using System;
using System.Linq;
using StardewValley;

namespace StardewModdingAPI.Utilities
{
    /// <summary>Represents a Stardew Valley date.</summary>
    public class SDate : IEquatable<SDate>
    {
        /*********
        ** Fields
        *********/
        /// <summary>The internal season names in order.</summary>
        private readonly string[] Seasons = { "spring", "summer", "fall", "winter" };

        /// <summary>The number of seasons in a year.</summary>
        private int SeasonsInYear => this.Seasons.Length;

        /// <summary>The number of days in a season.</summary>
        private readonly int DaysInSeason = 28;


        /*********
        ** Accessors
        *********/
        /// <summary>The day of month.</summary>
        public int Day { get; }

        /// <summary>The season name.</summary>
        public string Season { get; }

        /// <summary>The year.</summary>
        public int Year { get; }

        /// <summary>The day of week.</summary>
        public DayOfWeek DayOfWeek { get; }

        /// <summary>The number of days since the game began (starting at 1 for the first day of spring in Y1).</summary>
        public int DaysSinceStart { get; }


        /*********
        ** Public methods
        *********/
        /// <summary>Construct an instance.</summary>
        /// <param name="day">The day of month.</param>
        /// <param name="season">The season name.</param>
        /// <exception cref="ArgumentException">One of the arguments has an invalid value (like day 35).</exception>
        public SDate(int day, string season)
            : this(day, season, Game1.year) { }

        /// <summary>Construct an instance.</summary>
        /// <param name="day">The day of month.</param>
        /// <param name="season">The season name.</param>
        /// <param name="year">The year.</param>
        /// <exception cref="ArgumentException">One of the arguments has an invalid value (like day 35).</exception>
        public SDate(int day, string season, int year)
            : this(day, season, year, allowDayZero: false) { }

        /// <summary>Get the current in-game date.</summary>
        public static SDate Now()
        {
            return new SDate(Game1.dayOfMonth, Game1.currentSeason, Game1.year, allowDayZero: true);
        }

        /// <summary>Get a new date with the given number of days added.</summary>
        /// <param name="offset">The number of days to add.</param>
        /// <returns>Returns the resulting date.</returns>
        /// <exception cref="ArithmeticException">The offset would result in an invalid date (like year 0).</exception>
        public SDate AddDays(int offset)
        {
            // get new hash code
            int hashCode = this.DaysSinceStart + offset;
            if (hashCode < 1)
                throw new ArithmeticException($"Adding {offset} days to {this} would result in a date before 01 spring Y1.");

            // get day
            int day = hashCode % 28;
            if (day == 0)
                day = 28;

            // get season index
            int seasonIndex = hashCode / 28;
            if (seasonIndex > 0 && hashCode % 28 == 0)
                seasonIndex -= 1;
            seasonIndex %= 4;

            // get year
            int year = (int)Math.Ceiling(hashCode / (this.Seasons.Length * this.DaysInSeason * 1m));

            // create date
            return new SDate(day, this.Seasons[seasonIndex], year);
        }

        /// <summary>Get a string representation of the date. This is mainly intended for debugging or console messages.</summary>
        public override string ToString()
        {
            return $"{this.Day:00} {this.Season} Y{this.Year}";
        }

        /****
        ** IEquatable
        ****/
        /// <summary>Get whether this instance is equal to another.</summary>
        /// <param name="other">The other value to compare.</param>
        public bool Equals(SDate other)
        {
            return this == other;
        }

        /// <summary>Get whether this instance is equal to another.</summary>
        /// <param name="obj">The other value to compare.</param>
        public override bool Equals(object obj)
        {
            return obj is SDate other && this == other;
        }

        /// <summary>Get a hash code which uniquely identifies a date.</summary>
        public override int GetHashCode()
        {
            return this.DaysSinceStart;
        }

        /****
        ** Operators
        ****/
        /// <summary>Get whether one date is equal to another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        /// <returns>The equality of the dates</returns>
        public static bool operator ==(SDate date, SDate other)
        {
            return date?.DaysSinceStart == other?.DaysSinceStart;
        }

        /// <summary>Get whether one date is not equal to another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        public static bool operator !=(SDate date, SDate other)
        {
            return date?.DaysSinceStart != other?.DaysSinceStart;
        }

