using Core.Entities;
using Core.Models;
using Core.Notes;

namespace Core.Parsers;

/// <summary>
/// Parses leftover CSV columns (not roster or ranking) into student note field merges.
/// </summary>
public class StudentNotesFieldParser : CsvParserBase
{
    private static readonly HashSet<string> ReservedHeaders = new(StringComparer.OrdinalIgnoreCase)
    {
        "Student Name",
        "Grade",
        "TSA year",
        "State ID",
        "Regional ID",
        "National ID",
        "Officer",
        "TOTAL # OF EVENTS"
    };

    public StudentNotesFieldParser(FileSystemInfo csvFile, bool ignoreBlankLines = true) : base(csvFile, ignoreBlankLines)
    {
    }

    public StudentNotesFieldParser(StreamReader reader, bool ignoreBlankLines = true) : base(reader, ignoreBlankLines)
    {
    }

    /// <summary>
    /// Roster and ranking columns that must not become imported note fields.
    /// </summary>
    public static bool IsReservedHeader(string? header)
    {
        if (string.IsNullOrWhiteSpace(header))
            return true;

        var trimmed = header.Trim();
        if (ReservedHeaders.Contains(trimmed))
            return true;

        return int.TryParse(trimmed, out var rank)
               && rank >= 1
               && rank <= StudentEventRanking.MaxRank;
    }

    /// <summary>
    /// Leftover field names from a header row after reserved columns are removed.
    /// </summary>
    public static List<string> GetLeftoverFieldNames(IEnumerable<string?> headers) =>
        headers
            .Where(h => !IsReservedHeader(h))
            .Select(h => h!.Trim())
            .ToList();

    /// <summary>
    /// Reads the header row and returns leftover field names without processing data rows.
    /// </summary>
    public List<string> PeekLeftoverFieldNames()
    {
        CsvReader.Read();
        CsvReader.ReadHeader();
        return GetLeftoverFieldNames(CsvReader.HeaderRecord ?? []);
    }

    public StudentNotesImportResult Parse(
        ICollection<Student> students,
        IReadOnlyDictionary<int, string?> existingNotesByStudentId)
    {
        var result = new StudentNotesImportResult();

        CsvReader.Read();
        CsvReader.ReadHeader();

        if (CsvReader.HeaderRecord is null ||
            !CsvReader.HeaderRecord.Contains("Student Name", StringComparer.OrdinalIgnoreCase))
        {
            result.Errors.Add("CSV must include a 'Student Name' column.");
            return result;
        }

        var fieldNames = GetLeftoverFieldNames(CsvReader.HeaderRecord);
        result.FieldNames = fieldNames;

        if (fieldNames.Count == 0)
            result.Warnings.Add("No leftover field columns were found besides roster and ranking columns.");

        Dictionary<int, PendingStudentFields> pendingByStudentId = [];

        while (CsvReader.Read())
        {
            var rowNumber = CsvReader.Context.Parser?.Row ?? 0;
            var name = CsvReader.GetField("Student Name")?.Trim();
            if (string.IsNullOrEmpty(name))
                continue;

            var studentMatch = FuzzyStudentMatcher.Find(students, name);
            if (studentMatch is null)
            {
                result.Issues.Add(new StudentNotesImportIssue
                {
                    RowNumber = rowNumber,
                    RawStudentName = name,
                    Message = $"No student matched '{name}'."
                });
                continue;
            }

            var (student, score) = studentMatch.Value;
            if (!pendingByStudentId.TryGetValue(student.Id, out var pending))
            {
                pending = new PendingStudentFields(student);
                pendingByStudentId[student.Id] = pending;
            }

            pending.RawStudentName = name;
            pending.RowNumber = rowNumber;
            pending.StudentScore = score;

            foreach (var fieldName in fieldNames)
            {
                var raw = CsvReader.GetField(fieldName);
                pending.Fields[fieldName.Trim()] = raw ?? string.Empty;
            }
        }

        foreach (var pending in pendingByStudentId.Values)
        {
            List<ImportedField> incoming = [.. pending.Fields.Select(pair => new ImportedField(pair.Key, pair.Value))];
            existingNotesByStudentId.TryGetValue(pending.Student.Id, out var existingMarkdown);
            var merge = ImportedFieldsTable.Merge(existingMarkdown, incoming);

            result.Matches.Add(new StudentNotesImportMatch
            {
                Student = pending.Student,
                RawStudentName = pending.RawStudentName,
                RowNumber = pending.RowNumber,
                StudentScore = pending.StudentScore,
                IncomingFields = incoming,
                Merge = merge
            });
        }

        if (result.Matches.Count == 0 && result.Errors.Count == 0)
            result.Warnings.Add("No students were matched from the CSV.");

        return result;
    }

    private sealed class PendingStudentFields(Student student)
    {
        public Student Student { get; } = student;

        public string RawStudentName { get; set; } = string.Empty;

        public int RowNumber { get; set; }

        public int StudentScore { get; set; }

        public Dictionary<string, string> Fields { get; } = new(StringComparer.OrdinalIgnoreCase);
    }
}