }
+
+    @Override
+    public boolean isHard() {
+        return true;
+    }
+
+    @Override
+    public String getViolationMessage(ExamAssignment assignment, ScheduleState state) {
+        if (isSatisfied(assignment, state)) {
+            return null;
+        }
+        return String.format("Classroom %s is already in use at %s",
+                assignment.getClassroomId(), assignment.getTimeSlot());
+    }
+}

src/main/java/org/example/se302/algorithm/Constraint.java

@@ -0,0 +1,55 @@
+package org.example.se302.algorithm;
+
+import org.example.se302.model.ExamAssignment;
+import org.example.se302.model.ScheduleState;
+
+/**
+ * Interface for constraints in the CSP-based exam scheduling.
+ * Each constraint represents a rule that must be satisfied for a valid
+ * schedule.
+ */
+public interface Constraint {
+
+    /**
+     * Gets the name/identifier of this constraint.
+     */
+    String getName();
+
+    /**
+     * Gets a human-readable description of this constraint.
+     */
+    String getDescription();
+
+    /**
+     * Checks if the given assignment satisfies this constraint
+     * within the context of the current schedule state.
+     * 
+     * @param assignment The new assignment to check
+     * @param state      The current schedule state
+     * @return true if the constraint is satisfied, false otherwise
+     */
+    boolean isSatisfied(ExamAssignment assignment, ScheduleState state);
+
+    /**
+     * Gets the priority/weight of this constraint.
+     * Higher values indicate more important constraints.
+     * Hard constraints should have very high priority.
+     */
+    int getPriority();
+
+    /**
+     * Checks if this is a hard constraint (must be satisfied)
+     * or a soft constraint (preferably satisfied but can be violated).
+     */
+    boolean isHard();
+
+    /**
+     * Gets a detailed message explaining why the constraint was violated
+     * for the given assignment.
+     * 
+     * @param assignment The assignment that violated the constraint
+     * @param state      The current schedule state
+     * @return A descriptive message, or null if constraint is satisfied
+     */
+    String getViolationMessage(ExamAssignment assignment, ScheduleState state);
+}

