MonoGame.Samples/RolePlayingGame/Core/Combat/CombatEngine.cs

//-----------------------------------------------------------------------------
// CombatEngine.cs
//
// Microsoft XNA Community Game Platform
// Copyright (C) Microsoft Corporation. All rights reserved.
//-----------------------------------------------------------------------------

using System;
using System.Collections.Generic;
using System.IO;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using RolePlaying.Data;

namespace RolePlaying
{
    /// <summary>
    /// The runtime execution engine for the combat system.
    /// </summary>
    class CombatEngine
    {


        /// <summary>
        /// The singleton of the combat engine.
        /// </summary>
        private static CombatEngine singleton = null;


        /// <summary>
        /// Check to see if there is a combat going on, and throw an exception if not.
        /// </summary>
        private static void CheckSingleton()
        {
            if (singleton == null)
            {
                throw new InvalidOperationException(
                    "There is no active combat at this time.");
            }
        }






        /// <summary>
        /// If true, the combat engine is active and the user is in combat.
        /// </summary>
        public static bool IsActive
        {
          get { return (singleton != null); }
        }


        /// <summary>
        /// If true, it is currently the players' turn.
        /// </summary>
        private bool isPlayersTurn;

        /// <summary>
        /// If true, it is currently the players' turn.
        /// </summary>
        public static bool IsPlayersTurn
        {
            get 
            {
                CheckSingleton();
                return singleton.isPlayersTurn; 
            }
        }






        /// <summary>
        /// The fixed combat used to generate this fight, if any.
        /// </summary>
        /// <remarks>
        /// Used for rewards.  Null means it was a random fight with no special rewards.
        /// </remarks>
        private MapEntry<FixedCombat> fixedCombatEntry;

        /// <summary>
        /// The fixed combat used to generate this fight, if any.
        /// </summary>
        /// <remarks>
        /// Used for rewards.  Null means it was a random fight with no special rewards.
        /// </remarks>
        public static MapEntry<FixedCombat> FixedCombatEntry
        {
            get { return (singleton == null ? null : singleton.fixedCombatEntry); }
        }






        /// <summary>
        /// The players involved in the current combat.
        /// </summary>
        private List<CombatantPlayer> players = null;

        /// <summary>
        /// The players involved in the current combat.
        /// </summary>
        public static List<CombatantPlayer> Players
        {
            get 
            {
                CheckSingleton();
                return singleton.players; 
            }
        }


        private int highlightedPlayer;


        /// <summary>
        /// The positions of the players on screen.
        /// </summary>
        private static readonly Vector2[] PlayerPositions = new Vector2[5]
        {
            new Vector2(850f, 345f),
            new Vector2(980f, 260f),
            new Vector2(940f, 440f),
            new Vector2(1100f, 200f),
            new Vector2(1100f, 490f)
        };


        /// <summary>
        /// Start the given player's combat turn.
        /// </summary>
        private void BeginPlayerTurn(CombatantPlayer player)
        {
            // check the parameter
            if (player == null)
            {
                throw new ArgumentNullException("player");
            }

            // set the highlight sprite
            highlightedCombatant = player;
            primaryTargetedCombatant = null;
            secondaryTargetedCombatants.Clear();

            Session.Hud.ActionText = "Choose an Action";
        }


        /// <summary>
        /// Begin the players' turn in this combat round.
        /// </summary>
        private void BeginPlayersTurn()
        {
            // set the player-turn
            isPlayersTurn = true;
 
            // reset each player for the next combat turn
            foreach (CombatantPlayer player in players)
            {
                // reset the animation of living players
                if (!player.IsDeadOrDying)
                {
                    player.State = Character.CharacterState.Idle;
                }
                // reset the turn-taken flag
                player.IsTurnTaken = false;
                // clear the combat action
                player.CombatAction = null;
                // advance each player
                player.AdvanceRound();
            }

            // set the action text on the HUD
            Session.Hud.ActionText = "Your Party's Turn";

            // find the first player who is alive
            highlightedPlayer = 0;
            CombatantPlayer firstPlayer = players[highlightedPlayer];
            while (firstPlayer.IsTurnTaken || firstPlayer.IsDeadOrDying)
            {
                highlightedPlayer = (highlightedPlayer + 1) % players.Count;
                firstPlayer = players[highlightedPlayer];
            }

            // start the first player's turn
            BeginPlayerTurn(firstPlayer);
        }


        /// <summary>
        /// Check for whether all players have taken their turn.
        /// </summary>
        private bool IsPlayersTurnComplete
        {
            get
            {
                return players.TrueForAll(delegate(CombatantPlayer player)
                {
                    return (player.IsTurnTaken || player.IsDeadOrDying);
                });
            }
        }


        /// <summary>
        /// Check for whether the players have been wiped out and defeated.
        /// </summary>
        private bool ArePlayersDefeated
        {
            get 
            {
                return players.TrueForAll(delegate(CombatantPlayer player)
                {
                    return (player.State == Character.CharacterState.Dead);
                });
            }
        }


        /// <summary>
        /// Retrieves the first living player, if any.
        /// </summary>
        private CombatantPlayer FirstPlayerTarget
        {
            get
            {
                // if there are no living players, then this is moot
                if (ArePlayersDefeated)
                {
                    return null;
                }

                int playerIndex = 0;
                while ((playerIndex < players.Count) &&
                    players[playerIndex].IsDeadOrDying)
                {
                    playerIndex++;
                }
                return players[playerIndex];
            }
        }






