using System.Diagnostics.Metrics;

namespace StellaOps.TestKit.Observability;

/// <summary>
/// Assertion helpers for metrics contract testing.
/// </summary>
/// <remarks>
/// These assertions validate that metrics conform to expected contracts:
/// metric existence, label cardinality, monotonicity, and naming conventions.
///
/// Usage:
/// <code>
/// var capture = new MetricsCapture("MyService");
/// await service.ProcessAsync();
///
/// MetricsContractAssert.MetricExists(capture, "requests_total");
/// MetricsContractAssert.LabelCardinalityBounded(capture, "http_requests_total", maxLabels: 50);
/// MetricsContractAssert.CounterMonotonic(capture, "processed_items_total");
/// </code>
/// </remarks>
public static class MetricsContractAssert
{
    /// <summary>
    /// Asserts that a metric with the specified name exists.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="metricName">The expected metric name.</param>
    /// <exception cref="ContractViolationException">Thrown when metric doesn't exist.</exception>
    public static void MetricExists(MetricsCapture capture, string metricName)
    {
        ArgumentNullException.ThrowIfNull(capture);

        if (!capture.HasMetric(metricName))
        {
            throw new ContractViolationException(
                $"Expected metric '{metricName}' not found. " +
                $"Available metrics: [{string.Join(", ", capture.MetricNames)}]");
        }
    }

    /// <summary>
    /// Asserts that a metric's label cardinality is within bounds.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="metricName">The metric to check.</param>
    /// <param name="maxLabels">Maximum allowed unique label combinations.</param>
    /// <exception cref="ContractViolationException">Thrown when cardinality exceeds threshold.</exception>
    public static void LabelCardinalityBounded(MetricsCapture capture, string metricName, int maxLabels)
    {
        ArgumentNullException.ThrowIfNull(capture);

        var cardinality = capture.GetLabelCardinality(metricName);

        if (cardinality > maxLabels)
        {
            throw new ContractViolationException(
                $"Metric '{metricName}' has cardinality {cardinality}, exceeds max {maxLabels}. " +
                "High cardinality metrics cause storage and performance issues.");
        }
    }

    /// <summary>
    /// Asserts that a counter metric is monotonically increasing.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="metricName">The counter metric to check.</param>
    /// <exception cref="ContractViolationException">Thrown when counter decreases.</exception>
    public static void CounterMonotonic(MetricsCapture capture, string metricName)
    {
        ArgumentNullException.ThrowIfNull(capture);

        var values = capture.GetValues(metricName);
        double? previous = null;

        foreach (var value in values)
        {
            if (previous.HasValue && value < previous.Value)
            {
                throw new ContractViolationException(
                    $"Counter '{metricName}' is not monotonic: decreased from {previous} to {value}");
            }
            previous = value;
        }
    }

    /// <summary>
    /// Asserts that a gauge metric stays within expected bounds.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="metricName">The gauge metric to check.</param>
    /// <param name="minValue">Minimum acceptable value.</param>
    /// <param name="maxValue">Maximum acceptable value.</param>
    /// <exception cref="ContractViolationException">Thrown when gauge exceeds bounds.</exception>
    public static void GaugeInBounds(MetricsCapture capture, string metricName, double minValue, double maxValue)
    {
        ArgumentNullException.ThrowIfNull(capture);

        var values = capture.GetValues(metricName);

        foreach (var value in values)
        {
            if (value < minValue || value > maxValue)
            {
                throw new ContractViolationException(
                    $"Gauge '{metricName}' value {value} outside bounds [{minValue}, {maxValue}]");
            }
        }
    }

    /// <summary>
    /// Asserts that metric names follow the expected naming convention.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="pattern">Regex pattern for metric names (e.g., "^[a-z_]+_total$" for counters).</param>
    /// <exception cref="ContractViolationException">Thrown when metric names don't match pattern.</exception>
    public static void MetricNamesMatchPattern(MetricsCapture capture, string pattern)
    {
        ArgumentNullException.ThrowIfNull(capture);

        var regex = new System.Text.RegularExpressions.Regex(pattern);
        var violating = capture.MetricNames.Where(name => !regex.IsMatch(name)).ToList();

        if (violating.Count > 0)
        {
            throw new ContractViolationException(
                $"Metric names violate naming convention '{pattern}': [{string.Join(", ", violating)}]");
        }
    }

    /// <summary>
    /// Asserts that required metrics are present.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="metricNames">Required metric names.</param>
    /// <exception cref="ContractViolationException">Thrown when required metrics are missing.</exception>
    public static void HasRequiredMetrics(MetricsCapture capture, params string[] metricNames)
    {
        ArgumentNullException.ThrowIfNull(capture);

        var missing = metricNames.Where(name => !capture.HasMetric(name)).ToList();

        if (missing.Count > 0)
        {
            throw new ContractViolationException(
                $"Missing required metrics: [{string.Join(", ", missing)}]");
        }
    }

