using QuestionsApi.Models;

namespace QuestionsApi.Services;

public static class QuestionValidator
{
    public static string? Validate(UpsertQuestionRequest req)
    {
        if (string.IsNullOrWhiteSpace(req.ExternalId)) return "externalId required";
        if (string.IsNullOrWhiteSpace(req.Stem)) return "stem required";
        if (string.IsNullOrWhiteSpace(req.LeadIn)) return "leadIn required";
        if (req.Choices.Count < (req.IsPublished ? 5 : 2))
            return req.IsPublished ? "published questions require at least five choices" : "at least two choices required";
        if (req.Choices.Count > 20) return "questions support at most twenty choices";
        if (req.Choices.Any(choice => string.IsNullOrWhiteSpace(choice.Id) || string.IsNullOrWhiteSpace(choice.Text)))
            return "choice ids and text are required";

        var choiceIds = req.Choices.Select(choice => choice.Id.Trim()).ToList();
        if (choiceIds.Distinct(StringComparer.Ordinal).Count() != choiceIds.Count)
            return "choice ids must be unique";
        if (string.IsNullOrWhiteSpace(req.CorrectChoiceId)) return "correctChoiceId required";
        if (!choiceIds.Contains(req.CorrectChoiceId.Trim(), StringComparer.Ordinal))
            return "correctChoiceId must match a choice id";
        if (string.IsNullOrWhiteSpace(req.Explanation)) return "explanation required";

        var choiceIdSet = choiceIds.ToHashSet(StringComparer.Ordinal);
        if (!choiceIdSet.SetEquals(req.ChoiceExplanations.Keys))
            return "choiceExplanations must contain exactly one entry for every choice";
        var incorrectChoiceIds = choiceIdSet
            .Where(id => id != req.CorrectChoiceId.Trim())
            .ToHashSet(StringComparer.Ordinal);
        if (!incorrectChoiceIds.SetEquals(req.WrongAnswerFollowups.Keys))
            return "wrongAnswerFollowups must contain exactly one entry for every incorrect choice";
        if (req.ChoiceSelectionPercentages.Keys.Any(id => !choiceIdSet.Contains(id)))
            return "choiceSelectionPercentages keys must match choice ids";
        if (req.ChoiceSelectionPercentages.Any(entry => entry.Value is < 0 or > 100))
            return "choiceSelectionPercentages values must be between 0 and 100";

        foreach (var entry in req.WrongAnswerFollowups)
        {
            if (string.IsNullOrWhiteSpace(entry.Value.Prompt)) return "wrongAnswerFollowups prompt required";
            if (string.IsNullOrWhiteSpace(entry.Value.TeachingPoint)) return "wrongAnswerFollowups teachingPoint required";
            if (entry.Value.Choices.Count == 0) return "wrongAnswerFollowups choices required";
            if (entry.Value.Choices.Any(choice => string.IsNullOrWhiteSpace(choice.Id) || string.IsNullOrWhiteSpace(choice.Text)))
                return "wrongAnswerFollowups choice ids and text are required";
            if (entry.Value.Choices.Select(choice => choice.Id).Distinct(StringComparer.Ordinal).Count() != entry.Value.Choices.Count)
                return "wrongAnswerFollowups choice ids must be unique";
            if (!entry.Value.Choices.Any(choice => choice.Id == entry.Value.CorrectChoiceId))
                return "wrongAnswerFollowups correctChoiceId must match a follow-up choice id";
        }

        if (string.IsNullOrWhiteSpace(req.System)) return "system required";
        if (string.IsNullOrWhiteSpace(req.Discipline)) return "discipline required";
        if (req.Difficulty is < 1 or > 5) return "difficulty must be between 1 and 5";
        return null;
    }
}