        /// <summary>
        /// The monsters involved in the current combat.
        /// </summary>
        private List<CombatantMonster> monsters = null;

        /// <summary>
        /// The monsters involved in the current combat.
        /// </summary>
        public static List<CombatantMonster> Monsters
        {
            get 
            {
                CheckSingleton();
                return singleton.monsters; 
            }
        }


        /// <summary>
        /// The positions of the monsters on the screen.
        /// </summary>
        private static readonly Vector2[] MonsterPositions = new Vector2[5]
        {
            new Vector2(480f, 345f),
            new Vector2(345f, 260f),
            new Vector2(370f, 440f),
            new Vector2(225f, 200f),
            new Vector2(225f, 490f)
        };


        /// <summary>
        /// Start the given player's combat turn.
        /// </summary>
        private void BeginMonsterTurn(CombatantMonster monster)
        {
            // if it's null, find a random living monster who has yet to take their turn
            if (monster == null)
            {
                // don't bother if all monsters have finished
                if (IsMonstersTurnComplete)
                {
                    return;
                }
                // pick random living monsters who haven't taken their turn
                do
                {
                    monster = monsters[Session.Random.Next(monsters.Count)];
                }
                while (monster.IsTurnTaken || monster.IsDeadOrDying);
            }

            // set the highlight sprite
            highlightedCombatant = monster;
            primaryTargetedCombatant = null;
            secondaryTargetedCombatants.Clear();

            // choose the action immediate
            monster.CombatAction = monster.ArtificialIntelligence.ChooseAction();
        }
        
        
        /// <summary>
        /// Begin the monsters' turn in this combat round.
        /// </summary>
        private void BeginMonstersTurn()
        {
            // set the monster-turn
            isPlayersTurn = false;

            // reset each monster for the next combat turn
            foreach (CombatantMonster monster in monsters)
            {
                // reset the animations back to idle
                monster.Character.State = Character.CharacterState.Idle;
                // reset the turn-taken flag
                monster.IsTurnTaken = false;
                // clear the combat action
                monster.CombatAction = null;
                // advance the combatants
                monster.AdvanceRound();
            }
            
            // set the action text on the HUD
            Session.Hud.ActionText = "Enemy Party's Turn";

            // start a Session.Random monster's turn
            BeginMonsterTurn(null);
        }


        /// <summary>
        /// Check for whether all monsters have taken their turn.
        /// </summary>
        private bool IsMonstersTurnComplete
        {
            get
            {
                return monsters.TrueForAll(delegate(CombatantMonster monster)
                {
                    return (monster.IsTurnTaken || monster.IsDeadOrDying);
                });
            }
        }


        /// <summary>
        /// Check for whether the monsters have been wiped out and defeated.
        /// </summary>
        private bool AreMonstersDefeated
        {
            get
            {
                return monsters.TrueForAll(delegate(CombatantMonster monster)
                {
                    return (monster.State == Character.CharacterState.Dead);
                });
            }
        }


        /// <summary>
        /// Retrieves the first living monster, if any.
        /// </summary>
        private CombatantMonster FirstMonsterTarget
        {
            get
            {
                // if there are no living monsters, then this is moot
                if (AreMonstersDefeated)
                {
                    return null;
                }

                int monsterIndex = 0;
                while ((monsterIndex < monsters.Count) && 
                    monsters[monsterIndex].IsDeadOrDying)
                {
                    monsterIndex++;
                }
                return monsters[monsterIndex];
            }
        }






        /// <summary>
        /// The currently highlighted player, if any.
        /// </summary>
        private Combatant highlightedCombatant;

        /// <summary>
        /// The currently highlighted player, if any.
        /// </summary>
        public static Combatant HighlightedCombatant
        {
            get
            {
                CheckSingleton();
                return singleton.highlightedCombatant;
            }
        }


        /// <summary>
        /// The current primary target, if any.
        /// </summary>
        private Combatant primaryTargetedCombatant;

        /// <summary>
        /// The current primary target, if any.
        /// </summary>
        public static Combatant PrimaryTargetedCombatant
        {
            get
            {
                CheckSingleton();
                return singleton.primaryTargetedCombatant;
            }
        }


        /// <summary>
        /// The current secondary targets, if any.
        /// </summary>
        private List<Combatant> secondaryTargetedCombatants = new List<Combatant>();

        /// <summary>
        /// The current secondary targets, if any.
        /// </summary>
        public static List<Combatant> SecondaryTargetedCombatants
        {
            get
            {
                CheckSingleton();
                return singleton.secondaryTargetedCombatants;
            }
        }


        /// <summary>
        /// Retrieves the first living enemy, if any.
        /// </summary>
        public static Combatant FirstEnemyTarget
        {
            get
            {
                CheckSingleton();

                if (IsPlayersTurn)
                {
                    return singleton.FirstMonsterTarget;
                }
                else
                {
                    return singleton.FirstPlayerTarget;
                }
            }
        }


        /// <summary>
        /// Retrieves the first living ally, if any.
        /// </summary>
        public static Combatant FirstAllyTarget
        {
            get
            {
                CheckSingleton();

                if (IsPlayersTurn)
                {
                    return singleton.FirstPlayerTarget;
                }
                else
                {
                    return singleton.FirstMonsterTarget;
                }
            }
        }
        