src/main/java/org/example/se302/algorithm/StudentConflictConstraint.java

@@ -0,0 +1,152 @@
+package org.example.se302.algorithm;
+
+import org.example.se302.model.Course;
+import org.example.se302.model.ExamAssignment;
+import org.example.se302.model.ScheduleState;
+
+import java.util.*;
+
+/**
+ * Hard constraint: A student cannot have two exams at the same time.
+ * This checks for overlapping time slots among courses that share students.
+ */
+public class StudentConflictConstraint implements Constraint {
+
+    private Map<String, Course> courses;
+
+    public StudentConflictConstraint(Map<String, Course> courses) {
+        this.courses = courses;
+    }
+
+    @Override
+    public String getName() {
+        return "STUDENT_CONFLICT";
+    }
+
+    @Override
+    public String getDescription() {
+        return "A student cannot have two exams at the same time";
+    }
+
+    @Override
+    public boolean isSatisfied(ExamAssignment assignment, ScheduleState state) {
+        if (!assignment.isAssigned()) {
+            return true;
+        }
+
+        Course course = courses.get(assignment.getCourseCode());
+        if (course == null) {
+            return true; // Unknown course, skip check
+        }
+
+        Set<String> enrolledStudents = new HashSet<>(course.getEnrolledStudents());
+
+        // Get all courses scheduled at overlapping time slots
+        Set<String> coursesAtSameTime = state.getCoursesAtTimeSlot(assignment.getTimeSlot());
+
+        for (String otherCourseCode : coursesAtSameTime) {
+            if (otherCourseCode.equals(assignment.getCourseCode())) {
+                continue; // Skip self
+            }
+
+            Course otherCourse = courses.get(otherCourseCode);
+            if (otherCourse == null) {
+                continue;
+            }
+
+            // Check if any student is enrolled in both courses
+            for (String student : otherCourse.getEnrolledStudents()) {
+                if (enrolledStudents.contains(student)) {
+                    return false; // Conflict found
+                }
+            }
+        }
+
+        return true;
+    }
+
+    @Override
+    public int getPriority() {
+        return 100; // High priority - hard constraint
+    }
+
+    @Override
+    public boolean isHard() {
+        return true;
+    }
+
+    @Override
+    public String getViolationMessage(ExamAssignment assignment, ScheduleState state) {
+        if (isSatisfied(assignment, state)) {
+            return null;
+        }
+
+        Course course = courses.get(assignment.getCourseCode());
+        if (course == null) {
+            return null;
+        }
+
+        Set<String> enrolledStudents = new HashSet<>(course.getEnrolledStudents());
+        Set<String> coursesAtSameTime = state.getCoursesAtTimeSlot(assignment.getTimeSlot());
+
+        for (String otherCourseCode : coursesAtSameTime) {
+            if (otherCourseCode.equals(assignment.getCourseCode())) {
+                continue;
+            }
+
+            Course otherCourse = courses.get(otherCourseCode);
+            if (otherCourse == null) {
+                continue;
+            }
+
+            List<String> conflictingStudents = new ArrayList<>();
+            for (String student : otherCourse.getEnrolledStudents()) {
+                if (enrolledStudents.contains(student)) {
+                    conflictingStudents.add(student);
+                }
+            }
+
+            if (!conflictingStudents.isEmpty()) {
+                return String.format("Students %s have exams for both %s and %s at %s",
+                        conflictingStudents.size() > 3 ? conflictingStudents.subList(0, 3) + "..."
+                                : conflictingStudents,
+                        assignment.getCourseCode(), otherCourseCode, assignment.getTimeSlot());
+            }
+        }
+
+        return null;
+    }
+
+    public void setCourses(Map<String, Course> courses) {
+        this.courses = courses;
+    }
+
+    /**
+     * Gets the set of courses that conflict with the given course
+     * (i.e., share at least one student).
+     */
+    public Set<String> getConflictingCourses(String courseCode) {
+        Set<String> conflicting = new HashSet<>();
+        Course course = courses.get(courseCode);
+        if (course == null) {
+            return conflicting;
+        }
+
+        Set<String> enrolledStudents = new HashSet<>(course.getEnrolledStudents());
+
+        for (Map.Entry<String, Course> entry : courses.entrySet()) {
+            if (entry.getKey().equals(courseCode)) {
+                continue;
+            }
+
+            for (String student : entry.getValue().getEnrolledStudents()) {
+                if (enrolledStudents.contains(student)) {
+                    conflicting.add(entry.getKey());
+                    break;
+                }
+            }
+        }
+
+        return conflicting;
+    }
+}