    /// <summary>
    /// Asserts that no metrics have unbounded label values.
    /// </summary>
    /// <param name="capture">The metrics capture.</param>
    /// <param name="forbiddenLabelPatterns">Patterns indicating unbounded values (e.g., IDs, timestamps).</param>
    /// <exception cref="ContractViolationException">Thrown when unbounded labels are detected.</exception>
    public static void NoUnboundedLabels(MetricsCapture capture, params System.Text.RegularExpressions.Regex[] forbiddenLabelPatterns)
    {
        ArgumentNullException.ThrowIfNull(capture);

        foreach (var metricName in capture.MetricNames)
        {
            var labels = capture.GetLabels(metricName);

            foreach (var (labelName, labelValues) in labels)
            {
                foreach (var value in labelValues)
                {
                    foreach (var pattern in forbiddenLabelPatterns)
                    {
                        if (pattern.IsMatch(value))
                        {
                            throw new ContractViolationException(
                                $"Metric '{metricName}' has potentially unbounded label '{labelName}': " +
                                $"value '{value}' matches pattern '{pattern}'");
                        }
                    }
                }
            }
        }
    }
}

/// <summary>
/// Captures metrics for contract testing.
/// </summary>
public sealed class MetricsCapture : IDisposable
{
    private readonly Dictionary<string, List<MetricMeasurement>> _measurements = new();
    private readonly MeterListener _listener;
    private bool _disposed;

    /// <summary>
    /// Creates a new metrics capture.
    /// </summary>
    /// <param name="meterName">Optional meter name filter.</param>
    public MetricsCapture(string? meterName = null)
    {
        _listener = new MeterListener
        {
            InstrumentPublished = (instrument, listener) =>
            {
                if (meterName == null || instrument.Meter.Name == meterName)
                {
                    listener.EnableMeasurementEvents(instrument);
                }
            }
        };

        _listener.SetMeasurementEventCallback<double>(OnMeasurement);
        _listener.SetMeasurementEventCallback<long>(OnMeasurementLong);
        _listener.SetMeasurementEventCallback<int>(OnMeasurementInt);

        _listener.Start();
    }

    private void OnMeasurement(Instrument instrument, double measurement,
        ReadOnlySpan<KeyValuePair<string, object?>> tags, object? state)
    {
        RecordMeasurement(instrument.Name, measurement, tags);
    }

    private void OnMeasurementLong(Instrument instrument, long measurement,
        ReadOnlySpan<KeyValuePair<string, object?>> tags, object? state)
    {
        RecordMeasurement(instrument.Name, measurement, tags);
    }

    private void OnMeasurementInt(Instrument instrument, int measurement,
        ReadOnlySpan<KeyValuePair<string, object?>> tags, object? state)
    {
        RecordMeasurement(instrument.Name, measurement, tags);
    }

    private void RecordMeasurement(string name, double value, ReadOnlySpan<KeyValuePair<string, object?>> tags)
    {
        lock (_measurements)
        {
            if (!_measurements.TryGetValue(name, out var list))
            {
                list = new List<MetricMeasurement>();
                _measurements[name] = list;
            }

            list.Add(new MetricMeasurement
            {
                Value = value,
                Tags = tags.ToArray().ToDictionary(
                    t => t.Key,
                    t => t.Value?.ToString() ?? ""),
                Timestamp = DateTimeOffset.UtcNow
            });
        }
    }

    /// <summary>
    /// Gets all metric names that have been recorded.
    /// </summary>
    public IReadOnlyList<string> MetricNames
    {
        get
        {
            lock (_measurements)
            {
                return _measurements.Keys.ToList();
            }
        }
    }

    /// <summary>
    /// Checks if a metric has been recorded.
    /// </summary>
    public bool HasMetric(string name)
    {
        lock (_measurements)
        {
            return _measurements.ContainsKey(name);
        }
    }

    /// <summary>
    /// Gets all recorded values for a metric.
    /// </summary>
    public IReadOnlyList<double> GetValues(string metricName)
    {
        lock (_measurements)
        {
            if (_measurements.TryGetValue(metricName, out var list))
            {
                return list.Select(m => m.Value).ToList();
            }
            return Array.Empty<double>();
        }
    }

    /// <summary>
    /// Gets the cardinality (number of unique label combinations) for a metric.
    /// </summary>
    public int GetLabelCardinality(string metricName)
    {
        lock (_measurements)
        {
            if (_measurements.TryGetValue(metricName, out var list))
            {
                return list
                    .Select(m => string.Join(",", m.Tags.OrderBy(t => t.Key).Select(t => $"{t.Key}={t.Value}")))
                    .Distinct()
                    .Count();
            }
            return 0;
        }
    }

    /// <summary>
    /// Gets all unique label values for a metric.
    /// </summary>
    public IReadOnlyDictionary<string, IReadOnlyList<string>> GetLabels(string metricName)
    {
        lock (_measurements)
        {
            if (!_measurements.TryGetValue(metricName, out var list))
            {
                return new Dictionary<string, IReadOnlyList<string>>();
            }

            var result = new Dictionary<string, HashSet<string>>();

            foreach (var measurement in list)
            {
                foreach (var (key, value) in measurement.Tags)
                {
                    if (!result.TryGetValue(key, out var values))
                    {
                        values = new HashSet<string>();
                        result[key] = values;
                    }
                    values.Add(value);
                }
            }

            return result.ToDictionary(
                kvp => kvp.Key,
                kvp => (IReadOnlyList<string>)kvp.Value.ToList());
        }
    }

    /// <inheritdoc />
    public void Dispose()
    {
        if (_disposed) return;
        _listener.Dispose();
        _disposed = true;
    }

    private sealed record MetricMeasurement
    {
        public double Value { get; init; }
        public Dictionary<string, string> Tags { get; init; } = new();
        public DateTimeOffset Timestamp { get; init; }
    }
}