        /// <summary>
        /// Set the primary and any secondary targets.
        /// </summary>
        /// <param name="primaryTarget">The desired primary target.</param>
        /// <param name="adjacentTargets">
        /// The number of simultaneous, adjacent targets affected by this spell.
        /// </param>
        private void SetTargets(Combatant primaryTarget, int adjacentTargets)
        {
            // set the primary target
            this.primaryTargetedCombatant = primaryTarget;

            // set any secondary targets
            this.secondaryTargetedCombatants.Clear();
            if ((primaryTarget != null) && (adjacentTargets > 0))
            {
                // find out which side is targeted
                bool isPlayerTarget = primaryTarget is CombatantPlayer;
                // find the index
                int primaryTargetIndex = 0;
                if (isPlayerTarget)
                {
                    primaryTargetIndex = players.FindIndex(
                        delegate(CombatantPlayer player)
                        {
                            return (player == primaryTarget);
                        });
                }
                else
                {
                    primaryTargetIndex = monsters.FindIndex(
                        delegate(CombatantMonster monster)
                        {
                            return (monster == primaryTarget);
                        });
                }
                // add the surrounding indices
                for (int i = 1; i <= adjacentTargets; i++)
                {
                    int leftIndex = primaryTargetIndex - i;
                    if (leftIndex >= 0)
                    {
                        secondaryTargetedCombatants.Add(
                            isPlayerTarget ? players[leftIndex] as Combatant : 
                                monsters[leftIndex] as Combatant);
                    }
                    int rightIndex = primaryTargetIndex + i;
                    if (rightIndex < (isPlayerTarget ? players.Count : monsters.Count))
                    {
                        secondaryTargetedCombatants.Add(
                            isPlayerTarget ? players[rightIndex] as Combatant :
                            monsters[rightIndex] as Combatant);
                    }
                }
            }
        }






        /// <summary>
        /// A combat effect sprite, typically used for damage or healing numbers.
        /// </summary>
        private class CombatEffect
        {


            /// <summary>
            /// The starting position of the effect on the screen.
            /// </summary>
            public Vector2 OriginalPosition;


            /// <summary>
            /// The current position of the effect on the screen.
            /// </summary>
            protected Vector2 position;

            /// <summary>
            /// The current position of the effect on the screen.
            /// </summary>
            public Vector2 Position
            {
                get { return position; }
            }






            /// <summary>
            /// The text that appears on top of the effect.
            /// </summary>
            protected string text = String.Empty;

            /// <summary>
            /// The text that appears on top of the effect.
            /// </summary>
            public string Text
            {
                get { return text; }
                set
                {
                    text = value;
                    // recalculate the origin
                    if (String.IsNullOrEmpty(text))
                    {
                        textOrigin = Vector2.Zero;
                    }
                    else
                    {
                        Vector2 textSize = Fonts.DamageFont.MeasureString(text);
                        textOrigin = new Vector2(
                            (float)Math.Ceiling(textSize.X / 2f), 
                            (float)Math.Ceiling(textSize.Y / 2f));
                    }
                }
            }

            /// <summary>
            /// The drawing origin of the text on the effect.
            /// </summary>
            private Vector2 textOrigin = Vector2.Zero;






            /// <summary>
            /// The speed at which the effect rises on the screen.
            /// </summary>
            const int risePerSecond = 100;

            /// <summary>
            /// The amount which the effect rises on the screen.
            /// </summary>
            const int riseMaximum = 80;


            /// <summary>
            /// The amount which the effect has already risen on the screen.
            /// </summary>
            public float Rise = 0;


            /// <summary>
            /// If true, the effect has finished rising.
            /// </summary>
            private bool isRiseComplete = false;

            /// <summary>
            /// If true, the effect has finished rising.
            /// </summary>
            public bool IsRiseComplete
            {
                get { return isRiseComplete; }
            }






            /// <summary>
            /// Updates the combat effect.
            /// </summary>
            /// <param name="elapsedSeconds">
            /// The number of seconds elapsed since the last update.
            /// </param>
            public virtual void Update(float elapsedSeconds)
            {
                if (!isRiseComplete)
                {
                    Rise += ((float)risePerSecond * elapsedSeconds);
                    if (Rise > riseMaximum)
                    {
                        Rise = riseMaximum;
                        isRiseComplete = true;
                    }
                    position = new Vector2(
                        OriginalPosition.X, 
                        OriginalPosition.Y - Rise);
                }
            }






            /// <summary>
            /// Draw the combat effect.
            /// </summary>
            /// <param name="spriteBatch">The SpriteBatch used to draw.</param>
            /// <param name="texture">The texture for the effect.</param>
            public virtual void Draw(SpriteBatch spriteBatch, Texture2D texture)
            {
                // check the parameter
                if (spriteBatch == null)
                {
                    return;
                }
                // draw the texture
                if (texture != null)
                {
                    spriteBatch.Draw(texture, position, null, Color.White, 0f,
                        new Vector2(texture.Width / 2, texture.Height / 2), 1f, 
                        SpriteEffects.None, 0.3f * (float)Rise / 200f);
                }
                // draw the text
                if (!String.IsNullOrEmpty(Text))
                {
                    spriteBatch.DrawString(Fonts.DamageFont, text, position,
                        Color.White, 0f, new Vector2(textOrigin.X, textOrigin.Y), 1f,
                        SpriteEffects.None, 0.2f * (float)Rise / 200f);
                }
            }


        }




        /// <summary>
        /// The sprite texture for all damage combat effects.
        /// </summary>
        private Texture2D damageCombatEffectTexture;