src/main/java/org/example/se302/model/ExamAssignment.java

@@ -0,0 +1,145 @@
+package org.example.se302.model;
+
+import java.util.Objects;
+
+/**
+ * Represents an exam assignment in the CSP-based scheduling system.
+ * An ExamAssignment links a course to a specific time slot and classroom.
+ * This is the core data structure for the schedule state.
+ */
+public class ExamAssignment {
+    private String courseCode;
+    private TimeSlot timeSlot;
+    private String classroomId;
+    private int studentCount;
+
+    // Assignment status
+    private boolean isLocked; // If true, this assignment cannot be changed by the algorithm
+
+    /**
+     * Creates a new exam assignment.
+     * 
+     * @param courseCode  The course code for this exam
+     * @param timeSlot    The time slot assigned to this exam
+     * @param classroomId The classroom assigned to this exam
+     */
+    public ExamAssignment(String courseCode, TimeSlot timeSlot, String classroomId) {
+        this.courseCode = courseCode;
+        this.timeSlot = timeSlot;
+        this.classroomId = classroomId;
+        this.studentCount = 0;
+        this.isLocked = false;
+    }
+
+    /**
+     * Creates an unassigned exam (only course is known).
+     */
+    public ExamAssignment(String courseCode) {
+        this(courseCode, null, null);
+    }
+
+    /**
+     * Creates a deep copy of this assignment.
+     */
+    public ExamAssignment copy() {
+        ExamAssignment copy = new ExamAssignment(this.courseCode, this.timeSlot, this.classroomId);
+        copy.studentCount = this.studentCount;
+        copy.isLocked = this.isLocked;
+        return copy;
+    }
+
+    /**
+     * Checks if this assignment is complete (has both time slot and classroom).
+     */
+    public boolean isAssigned() {
+        return timeSlot != null && classroomId != null;
+    }
+
+    /**
+     * Checks if this assignment conflicts with another assignment.
+     * Two assignments conflict if:
+     * 1. Same classroom at overlapping time slots
+     * 2. (Checked separately) Same student in both courses at overlapping time
+     * slots
+     */
+    public boolean conflictsWith(ExamAssignment other) {
+        if (!this.isAssigned() || !other.isAssigned()) {
+            return false;
+        }
+
+        // Check classroom conflict
+        if (this.classroomId.equals(other.classroomId) &&
+                this.timeSlot.overlapsWith(other.timeSlot)) {
+            return true;
+        }
+
+        return false;
+    }
+
+    // Getters and Setters
+
+    public String getCourseCode() {
+        return courseCode;
+    }
+
+    public void setCourseCode(String courseCode) {
+        this.courseCode = courseCode;
+    }
+
+    public TimeSlot getTimeSlot() {
+        return timeSlot;
+    }
+
+    public void setTimeSlot(TimeSlot timeSlot) {
+        this.timeSlot = timeSlot;
+    }
+
+    public String getClassroomId() {
+        return classroomId;
+    }
+
+    public void setClassroomId(String classroomId) {
+        this.classroomId = classroomId;
+    }
+
+    public int getStudentCount() {
+        return studentCount;
+    }
+
+    public void setStudentCount(int studentCount) {
+        this.studentCount = studentCount;
+    }
+
+    public boolean isLocked() {
+        return isLocked;
+    }
+
+    public void setLocked(boolean locked) {
+        isLocked = locked;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        ExamAssignment that = (ExamAssignment) o;
+        return Objects.equals(courseCode, that.courseCode) &&
+                Objects.equals(timeSlot, that.timeSlot) &&
+                Objects.equals(classroomId, that.classroomId);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(courseCode, timeSlot, classroomId);
+    }
+
+    @Override
+    public String toString() {
+        if (!isAssigned()) {
+            return courseCode + " [Unassigned]";
+        }
+        return courseCode + " -> " + classroomId + " @ " + timeSlot;
+    }
+}

