using QuestionsApi.Models;
using System.Globalization;
using System.Security.Cryptography;
using System.Text;
using System.Text.RegularExpressions;

namespace QuestionsApi.Services;

public sealed class DynamicQuestionGenerator
{
    public const int CurrentVersion = 1;
    private static readonly Regex Placeholder = new(@"\{\{([a-zA-Z0-9_]+)\}\}", RegexOptions.Compiled);

    public DynamicGeneratedQuestion Generate(DynamicQuestionBlueprint blueprint, string seed, string nonce)
    {
        var draw = new DeterministicDraw($"{blueprint.ExternalId}|{blueprint.Version}|{CurrentVersion}|{seed}");
        var values = new Dictionary<string, string>(StringComparer.Ordinal);
        var numbers = new Dictionary<string, double>(StringComparer.Ordinal);
        foreach (var item in blueprint.Variables.OrderBy(value => value.Key, StringComparer.Ordinal))
        {
            var definition = item.Value;
            if (definition.Type == "enum")
            {
                if (definition.Values.Count == 0) throw new InvalidOperationException($"{item.Key} requires values");
                values[item.Key] = definition.Values[draw.Next(definition.Values.Count)];
                continue;
            }
            var step = definition.Step <= 0 ? 1 : definition.Step;
            var count = (int)Math.Floor((definition.Maximum - definition.Minimum) / step) + 1;
            if (count < 1) throw new InvalidOperationException($"{item.Key} range is invalid");
            var number = definition.Minimum + draw.Next(count) * step;
            number = Math.Round(number, Math.Clamp(definition.Precision, 0, 8));
            numbers[item.Key] = number;
            values[item.Key] = Format(number, definition.Precision);
        }
        foreach (var item in blueprint.Derived)
        {
            var definition = item.Value;
            var left = ResolveNumber(definition.Left, numbers);
            var right = ResolveNumber(definition.Right, numbers);
            var number = definition.Operation switch
            {
                "add" => left + right,
                "subtract" => left - right,
                "multiply" => left * right,
                "divide" when Math.Abs(right) > double.Epsilon => left / right,
                _ => throw new InvalidOperationException($"{item.Key} operation is invalid")
            };
            number = Math.Round(number, Math.Clamp(definition.Precision, 0, 8));
            numbers[item.Key] = number;
            values[item.Key] = Format(number, definition.Precision);
        }
        foreach (var constraint in blueprint.Constraints)
        {
            if (!numbers.TryGetValue(constraint.Field, out var actual))
                throw new InvalidOperationException($"constraint field {constraint.Field} is not numeric");
            var valid = constraint.Operator switch
            {
                "lt" => actual < constraint.Value,
                "lte" => actual <= constraint.Value,
                "gt" => actual > constraint.Value,
                "gte" => actual >= constraint.Value,
                "eq" => Math.Abs(actual - constraint.Value) < 0.0000001,
                "neq" => Math.Abs(actual - constraint.Value) >= 0.0000001,
                _ => false
            };
            if (!valid) throw new InvalidOperationException($"generated value violates {constraint.Field} {constraint.Operator}");
        }

        var distractors = blueprint.Distractors.OrderBy(_ => draw.Next(int.MaxValue)).Take(4).ToList();
        if (distractors.Count != 4) throw new InvalidOperationException("at least four distractors are required");
        var answerTemplates = distractors.Append(blueprint.CorrectAnswer)
            .OrderBy(_ => draw.Next(int.MaxValue)).ToList();
        var choices = answerTemplates.Select(answer => new QuestionChoice
        {
            Id = answer.Id,
            Text = Render(answer.TextTemplate, values)
        }).ToList();
        if (choices.Select(value => value.Id).Distinct(StringComparer.Ordinal).Count() != 5
            || choices.Select(value => value.Text).Distinct(StringComparer.OrdinalIgnoreCase).Count() != 5)
            throw new InvalidOperationException("generated choices must be unique");