        /// <summary>
        /// All current damage combat effects.
        /// </summary>
        private List<CombatEffect> damageCombatEffects = new List<CombatEffect>();


        /// <summary>
        /// Adds a new damage combat effect to the scene.
        /// </summary>
        /// <param name="position">The position that the effect starts at.</param>
        /// <param name="damage">The damage statistics.</param>
        public static void AddNewDamageEffects(Vector2 position, 
            StatisticsValue damage)
        {
            int startingRise = 0;

            CheckSingleton();

            if (damage.HealthPoints != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "HP\n" + damage.HealthPoints.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }

            if (damage.MagicPoints != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MP\n" + damage.MagicPoints.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }

            if (damage.PhysicalOffense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "PO\n" + damage.PhysicalOffense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }

            if (damage.PhysicalDefense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "PD\n" + damage.PhysicalDefense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }

            if (damage.MagicalOffense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MO\n" + damage.MagicalOffense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }

            if (damage.MagicalDefense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MD\n" + damage.MagicalDefense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.damageCombatEffects.Add(combatEffect);
            }
        }






        /// <summary>
        /// The sprite texture for all healing combat effects.
        /// </summary>
        private Texture2D healingCombatEffectTexture;


        /// <summary>
        /// All current healing combat effects.
        /// </summary>
        private List<CombatEffect> healingCombatEffects = new List<CombatEffect>();


        /// <summary>
        /// Adds a new healing combat effect to the scene.
        /// </summary>
        /// <param name="position">The position that the effect starts at.</param>
        /// <param name="damage">The healing statistics.</param>
        public static void AddNewHealingEffects(Vector2 position,
            StatisticsValue healing)
        {
            int startingRise = 0;

            CheckSingleton();

            if (healing.HealthPoints != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "HP\n" + healing.HealthPoints.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }

            if (healing.MagicPoints != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MP\n" + healing.MagicPoints.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }

            if (healing.PhysicalOffense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "PO\n" + healing.PhysicalOffense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }

            if (healing.PhysicalDefense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "PD\n" + healing.PhysicalDefense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }

            if (healing.MagicalOffense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MO\n" + healing.MagicalOffense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }

            if (healing.MagicalDefense != 0)
            {
                CombatEffect combatEffect = new CombatEffect();
                combatEffect.OriginalPosition = position;
                combatEffect.Text = "MD\n" + healing.MagicalDefense.ToString();
                combatEffect.Rise = startingRise;
                startingRise -= 5;
                singleton.healingCombatEffects.Add(combatEffect);
            }
        }




        /// <summary>
        /// Load the graphics data for the combat effect sprites.
        /// </summary>
        private void CreateCombatEffectSprites()
        {
            ContentManager content = Session.ScreenManager.Game.Content;

            damageCombatEffectTexture =
                content.Load<Texture2D>(@"Textures\Combat\DamageIcon");
            healingCombatEffectTexture =
                content.Load<Texture2D>(@"Textures\Combat\HealingIcon");
        }


        /// <summary>
        /// Draw all combat effect sprites.
        /// </summary>
        private void DrawCombatEffects(GameTime gameTime)
        {
            float elapsedSeconds = (float)gameTime.ElapsedGameTime.TotalSeconds;
            SpriteBatch spriteBatch = Session.ScreenManager.SpriteBatch;

            // update all effects
            foreach (CombatEffect combatEffect in damageCombatEffects)
            {
                combatEffect.Update(elapsedSeconds);
            }
            foreach (CombatEffect combatEffect in healingCombatEffects)
            {
                combatEffect.Update(elapsedSeconds);
            }

            // draw the damage effects
            if (damageCombatEffectTexture != null)
            {
                foreach (CombatEffect combatEffect in damageCombatEffects)
                {
                    combatEffect.Draw(spriteBatch, damageCombatEffectTexture);
                }
            }

            // draw the healing effects
            if (healingCombatEffectTexture != null)
            {
                foreach (CombatEffect combatEffect in healingCombatEffects)
                {
                    combatEffect.Draw(spriteBatch, healingCombatEffectTexture);
                }
            }

            // remove all complete effects
            Predicate<CombatEffect> removeCompleteEffects =
                delegate(CombatEffect combatEffect)
                {
                    return combatEffect.IsRiseComplete;
                };
            damageCombatEffects.RemoveAll(removeCompleteEffects);
            healingCombatEffects.RemoveAll(removeCompleteEffects);
        }






        /// <summary>
        /// The animating sprite that draws over the highlighted character.
        /// </summary>
        private AnimatingSprite highlightForegroundSprite = new AnimatingSprite();

        /// <summary>
        /// The animating sprite that draws behind the highlighted character.
        /// </summary>
        private AnimatingSprite highlightBackgroundSprite = new AnimatingSprite();

        /// <summary>
        /// The animating sprite that draws behind the primary target character.
        /// </summary>
        private AnimatingSprite primaryTargetSprite = new AnimatingSprite();

        /// <summary>
        /// The animating sprite that draws behind any secondary target characters.
        /// </summary>
        private AnimatingSprite secondaryTargetSprite = new AnimatingSprite();


        /// <summary>
        /// Create the selection sprite objects.
        /// </summary>
        private void CreateSelectionSprites()
        {
            ContentManager content = Session.ScreenManager.Game.Content;