src/main/java/org/example/se302/model/ScheduleState.java

@@ -0,0 +1,270 @@
+package org.example.se302.model;
+
+import java.util.*;
+
+/**
+ * Represents the complete state of an exam schedule.
+ * This is the main data structure used by the CSP solver to track
+ * the current schedule state and validate constraints.
+ */
+public class ScheduleState {
+    // All exam assignments indexed by course code
+    private Map<String, ExamAssignment> assignments;
+
+    // Quick lookup structures for constraint checking
+    private Map<String, Set<String>> classroomTimeSlotUsage; // classroomId -> set of timeSlot IDs
+    private Map<String, Set<String>> timeSlotCourseMapping; // timeSlotId -> set of course codes
+
+    // Available resources
+    private List<TimeSlot> availableTimeSlots;
+    private List<Classroom> availableClassrooms;
+
+    // Statistics
+    private int totalCourses;
+    private int assignedCourses;
+    private int constraintViolations;
+
+    public ScheduleState() {
+        this.assignments = new HashMap<>();
+        this.classroomTimeSlotUsage = new HashMap<>();
+        this.timeSlotCourseMapping = new HashMap<>();
+        this.availableTimeSlots = new ArrayList<>();
+        this.availableClassrooms = new ArrayList<>();
+        this.totalCourses = 0;
+        this.assignedCourses = 0;
+        this.constraintViolations = 0;
+    }
+
+    /**
+     * Creates a deep copy of this schedule state.
+     */
+    public ScheduleState copy() {
+        ScheduleState copy = new ScheduleState();
+
+        for (Map.Entry<String, ExamAssignment> entry : this.assignments.entrySet()) {
+            copy.assignments.put(entry.getKey(), entry.getValue().copy());
+        }
+
+        for (Map.Entry<String, Set<String>> entry : this.classroomTimeSlotUsage.entrySet()) {
+            copy.classroomTimeSlotUsage.put(entry.getKey(), new HashSet<>(entry.getValue()));
+        }
+
+        for (Map.Entry<String, Set<String>> entry : this.timeSlotCourseMapping.entrySet()) {
+            copy.timeSlotCourseMapping.put(entry.getKey(), new HashSet<>(entry.getValue()));
+        }
+
+        copy.availableTimeSlots = new ArrayList<>(this.availableTimeSlots);
+        copy.availableClassrooms = new ArrayList<>(this.availableClassrooms);
+        copy.totalCourses = this.totalCourses;
+        copy.assignedCourses = this.assignedCourses;
+        copy.constraintViolations = this.constraintViolations;
+
+        return copy;
+    }
+
+    /**
+     * Adds an exam assignment to the schedule.
+     * Updates the quick lookup structures.
+     */
+    public void addAssignment(ExamAssignment assignment) {
+        String courseCode = assignment.getCourseCode();
+        assignments.put(courseCode, assignment);
+        totalCourses++;
+
+        if (assignment.isAssigned()) {
+            updateLookupStructures(assignment, true);
+            assignedCourses++;
+        }
+    }
+
+    /**
+     * Updates an existing assignment with new time slot and classroom.
+     */
+    public boolean updateAssignment(String courseCode, TimeSlot timeSlot, String classroomId) {
+        ExamAssignment assignment = assignments.get(courseCode);
+        if (assignment == null) {
+            return false;
+        }
+
+        if (assignment.isLocked()) {
+            return false; // Cannot modify locked assignments
+        }
+
+        // Remove old assignment from lookup structures
+        if (assignment.isAssigned()) {
+            updateLookupStructures(assignment, false);
+            assignedCourses--;
+        }
+
+        // Update assignment
+        assignment.setTimeSlot(timeSlot);
+        assignment.setClassroomId(classroomId);
+
+        // Add new assignment to lookup structures
+        if (assignment.isAssigned()) {
+            updateLookupStructures(assignment, true);
+            assignedCourses++;
+        }
+
+        return true;
+    }
+
+    /**
+     * Removes an assignment (resets it to unassigned state).
+     */
+    public boolean removeAssignment(String courseCode) {
+        ExamAssignment assignment = assignments.get(courseCode);
+        if (assignment == null || assignment.isLocked()) {
+            return false;
+        }
+
+        if (assignment.isAssigned()) {
+            updateLookupStructures(assignment, false);
+            assignedCourses--;
+        }
+
+        assignment.setTimeSlot(null);
+        assignment.setClassroomId(null);
+
+        return true;
+    }
+
+    /**
+     * Updates the quick lookup structures when an assignment changes.
+     */
+    private void updateLookupStructures(ExamAssignment assignment, boolean add) {
+        if (!assignment.isAssigned())
+            return;
+
+        String classroomId = assignment.getClassroomId();
+        String timeSlotId = assignment.getTimeSlot().getId();
+        String courseCode = assignment.getCourseCode();
+
+        if (add) {
+            classroomTimeSlotUsage.computeIfAbsent(classroomId, k -> new HashSet<>()).add(timeSlotId);
+            timeSlotCourseMapping.computeIfAbsent(timeSlotId, k -> new HashSet<>()).add(courseCode);
+        } else {
+            Set<String> slots = classroomTimeSlotUsage.get(classroomId);
+            if (slots != null) {
+                slots.remove(timeSlotId);
+            }
+
+            Set<String> courses = timeSlotCourseMapping.get(timeSlotId);
+            if (courses != null) {
+                courses.remove(courseCode);
+            }
+        }
+    }
+
+    /**
+     * Checks if a classroom is available at a given time slot.
+     */
+    public boolean isClassroomAvailable(String classroomId, TimeSlot timeSlot) {
+        Set<String> usedSlots = classroomTimeSlotUsage.get(classroomId);
+        if (usedSlots == null) {
+            return true;
+        }
+
+        // Check for overlapping time slots
+        for (ExamAssignment assignment : assignments.values()) {
+            if (assignment.isAssigned() &&
+                    assignment.getClassroomId().equals(classroomId) &&
+                    assignment.getTimeSlot().overlapsWith(timeSlot)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Gets all courses scheduled at a specific time slot.
+     */
+    public Set<String> getCoursesAtTimeSlot(TimeSlot timeSlot) {
+        Set<String> courses = new HashSet<>();
+        for (ExamAssignment assignment : assignments.values()) {
+            if (assignment.isAssigned() &&
+                    assignment.getTimeSlot().overlapsWith(timeSlot)) {
+                courses.add(assignment.getCourseCode());
+            }
+        }
+        return courses;
+    }
+
+    /**
+     * Gets all unassigned courses.
+     */
+    public List<ExamAssignment> getUnassignedCourses() {
+        List<ExamAssignment> unassigned = new ArrayList<>();
+        for (ExamAssignment assignment : assignments.values()) {
+            if (!assignment.isAssigned()) {
+                unassigned.add(assignment);
+            }
+        }
+        return unassigned;
+    }
+
+    /**
+     * Checks if the schedule is complete (all courses are assigned).
+     */
+    public boolean isComplete() {
+        return assignedCourses == totalCourses;
+    }
+
+    /**
+     * Gets the completion percentage.
+     */
+    public double getCompletionPercentage() {
+        if (totalCourses == 0)
+            return 100.0;
+        return (assignedCourses * 100.0) / totalCourses;
+    }
+
+    // Getters and Setters
+
+    public Map<String, ExamAssignment> getAssignments() {
+        return Collections.unmodifiableMap(assignments);
+    }
+
+    public ExamAssignment getAssignment(String courseCode) {
+        return assignments.get(courseCode);
+    }
+
+    public List<TimeSlot> getAvailableTimeSlots() {
+        return availableTimeSlots;
+    }
+
+    public void setAvailableTimeSlots(List<TimeSlot> availableTimeSlots) {
+        this.availableTimeSlots = availableTimeSlots;
+    }
+
+    public List<Classroom> getAvailableClassrooms() {
+        return availableClassrooms;
+    }
+
+    public void setAvailableClassrooms(List<Classroom> availableClassrooms) {
+        this.availableClassrooms = availableClassrooms;
+    }
+
+    public int getTotalCourses() {
+        return totalCourses;
+    }
+
+    public int getAssignedCourses() {
+        return assignedCourses;
+    }
+
+    public int getConstraintViolations() {
+        return constraintViolations;
+    }
+
+    public void setConstraintViolations(int constraintViolations) {
+        this.constraintViolations = constraintViolations;
+    }
+
+    @Override
+    public String toString() {
+        return String.format("ScheduleState[%d/%d assigned, %.1f%% complete, %d violations]",
+                assignedCourses, totalCourses, getCompletionPercentage(), constraintViolations);
+    }
+}

