using Core.Entities;

namespace Core.YearTransition;

/// <summary>
/// Inputs for building a year-transition plan.
/// </summary>
public sealed class YearTransitionRequest
{
    public required IReadOnlyList<Student> Students { get; init; }
    public required IReadOnlySet<int> ReturningStudentIds { get; init; }
    public IReadOnlyDictionary<OfficerRole, int?> OfficerAssignments { get; init; }
        = new Dictionary<OfficerRole, int?>();
    public required int GraduatingGrade { get; init; }
    public required string TargetCompetitionYear { get; init; }
    public IReadOnlyList<string> PastedNames { get; init; } = [];
}

/// <summary>
/// Preview of a year transition before it is applied.
/// </summary>
public sealed class YearTransitionPlan
{
    public required string TargetCompetitionYear { get; init; }
    public required int GraduatingGrade { get; init; }
    public required IReadOnlyList<StudentPromotion> Promotions { get; init; }
    public required IReadOnlyList<Student> StudentsToRemove { get; init; }
    public required IReadOnlyList<OfficerAssignmentChange> OfficerChanges { get; init; }
    public required IReadOnlyList<string> UnmatchedPastedNames { get; init; }
    public required IReadOnlyList<string> AmbiguousPastedNames { get; init; }
    public required IReadOnlyList<string> Warnings { get; init; }

    public int ReturningCount => Promotions.Count;
    public int RemovalCount => StudentsToRemove.Count;
}

public sealed class StudentPromotion
{
    public required Student Student { get; init; }
    public required int PreviousGrade { get; init; }
    public required int NewGrade { get; init; }
    public required int PreviousTsaYear { get; init; }
    public required int NewTsaYear { get; init; }
    public OfficerRole? PreviousOfficerRole { get; init; }
    public OfficerRole? NewOfficerRole { get; init; }
}

public sealed class OfficerAssignmentChange
{
    public required OfficerRole Role { get; init; }
    public Student? NewOfficer { get; init; }
    public Student? PreviousOfficer { get; init; }
}

/// <summary>
/// Pure planner for year-end student promotion, graduation, and officer assignment.
/// </summary>
public static class YearTransitionPlanner
{
    private const int AbsoluteMaxGrade = 12;

    /// <summary>
    /// Students at or above the graduating grade are suggested as non-returning.
    /// </summary>
    public static bool SuggestReturning(Student student, int graduatingGrade) =>
        student.Grade < graduatingGrade;

    /// <summary>
    /// Parses pasted name lines and matches them to students.
    /// </summary>
    public static NameMatchResult MatchPastedNames(
        IReadOnlyList<Student> students,
        IEnumerable<string> pastedLines)
    {
        var matchedIds = new HashSet<int>();
        var unmatched = new List<string>();
        var ambiguous = new List<string>();

        foreach (var rawLine in pastedLines)
        {
            var line = rawLine.Trim();
            if (string.IsNullOrWhiteSpace(line))
                continue;

            var matches = FindNameMatches(students, line).ToList();
            if (matches.Count == 0)
            {
                unmatched.Add(line);
            }
            else if (matches.Count > 1)
            {
                ambiguous.Add(line);
                foreach (var match in matches)
                    matchedIds.Add(match.Id);
            }
            else
            {
                matchedIds.Add(matches[0].Id);
            }
        }

        return new NameMatchResult(matchedIds, unmatched, ambiguous);
    }

    public static YearTransitionPlan Build(YearTransitionRequest request)
    {
        ArgumentNullException.ThrowIfNull(request);
        if (request.GraduatingGrade is < 5 or > AbsoluteMaxGrade)
            throw new ArgumentOutOfRangeException(nameof(request), "Graduating grade must be between 5 and 12.");

        var studentsById = request.Students.ToDictionary(s => s.Id);
        var warnings = new List<string>();
        var promotions = new List<StudentPromotion>();
        var toRemove = new List<Student>();

        var nameMatch = MatchPastedNames(request.Students, request.PastedNames);
        warnings.AddRange(nameMatch.AmbiguousNames.Select(n =>
            $"Pasted name '{n}' matched more than one student."));

        // Officer role -> student id from request (null = vacant)
        var newOfficerByRole = Enum.GetValues<OfficerRole>()
            .ToDictionary(
                role => role,
                role => request.OfficerAssignments.TryGetValue(role, out var id) ? id : null);

        // Detect same student assigned to multiple offices
        var assignedStudentIds = newOfficerByRole.Values
            .Where(id => id.HasValue)
            .Select(id => id!.Value)
            .ToList();
        var duplicateOfficerStudents = assignedStudentIds
            .GroupBy(id => id)
            .Where(g => g.Count() > 1)
            .Select(g => g.Key)
            .ToHashSet();

        foreach (var studentId in duplicateOfficerStudents)
        {
            if (studentsById.TryGetValue(studentId, out var student))
            {
                warnings.Add($"{student.LastNameFirstName} is assigned to more than one officer role.");
            }
        }