        var question = new Question
        {
            Id = $"dynamic-{nonce}",
            ExternalId = $"dynamic-{nonce}",
            Stem = Render(blueprint.StemTemplate, values),
            LeadIn = Render(blueprint.LeadIn, values),
            Choices = choices,
            CorrectChoiceId = blueprint.CorrectAnswer.Id,
            Explanation = Render(blueprint.ExplanationTemplate, values),
            KeyTakeaway = Render(blueprint.KeyTakeaway, values),
            UsmlePearl = Render(blueprint.UsmlePearl, values),
            ChoiceExplanations = answerTemplates.ToDictionary(
                value => value.Id,
                value => Render(value.ExplanationTemplate, values)),
            Discipline = blueprint.Discipline,
            System = blueprint.System,
            PerformanceCategory = blueprint.PerformanceCategory,
            Difficulty = blueprint.Difficulty,
            Labs = blueprint.Labs.Select(lab => new QuestionLab
            {
                Name = lab.Name,
                Value = Render(lab.ValueTemplate, values),
                Unit = lab.Unit,
                Reference = lab.Reference
            }).ToList(),
            Tags = [.. blueprint.Tags, "dynamic", $"objective:{blueprint.ObjectiveId}"],
            References = blueprint.References,
            IsPublished = true
        };
        return new DynamicGeneratedQuestion { Question = question, Blueprint = blueprint };
    }

    public static string? Validate(DynamicQuestionBlueprint blueprint)
    {
        if (string.IsNullOrWhiteSpace(blueprint.ExternalId)) return "externalId required";
        if (string.IsNullOrWhiteSpace(blueprint.ObjectiveId)) return "objectiveId required";
        if (blueprint.Version < 1) return "version must be positive";
        if (string.IsNullOrWhiteSpace(blueprint.StemTemplate) || string.IsNullOrWhiteSpace(blueprint.LeadIn)) return "stemTemplate and leadIn required";
        if (string.IsNullOrWhiteSpace(blueprint.CorrectAnswer.Id) || string.IsNullOrWhiteSpace(blueprint.CorrectAnswer.TextTemplate)) return "correctAnswer required";
        if (blueprint.Distractors.Count < 4) return "at least four distractors required";
        var ids = blueprint.Distractors.Select(value => value.Id).Append(blueprint.CorrectAnswer.Id).ToList();
        if (ids.Any(string.IsNullOrWhiteSpace) || ids.Distinct(StringComparer.Ordinal).Count() != ids.Count) return "answer ids must be unique";
        if (blueprint.References.Count == 0) return "at least one reference required";
        try
        {
            var generator = new DynamicQuestionGenerator();
            for (var index = 0; index < 100; index++)
                generator.Generate(blueprint, $"validation-{index}", $"validation-{index}");
        }
        catch (Exception exception) { return exception.Message; }
        return null;
    }

    private static string Render(string template, IReadOnlyDictionary<string, string> values)
    {
        var rendered = Placeholder.Replace(template ?? "", match =>
            values.TryGetValue(match.Groups[1].Value, out var value) ? value : match.Value);
        if (Placeholder.IsMatch(rendered)) throw new InvalidOperationException($"unresolved template in {rendered}");
        return rendered;
    }

    private static double ResolveNumber(string operand, IReadOnlyDictionary<string, double> numbers) =>
        numbers.TryGetValue(operand, out var value)
            ? value
            : double.TryParse(operand, NumberStyles.Float, CultureInfo.InvariantCulture, out value)
                ? value
                : throw new InvalidOperationException($"unknown numeric operand {operand}");

    private static string Format(double value, int precision) =>
        value.ToString(precision > 0 ? $"F{precision}" : "0", CultureInfo.InvariantCulture);

    private sealed class DeterministicDraw(string source)
    {
        private int _counter;
        public int Next(int maximum)
        {
            if (maximum <= 0) throw new ArgumentOutOfRangeException(nameof(maximum));
            var bytes = SHA256.HashData(Encoding.UTF8.GetBytes($"{source}|{_counter++}"));
            return (int)(BitConverter.ToUInt32(bytes, 0) % (uint)maximum);
        }
    }
}