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

using System.Text.RegularExpressions;

namespace QuestionsApi.Services.Flashcards;

public sealed record ClozeDeletion(int Number, string Answer, string? Hint);

// {{c1::answer}} or {{c1::answer::hint}}. Numbers need not be contiguous or start at 1;
// each distinct number becomes its own independently scheduled card (a "sibling").
public static partial class FlashcardClozeParser
{
    [GeneratedRegex(@"\{\{c(\d+)::((?:(?!\}\}).)*)\}\}", RegexOptions.Singleline)]
    private static partial Regex ClozeRegex();

    public static List<ClozeDeletion> Parse(string text)
    {
        if (string.IsNullOrEmpty(text)) return [];
        var results = new List<ClozeDeletion>();
        foreach (Match match in ClozeRegex().Matches(text))
        {
            if (!int.TryParse(match.Groups[1].Value, out var number) || number < 1)
                continue;
            var inner = match.Groups[2].Value;
            var parts = inner.Split("::", 2);
            var answer = parts[0].Trim();
            var hint = parts.Length > 1 ? parts[1].Trim() : null;
            if (answer.Length == 0) continue;
            results.Add(new ClozeDeletion(number, answer, string.IsNullOrEmpty(hint) ? null : hint));
        }
        return results;
    }

    public static List<int> DistinctClozeNumbers(string text) =>
        Parse(text).Select(v => v.Number).Distinct().OrderBy(v => v).ToList();

    public static bool HasValidCloze(string text) => DistinctClozeNumbers(text).Count > 0;

    // Renders the front for a given cloze number: that number's answer is hidden (replaced
    // by its hint or "..."), all other cloze numbers are revealed as their answer text.
    public static string RenderFront(string text, int clozeNumber) =>
        ClozeRegex().Replace(text, match =>
        {
            if (!int.TryParse(match.Groups[1].Value, out var number)) return match.Value;
            var inner = match.Groups[2].Value;
            var parts = inner.Split("::", 2);
            if (number != clozeNumber) return parts[0].Trim();
            var hint = parts.Length > 1 ? parts[1].Trim() : "";
            return hint.Length > 0 ? $"[{hint}]" : "[...]";
        });

    public static string RenderBack(string text, int clozeNumber) =>
        ClozeRegex().Replace(text, match =>
        {
            var inner = match.Groups[2].Value;
            var parts = inner.Split("::", 2);
            return parts[0].Trim();
        });
}