        // Build new officer role lookup for returning students
        var newRoleByStudentId = new Dictionary<int, OfficerRole>();
        foreach (var (role, studentId) in newOfficerByRole)
        {
            if (!studentId.HasValue)
                continue;

            if (!request.ReturningStudentIds.Contains(studentId.Value))
            {
                var name = studentsById.TryGetValue(studentId.Value, out var s)
                    ? s.LastNameFirstName
                    : $"Id {studentId.Value}";
                warnings.Add($"{role} is assigned to {name}, who is not marked returning.");
                continue;
            }

            if (duplicateOfficerStudents.Contains(studentId.Value))
                continue;

            newRoleByStudentId[studentId.Value] = role;
        }

        foreach (var student in request.Students.OrderBy(s => s.LastName).ThenBy(s => s.FirstName))
        {
            if (request.ReturningStudentIds.Contains(student.Id))
            {
                var newGrade = Math.Min(AbsoluteMaxGrade,
                    Math.Min(request.GraduatingGrade, student.Grade + 1));
                var newTsaYear = student.TsaYear + 1;

                if (student.Grade >= request.GraduatingGrade)
                {
                    warnings.Add(
                        $"{student.LastNameFirstName} is at or above graduating grade {request.GraduatingGrade} but marked returning; grade stays at {newGrade}.");
                }

                OfficerRole? assignedRole = newRoleByStudentId.TryGetValue(student.Id, out var role)
                    ? role
                    : null;

                promotions.Add(new StudentPromotion
                {
                    Student = student,
                    PreviousGrade = student.Grade,
                    NewGrade = newGrade,
                    PreviousTsaYear = student.TsaYear,
                    NewTsaYear = newTsaYear,
                    PreviousOfficerRole = student.OfficerRole,
                    NewOfficerRole = assignedRole
                });
            }
            else
            {
                toRemove.Add(student);
            }
        }

        var officerChanges = Enum.GetValues<OfficerRole>()
            .Select(role =>
            {
                var previous = request.Students.FirstOrDefault(s => s.OfficerRole == role);
                Student? next = null;
                if (newOfficerByRole.TryGetValue(role, out var nextId) &&
                    nextId.HasValue &&
                    studentsById.TryGetValue(nextId.Value, out var nextStudent) &&
                    request.ReturningStudentIds.Contains(nextId.Value) &&
                    !duplicateOfficerStudents.Contains(nextId.Value))
                {
                    next = nextStudent;
                }

                return new OfficerAssignmentChange
                {
                    Role = role,
                    PreviousOfficer = previous,
                    NewOfficer = next
                };
            })
            .ToList();

        return new YearTransitionPlan
        {
            TargetCompetitionYear = request.TargetCompetitionYear,
            GraduatingGrade = request.GraduatingGrade,
            Promotions = promotions,
            StudentsToRemove = toRemove,
            OfficerChanges = officerChanges,
            UnmatchedPastedNames = nameMatch.UnmatchedNames,
            AmbiguousPastedNames = nameMatch.AmbiguousNames,
            Warnings = warnings
        };
    }

    private static IEnumerable<Student> FindNameMatches(IReadOnlyList<Student> students, string line)
    {
        var comparer = StringComparer.OrdinalIgnoreCase;

        // Exact full-name matches first
        var fullMatches = students.Where(s =>
            comparer.Equals(s.FirstNameLastName, line) ||
            comparer.Equals(s.LastNameFirstName, line)).ToList();
        if (fullMatches.Count > 0)
            return fullMatches;

        var (first, last) = ParseName(line);
        if (string.IsNullOrWhiteSpace(first) && string.IsNullOrWhiteSpace(last))
            return [];

        return students.Where(s =>
            comparer.Equals(s.FirstName.Trim(), first) &&
            comparer.Equals(s.LastName.Trim(), last));
    }

    /// <summary>
    /// Parses a name line into (first, last), using <see cref="Student.ParseNameParts"/> for
    /// "Last, First" and a last-space split for "First Last".
    /// </summary>
    private static (string First, string Last) ParseName(string fullName)
    {
        var trimmed = fullName.Trim();
        if (trimmed.Contains(','))
        {
            var parts = Student.ParseNameParts(trimmed);
            return (parts.Item1.Trim(), parts.Item2.Trim());
        }

        var lastSpace = trimmed.LastIndexOf(' ');
        if (lastSpace <= 0)
            return (trimmed, string.Empty);

        return (trimmed[..lastSpace].Trim(), trimmed[(lastSpace + 1)..].Trim());
    }
}

public sealed class NameMatchResult
{
    public NameMatchResult(
        IReadOnlySet<int> matchedStudentIds,
        IReadOnlyList<string> unmatchedNames,
        IReadOnlyList<string> ambiguousNames)
    {
        MatchedStudentIds = matchedStudentIds;
        UnmatchedNames = unmatchedNames;
        AmbiguousNames = ambiguousNames;
    }

    public IReadOnlySet<int> MatchedStudentIds { get; }
    public IReadOnlyList<string> UnmatchedNames { get; }
    public IReadOnlyList<string> AmbiguousNames { get; }
}