using System;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
using System.Xml.Serialization;

namespace ServerMonitorApp
{
    /// <summary>The possible statuses of a check.</summary>
    /// <remarks>
    /// The integer values of the "completed" statuses (Success, Information,
    /// Warning, Error) are in ascending order of severity.
    /// </remarks>
    public enum CheckStatus
    {
        Success,
        Information,
        Warning,
        Error,
        Running,
        Disabled,
    }

    /// <summary>Base class for checks that run against a server and return a result status.</summary>
    public abstract class Check
    {
        private static Type[] _checkTypes;

        public static Type[] CheckTypes
        {
            get
            {
                return _checkTypes ?? (_checkTypes = typeof(Check).Assembly.GetTypes()
                    .Where(t => t.IsSubclassOf(typeof(Check)))
                    .OrderBy(t => t.GetAttribute<DisplayWeightAttribute>()?.DisplayWeight).ToArray());
            }
        }

        /// <summary>The internal ID of the check.</summary>
        public int Id { get; set; }

        /// <summary>The display name check.</summary>
        public string Name { get; set; }

        /// <summary>The number of milliseconds the check must complete in before reporting failure.</summary>
        public int Timeout { get; set; }

        /// <summary>Whether the check will be executed on a schedule.</summary>
        public bool Enabled { get; set; }

        /// <summary>The schedule when the check will be executed.</summary>
        public Schedule Schedule { get; set; }

        /// <summary>The date and time the check was last executed.</summary>
        public DateTime LastRunTime { get; set; }

        /// <summary>The date and time the check was last executed on its schedule.</summary>
        public DateTime LastScheduledRunTime { get; set; }

        /// <summary>The date and time the check is currently scheduled to execute next.</summary>
        public DateTime NextRunTime { get; set; }

        /// <summary>The text output of the last execution of the check.</summary>
        public string LastMessage { get; set; }

        /// <summary>The current status of the check.</summary>
        public CheckStatus Status { get; set; }

        /// <summary>The status of the last check execution.</summary>
        public CheckStatus LastRunStatus { get; set; }

        /// <summary>The severity level reported when the check execution fails.</summary>
        public CheckStatus FailStatus { get; set; }

        /// <summary>The number of consecutive failed executions before the check begins reporting failure.</summary>
        public int MaxConsecutiveFailures { get; set; }

        /// <summary>The current number of consecutive times the check has failed.</summary>
        [XmlIgnore]
        public int ConsecutiveFailures { get; set; }

        /// <summary>The server the check belongs to.</summary>
        [XmlIgnore]
        public Server Server { get; set; }

        /// <summary>Check constructor.</summary>
        public Check()
        {
            // Set the default failure severity to Error.
            FailStatus = CheckStatus.Error;
        }

        /// <summary>Displays the check name.</summary>
        public override string ToString()
        {
            return Name;
        }

        /// <summary>Validates the check.</summary>
        /// <param name="saving">
        /// Whether the check is being saved.
        /// Checks are validated before being saved and before being executed. This parameter allows
        /// them to distinguish between these cases.
        /// </param>
        /// <returns>An empty string if the check is valid, or the reason the check is invalid.</returns>
        public virtual string Validate(bool saving = true)
        {
            string message = string.Empty;
            // Allow blank names if the check is being executed before saving.
            // This lets the user create a check and tinker with it without
            // needing to type a name unless they want to save it.
            if (Name.IsNullOrEmpty() && saving)
                message += "Name cannot be blank." + Environment.NewLine;
            return message;
        }

        /// <summary>Executes the check asynchronously.</summary>
        /// <param name="token">A token for cancelling the execution.</param>
        /// <param name="update">Whether the check status and last execution time should be updated when the check completes.</param>
        /// <returns>The result of the check execution.</returns>
        public async Task<CheckResult> ExecuteAsync(CancellationToken token = default(CancellationToken), bool update = true)
        {
            // Do nothing if the check has already been cancelled.
            if (token.IsCancellationRequested)
                return null;