        /// <summary>Get whether one date is more than another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        public static bool operator >(SDate date, SDate other)
        {
            return date?.DaysSinceStart > other?.DaysSinceStart;
        }

        /// <summary>Get whether one date is more than or equal to another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        public static bool operator >=(SDate date, SDate other)
        {
            return date?.DaysSinceStart >= other?.DaysSinceStart;
        }

        /// <summary>Get whether one date is less than or equal to another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        public static bool operator <=(SDate date, SDate other)
        {
            return date?.DaysSinceStart <= other?.DaysSinceStart;
        }

        /// <summary>Get whether one date is less than another.</summary>
        /// <param name="date">The base date to compare.</param>
        /// <param name="other">The other date to compare.</param>
        public static bool operator <(SDate date, SDate other)
        {
            return date?.DaysSinceStart < other?.DaysSinceStart;
        }


        /*********
        ** Private methods
        *********/
        /// <summary>Construct an instance.</summary>
        /// <param name="day">The day of month.</param>
        /// <param name="season">The season name.</param>
        /// <param name="year">The year.</param>
        /// <param name="allowDayZero">Whether to allow 0 spring Y1 as a valid date.</param>
        /// <exception cref="ArgumentException">One of the arguments has an invalid value (like day 35).</exception>
        private SDate(int day, string season, int year, bool allowDayZero)
        {
            // validate
            if (season == null)
                throw new ArgumentNullException(nameof(season));
            if (!this.Seasons.Contains(season))
                throw new ArgumentException($"Unknown season '{season}', must be one of [{string.Join(", ", this.Seasons)}].");
            if (day < 0 || day > this.DaysInSeason)
                throw new ArgumentException($"Invalid day '{day}', must be a value from 1 to {this.DaysInSeason}.");
            if (day == 0 && !(allowDayZero && this.IsDayZero(day, season, year)))
                throw new ArgumentException($"Invalid day '{day}', must be a value from 1 to {this.DaysInSeason}.");
            if (year < 1)
                throw new ArgumentException($"Invalid year '{year}', must be at least 1.");

            // initialise
            this.Day = day;
            this.Season = season;
            this.Year = year;
            this.DayOfWeek = this.GetDayOfWeek(day);
            this.DaysSinceStart = this.GetDaysSinceStart(day, season, year);

        }

        /// <summary>Get whether a date represents 0 spring Y1, which is the date during the in-game intro.</summary>
        /// <param name="day">The day of month.</param>
        /// <param name="season">The season name.</param>
        /// <param name="year">The year.</param>
        private bool IsDayZero(int day, string season, int year)
        {
            return day == 0 && season == "spring" && year == 1;
        }

        /// <summary>Get the day of week for a given date.</summary>
        /// <param name="day">The day of month.</param>
        private DayOfWeek GetDayOfWeek(int day)
        {
            switch (day % 7)
            {
                case 0:
                    return DayOfWeek.Sunday;
                case 1:
                    return DayOfWeek.Monday;
                case 2:
                    return DayOfWeek.Tuesday;
                case 3:
                    return DayOfWeek.Wednesday;
                case 4:
                    return DayOfWeek.Thursday;
                case 5:
                    return DayOfWeek.Friday;
                case 6:
                    return DayOfWeek.Saturday;
                default:
                    return 0;
            }
        }

        /// <summary>Get the number of days since the game began (starting at 1 for the first day of spring in Y1).</summary>
        /// <param name="day">The day of month.</param>
        /// <param name="season">The season name.</param>
        /// <param name="year">The year.</param>
        private int GetDaysSinceStart(int day, string season, int year)
        {
            // return the number of days since 01 spring Y1 (inclusively)
            int yearIndex = year - 1;
            return
                yearIndex * this.DaysInSeason * this.SeasonsInYear
                + this.GetSeasonIndex(season) * this.DaysInSeason
                + day;
        }

        /// <summary>Get a season index.</summary>
        /// <param name="season">The season name.</param>
        /// <exception cref="InvalidOperationException">The current season wasn't recognised.</exception>
        private int GetSeasonIndex(string season)
        {
            int index = Array.IndexOf(this.Seasons, season);
            if (index == -1)
                throw new InvalidOperationException($"The season '{season}' wasn't recognised.");
            return index;
        }
    }
}