using System;
using System.Collections.Generic;

namespace PKHeX.Core
{
    /// <summary>
    /// Level Up Learn Movepool Information
    /// </summary>
    public sealed class Learnset
    {
        /// <summary>
        /// Moves that can be learned.
        /// </summary>
        private readonly int[] Moves;

        /// <summary>
        /// Levels at which a move at a given index can be learned.
        /// </summary>
        private readonly int[] Levels;

        public Learnset(int[] moves, int[] levels)
        {
            Moves = moves;
            Levels = levels;
        }

        /// <summary>
        /// Returns the moves a Pokémon can learn between the specified level range.
        /// </summary>
        /// <param name="maxLevel">Maximum level</param>
        /// <param name="minLevel">Minimum level</param>
        /// <returns>Array of Move IDs</returns>
        public int[] GetMoves(int maxLevel, int minLevel = 0)
        {
            if (minLevel <= 1 && maxLevel >= 100)
                return Moves;
            if (minLevel > maxLevel)
                return Array.Empty<int>();
            int start = Array.FindIndex(Levels, z => z >= minLevel);
            if (start < 0)
                return Array.Empty<int>();
            int end = Array.FindLastIndex(Levels, z => z <= maxLevel);
            if (end < 0)
                return Array.Empty<int>();
            int[] result = new int[end - start + 1];
            Array.Copy(Moves, start, result, 0, result.Length);
            return result;
        }

        /// <summary>
        /// Adds the moves a Pokémon can learn between the specified level range.
        /// </summary>
        /// <param name="moves">Movepool</param>
        /// <param name="maxLevel">Maximum level</param>
        /// <param name="minLevel">Minimum level</param>
        /// <returns>Array of Move IDs</returns>
        public List<int> AddMoves(List<int> moves, int maxLevel, int minLevel = 0)
        {
            if (minLevel <= 1 && maxLevel >= 100)
            {
                moves.AddRange(Moves);
                return moves;
            }
            if (minLevel > maxLevel)
                return moves;
            int start = Array.FindIndex(Levels, z => z >= minLevel);
            if (start < 0)
                return moves;
            int end = Array.FindLastIndex(Levels, z => z <= maxLevel);
            if (end < 0)
                return moves;
            for (int i = start; i < end + 1; i++)
                moves.Add(Moves[i]);
            return moves;
        }

        /// <summary>
        /// Gets the moves a Pokémon can learn between the specified level range as a list.
        /// </summary>
        /// <param name="maxLevel">Maximum level</param>
        /// <param name="minLevel">Minimum level</param>
        /// <returns>Array of Move IDs</returns>
        public List<int> GetMoveList(int maxLevel, int minLevel = 0)
        {
            var list = new List<int>();
            return AddMoves(list, maxLevel, minLevel);
        }

        /// <summary>Returns the moves a Pokémon would have if it were encountered at the specified level.</summary>
        /// <remarks>In Generation 1, it is not possible to learn any moves lower than these encounter moves.</remarks>
        /// <param name="level">The level the Pokémon was encountered at.</param>
        /// <returns>Array of Move IDs</returns>
        public int[] GetEncounterMoves(int level)
        {
            const int count = 4;
            var moves = new int[count];
            return GetEncounterMoves(level, moves);
        }

        /// <summary>Returns the moves a Pokémon would have if it were encountered at the specified level.</summary>
        /// <remarks>In Generation 1, it is not possible to learn any moves lower than these encounter moves.</remarks>
        /// <param name="level">The level the Pokémon was encountered at.</param>
        /// <param name="moves">Move array to write to</param>
        /// <param name="ctr">Starting index to begin overwriting at</param>
        /// <returns>Array of Move IDs</returns>
        public int[] GetEncounterMoves(int level, int[] moves, int ctr = 0)
        {
            for (int i = 0; i < Moves.Length; i++)
            {
                if (Levels[i] > level)
                    break;

                int move = Moves[i];
                bool alreadyHasMove = Array.IndexOf(moves, move) >= 0;
                if (alreadyHasMove)
                    continue;

                moves[ctr++] = move;
                ctr &= 3;
            }
            return moves;
        }

        public IList<int> GetUniqueMovesLearned(IEnumerable<int> seed, int maxLevel, int minLevel = 0)
        {
            int start = Array.FindIndex(Levels, z => z >= minLevel);
            int end = Array.FindLastIndex(Levels, z => z <= maxLevel);
            var list = new List<int>(seed);
            for (int i = start; i <= end; i++)
            {
                if (!list.Contains(Moves[i]))
                    list.Add(Moves[i]);
            }
            return list;
        }

        /// <summary>Returns the index of the lowest level move if the Pokémon were encountered at the specified level.</summary>
        /// <remarks>Helps determine the minimum level an encounter can be at.</remarks>
        /// <param name="level">The level the Pokémon was encountered at.</param>
        /// <returns>Array of Move IDs</returns>
        public int GetMinMoveLevel(int level)
        {
            if (Levels.Length == 0)
                return 1;

            int end = Array.FindLastIndex(Levels, z => z <= level);
            return Math.Max(end - 4, 1);
        }

        private Dictionary<int, int>? Learn;

        private Dictionary<int, int> GetDictionary()
        {
            var dict = new Dictionary<int, int>();
            for (int i = 0; i < Moves.Length; i++)
            {
                if (!dict.ContainsKey(Moves[i]))
                    dict.Add(Moves[i], Levels[i]);
            }
            return dict;
        }

        /// <summary>Returns the level that a Pokémon can learn the specified move.</summary>
        /// <param name="move">Move ID</param>
        /// <returns>Level the move is learned at. If the result is below 0, the move cannot be learned by leveling up.</returns>
        public int GetLevelLearnMove(int move)
        {
            return (Learn ??= GetDictionary()).TryGetValue(move, out var level) ? level : -1;
        }

        /// <summary>Returns the level that a Pokémon can learn the specified move.</summary>
        /// <param name="move">Move ID</param>
        /// <param name="min">Minimum level to start looking at.</param>
        /// <returns>Level the move is learned at. If the result is below 0, the move cannot be learned by leveling up.</returns>
        public int GetLevelLearnMove(int move, int min)
        {
            for (int i = 0; i < Moves.Length; i++)
            {
                if (move != Moves[i])
                    continue;

                var lv = Levels[i];
                if (lv >= min)
                    return lv;
            }
            return -1;
        }

        public ReadOnlySpan<int> GetBaseEggMoves(int level)
        {
            // Count moves <= level
            var count = 0;
            foreach (var x in Levels)
            {
                if (x > level)
                    break;
                count++;
            }

            // Return a slice containing the moves <= level.
            if (count == 0)
                return ReadOnlySpan<int>.Empty;

            int start = 0;
            if (count > 4)
            {
                start = count - 4;
                count = 4;
            }
            return Moves.AsSpan(start, count);
        }
    }
}