            CheckResult result;
            DateTime startTime = DateTime.Now;
            try
            {
                // Execute the check.
                Task<CheckResult> checkTask = ExecuteCheckAsync(token);
                try
                {
                    // Wait for the check to complete or timeout, whichever happens first.
                    if (await Task.WhenAny(checkTask, Task.Delay(Timeout, token)) == checkTask)
                    {
                        // If the check completed before timing out, retrieve the result.
                        result = await checkTask;
                    }
                    else
                    {
                        // If the check timed out before completing, report failure.
                        result = Fail("Timed out.");
                    }
                }
                catch (TaskCanceledException)
                {
                    // If the check was cancelled, do not return a result so it will not be logged.
                    return null;
                }
            }
            catch (Exception e)
            {
                // If the execution threw an exception, report the exception as a failure.
                result = Fail(e.GetBaseException().Message);
            }
            result.StartTime = startTime;
            result.EndTime = DateTime.Now;
            if (update)
            {
                Status = result.CheckStatus;
                LastRunStatus = result.CheckStatus;
                LastMessage = result.Message;
                LastRunTime = result.EndTime;
            }
            return result;
        }

        /// <summary>Generates a successful check result.</summary>
        /// <param name="message">The execution result message.</param>
        /// <returns>A successful check result.</returns>
        public CheckResult Pass(string message)
        {
            return new CheckResult(this, CheckStatus.Success, message);
        }

        /// <summary>Generates a failed check result.</summary>
        /// <param name="message">The execution result message.</param>
        /// <returns>A failed check result.</returns>
        public CheckResult Fail(string message)
        {
            // The severity is controlled by the check's FailStatus setting.
            return new CheckResult(this, FailStatus, message);
        }

        /// <summary>Generates a failed check result from an exception.</summary>
        /// <param name="e">The exception that caused the failure.</param>
        /// <returns>A failed check result.</returns>
        protected CheckResult Fail(Exception e)
        {
            return Fail(e.GetBaseException().Message);
        }

        /// <summary>Generates a check result by comparing integer values for equality.</summary>
        /// <param name="expectedValue">The expected result value.</param>
        /// <param name="resultValue">The actual result value generated by the check execution.</param>
        /// <param name="description">Description of what the integer represents to use in the check result message. Example: "Exit code".</param>
        /// <returns>A successful check result if the values are equal, or a failed check result if they are unequal.</returns>
        protected CheckResult GetIntResult(int expectedValue, int resultValue, string description)
        {
            if (expectedValue == resultValue)
                return Pass(string.Format("{0}: {1}", description, resultValue));
            else
                return Fail(string.Format("{0}: {1} (expected: {2})", description, resultValue, expectedValue));
        }