src/main/java/org/example/se302/model/TimeSlot.java

@@ -0,0 +1,109 @@
+package org.example.se302.model;
+
+import java.time.LocalDate;
+import java.time.LocalTime;
+import java.time.format.DateTimeFormatter;
+import java.util.Objects;
+
+/**
+ * Represents a time slot for exam scheduling.
+ * A time slot is defined by a date and a specific time period.
+ */
+public class TimeSlot implements Comparable<TimeSlot> {
+    private LocalDate date;
+    private LocalTime startTime;
+    private LocalTime endTime;
+
+    private static final DateTimeFormatter DATE_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd");
+    private static final DateTimeFormatter TIME_FORMATTER = DateTimeFormatter.ofPattern("HH:mm");
+
+    public TimeSlot(LocalDate date, LocalTime startTime, LocalTime endTime) {
+        this.date = date;
+        this.startTime = startTime;
+        this.endTime = endTime;
+    }
+
+    /**
+     * Creates a time slot with a default duration of 2 hours.
+     */
+    public TimeSlot(LocalDate date, LocalTime startTime) {
+        this(date, startTime, startTime.plusHours(2));
+    }
+
+    public LocalDate getDate() {
+        return date;
+    }
+
+    public void setDate(LocalDate date) {
+        this.date = date;
+    }
+
+    public LocalTime getStartTime() {
+        return startTime;
+    }
+
+    public void setStartTime(LocalTime startTime) {
+        this.startTime = startTime;
+    }
+
+    public LocalTime getEndTime() {
+        return endTime;
+    }
+
+    public void setEndTime(LocalTime endTime) {
+        this.endTime = endTime;
+    }
+
+    /**
+     * Checks if this time slot overlaps with another time slot.
+     * Two time slots overlap if they are on the same day and their time ranges
+     * intersect.
+     */
+    public boolean overlapsWith(TimeSlot other) {
+        if (!this.date.equals(other.date)) {
+            return false;
+        }
+        // Check if time ranges overlap
+        return this.startTime.isBefore(other.endTime) && other.startTime.isBefore(this.endTime);
+    }
+
+    /**
+     * Returns a unique identifier for this time slot.
+     */
+    public String getId() {
+        return date.format(DATE_FORMATTER) + "_" + startTime.format(TIME_FORMATTER);
+    }
+
+    @Override
+    public int compareTo(TimeSlot other) {
+        int dateCompare = this.date.compareTo(other.date);
+        if (dateCompare != 0) {
+            return dateCompare;
+        }
+        return this.startTime.compareTo(other.startTime);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        TimeSlot timeSlot = (TimeSlot) o;
+        return Objects.equals(date, timeSlot.date) &&
+                Objects.equals(startTime, timeSlot.startTime) &&
+                Objects.equals(endTime, timeSlot.endTime);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(date, startTime, endTime);
+    }
+
+    @Override
+    public String toString() {
+        return date.format(DATE_FORMATTER) + " " +
+                startTime.format(TIME_FORMATTER) + "-" +
+                endTime.format(TIME_FORMATTER);
+    }
+}

