step3/api/QuestionsApi/Services/Flashcards/FlashcardCsv.cs

using System.Text;

namespace QuestionsApi.Services.Flashcards;

// Minimal RFC 4180 CSV reader/writer (quoted fields, embedded commas/newlines, "" escaping).
// UTF-8, text-only, matches the flashcard CSV contract exactly.
public static class FlashcardCsv
{
    public static readonly string[] Headers =
        ["stable_id", "deck", "type", "prompt", "answer", "extra", "tags", "source_question_external_id"];

    public static List<string[]> ParseRows(string text)
    {
        var rows = new List<string[]>();
        var field = new StringBuilder();
        var row = new List<string>();
        var inQuotes = false;
        var i = 0;
        var n = text.Length;

        while (i < n)
        {
            var c = text[i];
            if (inQuotes)
            {
                if (c == '"')
                {
                    if (i + 1 < n && text[i + 1] == '"') { field.Append('"'); i += 2; continue; }
                    inQuotes = false; i++; continue;
                }
                field.Append(c); i++; continue;
            }

            switch (c)
            {
                case '"':
                    inQuotes = true; i++; break;
                case ',':
                    row.Add(field.ToString()); field.Clear(); i++; break;
                case '\r':
                    i++; break;
                case '\n':
                    row.Add(field.ToString()); field.Clear();
                    rows.Add(row.ToArray()); row = [];
                    i++; break;
                default:
                    field.Append(c); i++; break;
            }
        }
        if (field.Length > 0 || row.Count > 0)
        {
            row.Add(field.ToString());
            rows.Add(row.ToArray());
        }
        return rows;
    }

    public static string WriteRow(IEnumerable<string> fields) => string.Join(",", fields.Select(Escape));

    private static string Escape(string field) =>
        field.IndexOfAny(['"', ',', '\n', '\r']) >= 0
            ? "\"" + field.Replace("\"", "\"\"") + "\""
            : field;
}