        /// <summary>Generates a check result by comparing string values.</summary>
        /// <param name="matchType">The comparison that will be used on the strings.</param>
        /// <param name="expectedPattern">The expected pattern to test the result against.</param>
        /// <param name="useRegex">Whether the expected pattern should be treated as a regular expression.</param>
        /// <param name="resultValue">The actual result value generated by the check execution.</param>
        /// <param name="description">Description of what the string represents to use in the check result message.</param>
        /// <returns>A successful check result if the string comparison succeeds, or a failed check result if it fails.</returns>
        protected CheckResult GetStringResult(MatchType matchType, string expectedPattern, bool useRegex, string resultValue, string description)
        {
            bool match;
            if (useRegex)
            {
                // If the match type is equals or not equals, modify the regex by
                // adding beginning and ending anchors if not already present
                // to prevent partial matches.
                if (matchType.In(MatchType.Equals, MatchType.NotEquals))
                {
                    if (!expectedPattern.StartsWith("^"))
                        expectedPattern = "^" + expectedPattern;
                    if (!expectedPattern.EndsWith("$"))
                        expectedPattern += "$";
                }
                // Execute the regex.
                Regex re = new Regex(expectedPattern, RegexOptions.Singleline);
                match = re.IsMatch(resultValue);
            }
            else
            {
                // Simple string comparisons.
                if (matchType.In(MatchType.Equals, MatchType.NotEquals))
                {
                    match = expectedPattern == resultValue;
                }
                else if (matchType.In(MatchType.Contains, MatchType.NotContains))
                {
                    match = resultValue.Contains(expectedPattern);
                }
                else
                {
                    // If the match type is greater or less than, the values must be numeric.
                    if (decimal.TryParse(expectedPattern, out decimal expectedNumeric) &&
                        decimal.TryParse(resultValue, out decimal resultNumeric))
                    {
                        // Compare the resulting decimals.
                        match = (matchType == MatchType.GreaterThan && resultNumeric > expectedNumeric) ||
                                (matchType == MatchType.LessThan    && resultNumeric < expectedNumeric);
                    }
                    else
                    {
                        return Fail(string.Format("{0} is not numeric: {1}", description, resultValue));
                    }
                }
            }

            // We have determined whether the result value matches the expected pattern.
            // Generate a check result accordingly.
            if (matchType.In(MatchType.Equals, MatchType.Contains))
            {
                // Equals, Contains: the strings are supposed to match.
                if (match)
                    return Pass(string.Format("{0} {1} the pattern: {2}", description, matchType.ToString().ToLower(), expectedPattern));
                else
                    return Fail(string.Format("{0} does not {1} the pattern: {2} ({0}: {3})", description, matchType.ToString().ToLower().TrimEnd('s'), expectedPattern, resultValue));
            }
            else if (matchType.In(MatchType.NotEquals, MatchType.NotContains))
            {
                // NotEquals, NotContains: the strings are not supposed to match.
                // So, fail if they do match and pass if they do not.
                if (match)
                    return Fail(string.Format("{0} {1} the pattern: {2} ({0}: {3})", description, matchType.ToString().ToLower().Replace("not", ""), expectedPattern, resultValue));
                else
                    return Pass(string.Format("{0} does not {1} the pattern: {2}", description, matchType.ToString().ToLower().TrimEnd('s').Replace("not", ""), expectedPattern));
            }
            else
            {
                // GreaterThan, LessThan
                if (match)
                    return Pass(string.Format("{0} ({1}) is {2} {3}", description, resultValue, matchType.ToString().ToLower().Replace("than", " than"), expectedPattern));
                else
                    return Fail(string.Format("{0} ({1}) is not {2} {3}", description, resultValue, matchType.ToString().ToLower().Replace("than", " than"), expectedPattern));
            }
        }

        /// <summary>Merges multiple execution results.</summary>
        /// <param name="results">The results to merge.</param>
        /// <returns>A single result containing the messages of all the input results, and a failure status if any of the input results failed.</returns>
        /// <remarks>
        /// Some checks may want to run several tests on the result of a remote command.
        /// After collecting their results, they can use this method to combine them into
        /// a single result that will be reported to the user.
        /// </remarks>
        protected CheckResult MergeResults(params CheckResult[] results)
        {
            StringBuilder message = new StringBuilder();
            bool failed = false;
            foreach (CheckResult result in results)
            {
                if (result == null)
                    continue;
                // Report failure if any of the results has failed.
                if (result.Failed)
                    failed = true;
                // Merge the result messages.
                message.AppendLine(result.Message);
            }
            return failed ? Fail(message.ToString().Trim()) : Pass(message.ToString().Trim());
        }

        /// <summary>Executes the check asynchronously.</summary>
        /// <param name="token">A token for cancelling the execution.</param>
        /// <returns>The result of the check execution.</returns>
        protected abstract Task<CheckResult> ExecuteCheckAsync(CancellationToken token);
    }
}