            Point frameDimensions = new Point(76, 58);
            highlightForegroundSprite.FramesPerRow = 6;
            highlightForegroundSprite.FrameDimensions = frameDimensions;
            highlightForegroundSprite.AddAnimation(
                new Animation("Selection", 1, 4, 100, true));
            highlightForegroundSprite.PlayAnimation(0);
            highlightForegroundSprite.SourceOffset =
                new Vector2(frameDimensions.X / 2f, 40f);
            highlightForegroundSprite.Texture =
                content.Load<Texture2D>(@"Textures\Combat\TilesheetSprangles");

            frameDimensions = new Point(102, 54);
            highlightBackgroundSprite.FramesPerRow = 4;
            highlightBackgroundSprite.FrameDimensions = frameDimensions;
            highlightBackgroundSprite.AddAnimation(
                new Animation("Selection", 1, 4, 100, true));
            highlightBackgroundSprite.PlayAnimation(0);
            highlightBackgroundSprite.SourceOffset =
                new Vector2(frameDimensions.X / 2f, frameDimensions.Y / 2f);
            highlightBackgroundSprite.Texture =
                content.Load<Texture2D>(@"Textures\Combat\CharSelectionRing");

            primaryTargetSprite.FramesPerRow = 4;
            primaryTargetSprite.FrameDimensions = frameDimensions;
            primaryTargetSprite.AddAnimation(
                new Animation("Selection", 1, 4, 100, true));
            primaryTargetSprite.PlayAnimation(0);
            primaryTargetSprite.SourceOffset =
                new Vector2(frameDimensions.X / 2f, frameDimensions.Y / 2f);
            primaryTargetSprite.Texture =
                content.Load<Texture2D>(@"Textures\Combat\Target1SelectionRing");

            secondaryTargetSprite.FramesPerRow = 4;
            secondaryTargetSprite.FrameDimensions = frameDimensions;
            secondaryTargetSprite.AddAnimation(
                new Animation("Selection", 1, 4, 100, true));
            secondaryTargetSprite.PlayAnimation(0);
            secondaryTargetSprite.SourceOffset =
                new Vector2(frameDimensions.X / 2f, frameDimensions.Y / 2f);
            secondaryTargetSprite.Texture =
                content.Load<Texture2D>(@"Textures\Combat\Target2SelectionRing");
        }


        /// <summary>
        /// Draw the highlight sprites.
        /// </summary>
        private void DrawSelectionSprites(GameTime gameTime)
        {
            SpriteBatch spriteBatch = Session.ScreenManager.SpriteBatch;
            Viewport viewport = Session.ScreenManager.GraphicsDevice.Viewport;
            
            // update the animations
            float elapsedSeconds = (float)gameTime.ElapsedGameTime.TotalSeconds;
            highlightForegroundSprite.UpdateAnimation(elapsedSeconds);
            highlightBackgroundSprite.UpdateAnimation(elapsedSeconds);
            primaryTargetSprite.UpdateAnimation(elapsedSeconds);
            secondaryTargetSprite.UpdateAnimation(elapsedSeconds);
            
            // draw the highlighted-player sprite, if any
            if (highlightedCombatant != null)
            {
                highlightBackgroundSprite.Draw(spriteBatch,
                    highlightedCombatant.Position,
                    1f - (highlightedCombatant.Position.Y - 1) / viewport.Height);
                highlightForegroundSprite.Draw(spriteBatch,
                    highlightedCombatant.Position,
                    1f - (highlightedCombatant.Position.Y + 1) / viewport.Height);
            }

            // draw the primary target sprite and name, if any
            if (primaryTargetedCombatant != null)
            {
                primaryTargetSprite.Draw(spriteBatch,
                    primaryTargetedCombatant.Position,
                    1f - (primaryTargetedCombatant.Position.Y - 1) / viewport.Height);
                if (primaryTargetedCombatant.Character is Monster)
                {
                    Fonts.DrawCenteredText(spriteBatch, Fonts.DamageFont,
#if DEBUG
                    primaryTargetedCombatant.Character.Name + "\n" +
                    primaryTargetedCombatant.Statistics.HealthPoints + "/" +
                    primaryTargetedCombatant.Character.CharacterStatistics.HealthPoints,
#else
                        primaryTargetedCombatant.Character.Name,
#endif
                        primaryTargetedCombatant.Position + new Vector2(0f, 42f),
                        Color.White);
                }
            }

            // draw the secondary target sprites on live enemies, if any
            foreach (Combatant combatant in secondaryTargetedCombatants)
            {
                if (combatant.IsDeadOrDying)
                {
                    continue;
                }
                secondaryTargetSprite.Draw(spriteBatch,
                    combatant.Position,
                    1f - (combatant.Position.Y - 1) / viewport.Height);
                if (combatant.Character is Monster)
                {
                    Fonts.DrawCenteredText(spriteBatch, Fonts.DamageFont,
#if DEBUG
                        combatant.Character.Name + "\n" +
                        combatant.Statistics.HealthPoints + "/" +
                        combatant.Character.CharacterStatistics.HealthPoints,
#else
                        combatant.Character.Name,
#endif
                        combatant.Position + new Vector2(0f, 42f), Color.White);
                }
            }
        }






        /// <summary>
        /// Varieties of delays that are interspersed throughout the combat flow.
        /// </summary>
        private enum DelayType
        {
            /// <summary>
            /// No delay at this time.
            /// </summary>
            NoDelay,

            /// <summary>
            /// Delay at the start of combat.
            /// </summary>
            StartCombat,

            /// <summary>
            /// Delay when one side turn's ends before the other side begins.
            /// </summary>
            EndRound,