src/main/resources/org/example/se302/view/import-view.fxml

@@ -4,73 +4,76 @@
 <?import javafx.scene.control.*?>
 <?import javafx.scene.layout.*?>
 
-<VBox xmlns="http://javafx.com/javafx"
-      xmlns:fx="http://javafx.com/fxml"
-      fx:controller="org.example.se302.controller.ImportController"
-      spacing="20" styleClass="content-pane">
-    <padding>
-        <Insets top="20" right="20" bottom="20" left="20"/>
-    </padding>
+<ScrollPane xmlns="http://javafx.com/javafx"
+            xmlns:fx="http://javafx.com/fxml"
+            fx:controller="org.example.se302.controller.ImportController"
+            fitToWidth="true" fitToHeight="true"
+            hbarPolicy="AS_NEEDED" vbarPolicy="AS_NEEDED">
+    <VBox spacing="20" styleClass="content-pane">
+        <padding>
+            <Insets top="20" right="20" bottom="20" left="20"/>
+        </padding>
 
-    <!-- Header -->
-    <Label text="Import Data from CSV Files" styleClass="section-title"/>
-    <Label text="Select CSV files to import student, course, classroom, and enrollment data." wrapText="true"/>
+        <!-- Header -->
+        <Label text="Import Data from CSV Files" styleClass="section-title"/>
+        <Label text="Select CSV files to import student, course, classroom, and enrollment data." wrapText="true"/>
 
