using System;
using System.Collections.Generic;
using System.Globalization;

namespace FileInterface.Scheduler
{
    /// <summary>
    /// Standard 5-field cron expression: minute hour day-of-month month day-of-week.
    /// Supported: *, */n, a,b,c, a-b, a-b/n. Day-of-week: 0 or 7 = Sunday.
    /// </summary>
    public sealed class CronExpression
    {
        private readonly HashSet<int> _minutes;
        private readonly HashSet<int> _hours;
        private readonly HashSet<int> _days;
        private readonly HashSet<int> _months;
        private readonly HashSet<int> _daysOfWeek;

        public string Expression { get; private set; }

        private CronExpression(string expression,
            HashSet<int> minutes,
            HashSet<int> hours,
            HashSet<int> days,
            HashSet<int> months,
            HashSet<int> daysOfWeek)
        {
            Expression = expression;
            _minutes = minutes;
            _hours = hours;
            _days = days;
            _months = months;
            _daysOfWeek = daysOfWeek;
        }

        public static CronExpression Parse(string expression)
        {
            if (String.IsNullOrWhiteSpace(expression))
                throw new FormatException("Cron expression is empty.");

            string[] parts = expression.Trim().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
            if (parts.Length != 5)
                throw new FormatException("Cron must contain exactly 5 fields: minute hour day month day-of-week.");

            return new CronExpression(
                expression.Trim(),
                ParseField(parts[0], 0, 59, false),
                ParseField(parts[1], 0, 23, false),
                ParseField(parts[2], 1, 31, false),
                ParseField(parts[3], 1, 12, false),
                ParseField(parts[4], 0, 7, true));
        }

        public bool IsMatch(DateTime localTime)
        {
            int dow = (int)localTime.DayOfWeek;
            return _minutes.Contains(localTime.Minute)
                && _hours.Contains(localTime.Hour)
                && _days.Contains(localTime.Day)
                && _months.Contains(localTime.Month)
                && _daysOfWeek.Contains(dow);
        }

        private static HashSet<int> ParseField(string text, int min, int max, bool normalizeSunday)
        {
            var result = new HashSet<int>();
            string[] segments = text.Split(',');
            foreach (string rawSegment in segments)
            {
                string segment = rawSegment.Trim();
                if (segment.Length == 0)
                    throw new FormatException("Empty cron field segment.");

                int step = 1;
                string rangePart = segment;
                int slash = segment.IndexOf('/');
                if (slash >= 0)
                {
                    rangePart = segment.Substring(0, slash);
                    string stepText = segment.Substring(slash + 1);
                    if (!Int32.TryParse(stepText, NumberStyles.Integer, CultureInfo.InvariantCulture, out step) || step <= 0)
                        throw new FormatException("Invalid cron step: " + segment);
                }

                int start;
                int end;
                if (rangePart == "*")
                {
                    start = min;
                    end = max;
                }
                else
                {
                    int dash = rangePart.IndexOf('-');
                    if (dash >= 0)
                    {
                        start = ParseNumber(rangePart.Substring(0, dash), min, max);
                        end = ParseNumber(rangePart.Substring(dash + 1), min, max);
                        if (start > end)
                            throw new FormatException("Cron range start is greater than end: " + segment);
                    }
                    else
                    {
                        start = ParseNumber(rangePart, min, max);
                        end = start;
                    }
                }

                for (int value = start; value <= end; value += step)
                {
                    int normalized = normalizeSunday && value == 7 ? 0 : value;
                    result.Add(normalized);
                }
            }

            if (result.Count == 0)
                throw new FormatException("Cron field produced no values: " + text);
            return result;
        }

        private static int ParseNumber(string text, int min, int max)
        {
            int value;
            if (!Int32.TryParse(text, NumberStyles.Integer, CultureInfo.InvariantCulture, out value))
                throw new FormatException("Invalid cron number: " + text);
            if (value < min || value > max)
                throw new FormatException("Cron value out of range: " + value + " (allowed " + min + "-" + max + ")");
            return value;
        }
    }
}