            /// <summary>
            /// Delay at the end of a character's turn before the next one begins.
            /// </summary>
            EndCharacterTurn,

            /// <summary>
            /// Delay before a flee is attempted.
            /// </summary>
            FleeAttempt,

            /// <summary>
            /// Delay when the party has fled from combat before combat ends.
            /// </summary>
            FleeSuccessful,
        }

        /// <summary>
        /// The current delay, if any (otherwise NoDelay).
        /// </summary>
        private DelayType delayType = DelayType.NoDelay;


        /// <summary>
        /// Returns true if the combat engine is delaying for any reason.
        /// </summary>
        public static bool IsDelaying
        {
            get 
            { 
                return (singleton == null ? false : 
                    singleton.delayType != DelayType.NoDelay); 
            }
        }


        /// <summary>
        /// The duration for all kinds of delays, in milliseconds.
        /// </summary>
        private const int totalDelay = 1000;


        /// <summary>
        /// The duration of the delay so far.
        /// </summary>
        private int currentDelay = 0;


        /// <summary>
        /// Update any delays in the combat system.
        /// </summary>
        /// <remarks>
        /// This function may cause combat to end, setting the singleton to null.
        /// </remarks>
        private void UpdateDelay(int elapsedMilliseconds)
        {
            if (delayType == DelayType.NoDelay)
            {
                return;
            }

            // increment the delay
            currentDelay += elapsedMilliseconds;

            // if the delay is ongoing, then we're done
            if (currentDelay < totalDelay)
            {
                return;
            }
            currentDelay = 0;

            // the delay has ended, so the operation implied by the DelayType happens
            switch (delayType)
            {
                case DelayType.StartCombat:
                    // determine who goes first and start combat
                    int whoseTurn = Session.Random.Next(2);
                    if (whoseTurn == 0)
                    {
                        BeginPlayersTurn();
                    }
                    else
                    {
                        BeginMonstersTurn();
                    }
                    delayType = DelayType.NoDelay;
                    break;

                case DelayType.EndCharacterTurn:
                    if (IsPlayersTurn)
                    {
                        // check to see if the players' turn is complete
                        if (IsPlayersTurnComplete)
                        {
                            delayType = DelayType.EndRound;
                            break;
                        }
                        // find the next player
                        int highlightedIndex = players.FindIndex(
                            delegate(CombatantPlayer player)
                            {
                                return (player == 
                                    highlightedCombatant as CombatantPlayer);
                            });
                        int nextIndex = (highlightedIndex + 1) % players.Count;
                        while (players[nextIndex].IsDeadOrDying ||
                            players[nextIndex].IsTurnTaken)
                        {
                            nextIndex = (nextIndex + 1) % players.Count;
                        }
                        BeginPlayerTurn(players[nextIndex]);
                    }
                    else
                    {
                        // check to see if the monsters' turn is complete
                        if (IsMonstersTurnComplete)
                        {
                            delayType = DelayType.EndRound;
                            break;
                        }
                        // find the next monster
                        BeginMonsterTurn(null);
                    }
                    delayType = DelayType.NoDelay;
                    break;

                case DelayType.EndRound:
                    // check for turn completion
                    if (IsPlayersTurn && IsPlayersTurnComplete)
                    {
                        BeginMonstersTurn();
                    }
                    else if (!IsPlayersTurn && IsMonstersTurnComplete)
                    {
                        BeginPlayersTurn();
                    }
                    delayType = DelayType.NoDelay;
                    break;

                case DelayType.FleeAttempt:
                    if (fleeThreshold <= 0)
                    {
                        delayType = DelayType.EndCharacterTurn;
                        Session.Hud.ActionText = "This Fight Cannot Be Escaped...";
                        if (highlightedCombatant != null)
                        {
                            highlightedCombatant.IsTurnTaken = true;
                        }
                    }
                    else if (CalculateFleeAttempt())
                    {
                        delayType = DelayType.FleeSuccessful;
                        Session.Hud.ActionText = "Your Party Has Fled!";
                    }
                    else
                    {
                        delayType = DelayType.EndCharacterTurn;
                        Session.Hud.ActionText = "Your Party Failed to Escape!";
                        if (highlightedCombatant != null)
                        {
                            highlightedCombatant.IsTurnTaken = true;
                        }
                    }
                    break;

                case DelayType.FleeSuccessful:
                    EndCombat(CombatEndingState.Fled);
                    delayType = DelayType.NoDelay;
                    break;
            }
        }






        /// <summary>
        /// Generates a list of CombatantPlayer objects from the party members.
        /// </summary>
        private static List<CombatantPlayer> GenerateCombatantsFromParty()
        {
            List<CombatantPlayer> generatedPlayers = new List<CombatantPlayer>();

            foreach (Player player in Session.Party.Players)
            {
                if (generatedPlayers.Count <= PlayerPositions.Length)
                {
                    generatedPlayers.Add(new CombatantPlayer(player));
                }
            }

            return generatedPlayers;
        }

        /// <summary>
        /// Start a new combat from the given FixedCombat object.
        /// </summary>
        public static void StartNewCombat(MapEntry<FixedCombat> fixedCombatEntry)
        {
            // check the parameter
            if (fixedCombatEntry == null)
            {
                throw new ArgumentNullException("fixedCombatEntry");
            }
            FixedCombat fixedCombat = fixedCombatEntry.Content;
            if (fixedCombat == null)
            {
                throw new ArgumentException("fixedCombatEntry has no content.");
            }