-    <Separator/>
+        <Separator/>
 
-    <!-- Students Import Section -->
-    <VBox spacing="8">
-        <Label text="Student Data" styleClass="subsection-title"/>
-        <HBox spacing="10" alignment="CENTER_LEFT">
-            <TextField fx:id="studentFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
-            <Button fx:id="studentBrowseButton" text="Browse..." onAction="#onBrowseStudents"/>
+        <!-- Students Import Section -->
+        <VBox spacing="8">
+            <Label text="Student Data" styleClass="subsection-title"/>
+            <HBox spacing="10" alignment="CENTER_LEFT">
+                <TextField fx:id="studentFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
+                <Button fx:id="studentBrowseButton" text="Browse..." onAction="#onBrowseStudents"/>
+            </HBox>
+            <Label fx:id="studentStatusLabel" text="Not Loaded" styleClass="status-label"/>
+        </VBox>
+
+        <!-- Courses Import Section -->
+        <VBox spacing="8">
+            <Label text="Course Data" styleClass="subsection-title"/>
+            <HBox spacing="10" alignment="CENTER_LEFT">
+                <TextField fx:id="courseFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
+                <Button fx:id="courseBrowseButton" text="Browse..." onAction="#onBrowseCourses"/>
+            </HBox>
+            <Label fx:id="courseStatusLabel" text="Not Loaded" styleClass="status-label"/>
+        </VBox>
+
+        <!-- Classrooms Import Section -->
+        <VBox spacing="8">
+            <Label text="Classroom Data" styleClass="subsection-title"/>
+            <HBox spacing="10" alignment="CENTER_LEFT">
+                <TextField fx:id="classroomFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
+                <Button fx:id="classroomBrowseButton" text="Browse..." onAction="#onBrowseClassrooms"/>
+            </HBox>
+            <Label fx:id="classroomStatusLabel" text="Not Loaded" styleClass="status-label"/>
+        </VBox>
+
+        <!-- Enrollments Import Section -->
+        <VBox spacing="8">
+            <Label text="Enrollment Data" styleClass="subsection-title"/>
+            <HBox spacing="10" alignment="CENTER_LEFT">
+                <TextField fx:id="enrollmentFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
+                <Button fx:id="enrollmentBrowseButton" text="Browse..." onAction="#onBrowseEnrollments"/>
+            </HBox>
+            <Label fx:id="enrollmentStatusLabel" text="Not Loaded" styleClass="status-label"/>
+        </VBox>
+
+        <Separator/>
+
+        <!-- Import Messages Area -->
+        <VBox spacing="5" VBox.vgrow="ALWAYS">
+            <Label text="Import Messages" styleClass="subsection-title"/>
+            <TextArea fx:id="messagesArea" editable="false" wrapText="true" VBox.vgrow="ALWAYS"
+                      promptText="Validation messages and import results will appear here..."/>
+        </VBox>
+
+        <!-- Action Buttons -->
+        <HBox spacing="10" alignment="CENTER_RIGHT">
+            <Button fx:id="importAllButton" text="Import All" onAction="#onImportAll"
+                    styleClass="primary-button" disable="true"/>
+            <Button text="Clear All" onAction="#onClearAll"/>
         </HBox>
-        <Label fx:id="studentStatusLabel" text="Not Loaded" styleClass="status-label"/>
     </VBox>
-
-    <!-- Courses Import Section -->
-    <VBox spacing="8">
-        <Label text="Course Data" styleClass="subsection-title"/>
-        <HBox spacing="10" alignment="CENTER_LEFT">
-            <TextField fx:id="courseFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
-            <Button fx:id="courseBrowseButton" text="Browse..." onAction="#onBrowseCourses"/>
-        </HBox>
-        <Label fx:id="courseStatusLabel" text="Not Loaded" styleClass="status-label"/>
-    </VBox>
-
-    <!-- Classrooms Import Section -->
-    <VBox spacing="8">
-        <Label text="Classroom Data" styleClass="subsection-title"/>
-        <HBox spacing="10" alignment="CENTER_LEFT">
-            <TextField fx:id="classroomFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
-            <Button fx:id="classroomBrowseButton" text="Browse..." onAction="#onBrowseClassrooms"/>
-        </HBox>
-        <Label fx:id="classroomStatusLabel" text="Not Loaded" styleClass="status-label"/>
-    </VBox>
-
-    <!-- Enrollments Import Section -->
-    <VBox spacing="8">
-        <Label text="Enrollment Data" styleClass="subsection-title"/>
-        <HBox spacing="10" alignment="CENTER_LEFT">
-            <TextField fx:id="enrollmentFileField" promptText="No file selected" editable="false" HBox.hgrow="ALWAYS"/>
-            <Button fx:id="enrollmentBrowseButton" text="Browse..." onAction="#onBrowseEnrollments"/>
-        </HBox>
-        <Label fx:id="enrollmentStatusLabel" text="Not Loaded" styleClass="status-label"/>
-    </VBox>
-
-    <Separator/>
-
-    <!-- Import Messages Area -->
-    <VBox spacing="5" VBox.vgrow="ALWAYS">
-        <Label text="Import Messages" styleClass="subsection-title"/>
-        <TextArea fx:id="messagesArea" editable="false" wrapText="true" VBox.vgrow="ALWAYS"
-                  promptText="Validation messages and import results will appear here..."/>
-    </VBox>
-
-    <!-- Action Buttons -->
-    <HBox spacing="10" alignment="CENTER_RIGHT">
-        <Button fx:id="importAllButton" text="Import All" onAction="#onImportAll"
-                styleClass="primary-button" disable="true"/>
-        <Button text="Clear All" onAction="#onClearAll"/>
-    </HBox>
-</VBox>
+</ScrollPane>