            // generate the monster combatant list
            List<CombatantMonster> generatedMonsters = new List<CombatantMonster>();
            foreach (ContentEntry<Monster> entry in fixedCombat.Entries)
            {
                for (int i = 0; i < entry.Count; i++)
                {
                    generatedMonsters.Add(
                        new CombatantMonster(entry.Content));
                }
            }

            // randomize the list of monsters
            List<CombatantMonster> randomizedMonsters = new List<CombatantMonster>();
            while ((generatedMonsters.Count > 0) && 
                (randomizedMonsters.Count <= MonsterPositions.Length))
            {
                int index = Session.Random.Next(generatedMonsters.Count);
                randomizedMonsters.Add(generatedMonsters[index]);
                generatedMonsters.RemoveAt(index);
            }

            // start the combat
            StartNewCombat(GenerateCombatantsFromParty(), randomizedMonsters, 0);
            singleton.fixedCombatEntry = fixedCombatEntry;
        }


        /// <summary>
        /// Start a new combat from the given RandomCombat object.
        /// </summary>
        public static void StartNewCombat(RandomCombat randomCombat)
        {
            // check the parameter
            if (randomCombat == null)
            {
                throw new ArgumentNullException("randomCombat");
            }

            // determine how many monsters will be in the combat
            int monsterCount = 
                randomCombat.MonsterCountRange.GenerateValue(Session.Random);

            // determine the total probability
            int totalWeight = 0;
            foreach (WeightedContentEntry<Monster> entry in randomCombat.Entries)
            {
                totalWeight += entry.Weight;
            }

            // generate each monster
            List<CombatantMonster> generatedMonsters = new List<CombatantMonster>();
            for (int i = 0; i < monsterCount; i++)
            {
                int monsterChoice = Session.Random.Next(totalWeight);
                foreach (WeightedContentEntry<Monster> entry in randomCombat.Entries)
                {
                    if (monsterChoice < entry.Weight)
                    {
                        generatedMonsters.Add(
                            new CombatantMonster(entry.Content));
                        break;
                    }
                    else
                    {
                        monsterChoice -= entry.Weight;
                    }
                }
            }

            // randomize the list of monsters
            List<CombatantMonster> randomizedMonsters = new List<CombatantMonster>();
            while ((generatedMonsters.Count > 0) && 
                (randomizedMonsters.Count <= MonsterPositions.Length))
            {
                int index = Session.Random.Next(generatedMonsters.Count);
                randomizedMonsters.Add(generatedMonsters[index]);
                generatedMonsters.RemoveAt(index);
            }

            // start the combat
            StartNewCombat(GenerateCombatantsFromParty(), randomizedMonsters, 
                randomCombat.FleeProbability);
        }


        /// <summary>
        /// Start a new combat between the party and a group of monsters.
        /// </summary>
        /// <param name="players">The player combatants.</param>
        /// <param name="monsters">The monster combatants.</param>
        /// <param name="fleeThreshold">The odds of success when fleeing.</param>
        private static void StartNewCombat(List<CombatantPlayer> players,
            List<CombatantMonster> monsters, int fleeThreshold)
        {
            // check if we are already in combat
            if (singleton != null)
            {
                throw new InvalidOperationException(
                    "There can only be one combat at a time.");
            }

            // create the new CombatEngine object
            singleton = new CombatEngine(players, monsters, fleeThreshold);
        }

        

        /// <summary>
        /// Construct a new CombatEngine object.
        /// </summary>
        /// <param name="players">The player combatants.</param>
        /// <param name="monsters">The monster combatants.</param>
        /// <param name="fleeThreshold">The odds of success when fleeing.</param>
        private CombatEngine(List<CombatantPlayer> players,
            List<CombatantMonster> monsters, int fleeThreshold)
        {
            // check the parameters
            if ((players == null) || (players.Count <= 0) || 
                (players.Count > PlayerPositions.Length))
            {
                throw new ArgumentException("players");
            }
            if ((monsters == null) || (monsters.Count <= 0) || 
                (monsters.Count > MonsterPositions.Length))
            {
                throw new ArgumentException("monsters");
            }

            // assign the parameters
            this.players = players;
            this.monsters = monsters;
            this.fleeThreshold = fleeThreshold;

            // assign positions
            for (int i = 0; i < players.Count; i++)
            {
                if (i >= PlayerPositions.Length)
                {
                    break;
                }
                players[i].Position =
                    players[i].OriginalPosition =
                    PlayerPositions[i];
            }
            for (int i = 0; i < monsters.Count; i++)
            {
                if (i >= MonsterPositions.Length)
                {
                    break;
                }
                monsters[i].Position =
                    monsters[i].OriginalPosition =
                    MonsterPositions[i];
            }

            // sort the monsters by the y coordinates, descending
            monsters.Sort(delegate(CombatantMonster monster1, CombatantMonster monster2)
            {
                return monster2.OriginalPosition.Y.CompareTo(
                    monster1.OriginalPosition.Y);
            });

            // create the selection sprites
            CreateSelectionSprites();

            // create the combat effect sprites
            CreateCombatEffectSprites();

            // start the first combat turn after a delay
            delayType = DelayType.StartCombat;

            // start the combat music
            AudioManager.PushMusic(TileEngine.Map.CombatMusicCueName);
        }






        public static void AttemptFlee()
        {
            CheckSingleton();

            if (!IsPlayersTurn)
            {
                throw new InvalidOperationException("Only the players may flee.");
            }


            singleton.delayType = DelayType.FleeAttempt;
            Session.Hud.ActionText = "Attempting to Escape...";
        }


        /// <summary>
        /// The odds of being able to flee this combat, from 0 to 100.
        /// </summary>
        private int fleeThreshold = 0;


        /// <summary>
        /// Calculate an attempted escape from the combat.
        /// </summary>
        /// <returns>If true, the escape succeeds.</returns>
        private bool CalculateFleeAttempt()
        {
            return (Session.Random.Next(100) < fleeThreshold);
        }






        /// <summary>
        /// End the combat
        /// </summary>
        /// <param name="combatEndState"></param>
        private void EndCombat(CombatEndingState combatEndingState)
        {
            // go back to the non-combat music
            AudioManager.PopMusic();

            switch (combatEndingState)
            {
                case CombatEndingState.Victory:
                    int experienceReward = 0;
                    int goldReward = 0;
                    List<Gear> gearRewards = new List<Gear>();
                    List<string> gearRewardNames = new List<string>();
                    // calculate the rewards from the monsters
                    foreach (CombatantMonster combatantMonster in monsters)
                    {
                        Monster monster = combatantMonster.Monster;
                        Session.Party.AddMonsterKill(monster);
                        experienceReward +=
                            monster.CalculateExperienceReward(Session.Random);
                        goldReward += monster.CalculateGoldReward(Session.Random);
                        gearRewardNames.AddRange(
                            monster.CalculateGearDrop(Session.Random));
                    }
                    foreach (string gearRewardName in gearRewardNames)
                    {
                        gearRewards.Add(Session.ScreenManager.Game.Content.Load<Gear>(
                            Path.Combine(@"Gear", gearRewardName)));
                    }
                    // add the reward screen
                    Session.ScreenManager.AddScreen(new RewardsScreen(
                        RewardsScreen.RewardScreenMode.Combat, experienceReward,
                        goldReward, gearRewards));
                    // remove the fixed combat entry, if this wasn't a random fight
                    if (FixedCombatEntry != null)
                    {
                        Session.RemoveFixedCombat(FixedCombatEntry);
                    }
                    break;

                case CombatEndingState.Loss: // game over
                    ScreenManager screenManager = Session.ScreenManager;
                    // end the session
                    Session.EndSession();
                    // add the game-over screen
                    screenManager.AddScreen(new GameOverScreen());
                    break;

                case CombatEndingState.Fled:
                    break;
            }

            // clear the singleton
            singleton = null;
        }


        /// <summary>
        /// Ensure that there is no combat happening right now.
        /// </summary>
        public static void ClearCombat()
        {
            // clear the singleton
            if (singleton != null)
            {
                singleton = null;
            }
        }






        /// <summary>
        /// Update the combat engine for this frame.
        /// </summary>
        public static void Update(GameTime gameTime)
        {
            // if there is no active combat, then there's nothing to update
            // -- this will be called every frame, so there should be no exception for 
            //    calling this method outside of combat
            if (singleton == null)
            {
                return;
            }

            // update the singleton
            singleton.UpdateCombatEngine(gameTime);
        }


        /// <summary>
        /// Update the combat engine for this frame.
        /// </summary>
        private void UpdateCombatEngine(GameTime gameTime)
        {
            // check for the end of combat
            if (ArePlayersDefeated)
            {
                EndCombat(CombatEndingState.Loss);
                return;
            }
            else if (AreMonstersDefeated)
            {
                EndCombat(CombatEndingState.Victory);
                return;
            }

            // update the target selections
            if ((highlightedCombatant != null) &&
                (highlightedCombatant.CombatAction != null))
            {
                SetTargets(highlightedCombatant.CombatAction.Target,
                    highlightedCombatant.CombatAction.AdjacentTargets);
            }

            // update the delay
            UpdateDelay(gameTime.ElapsedGameTime.Milliseconds);
            // UpdateDelay might cause combat to end due to a successful escape,
            // which will set the singleton to null.
            if (singleton == null)
            {
                return;
            }

            // update the players
            foreach (CombatantPlayer player in players)
            {
                player.Update(gameTime);
            }

            // update the monsters
            foreach (CombatantMonster monster in monsters)
            {
                monster.Update(gameTime);
            }

            // check for completion of the highlighted combatant
            if ((delayType == DelayType.NoDelay) &&
                (highlightedCombatant != null) && highlightedCombatant.IsTurnTaken)
            {
                delayType = DelayType.EndCharacterTurn;
            }

            // handle any player input
            HandleInput();
        }


        /// <summary>
        /// Handle player input that affects the combat engine.
        /// </summary>
        private void HandleInput()
        {
            // only accept input during the players' turn
            // -- exit game, etc. is handled by GameplayScreen
            if (!IsPlayersTurn || IsPlayersTurnComplete || 
                (highlightedCombatant == null))
            {
                return;
            }

#if DEBUG
            // cheat key
            if (InputManager.IsGamePadRightShoulderTriggered() ||
                InputManager.IsKeyTriggered(Keys.W))
            {
                EndCombat(CombatEndingState.Victory);
                return;
            }
#endif
            // handle input while choosing an action
            if (highlightedCombatant.CombatAction != null)
            {
                // skip if its turn is over or the action is already going
                if (highlightedCombatant.IsTurnTaken || 
                    (highlightedCombatant.CombatAction.Stage != 
                     CombatAction.CombatActionStage.NotStarted))
                {
                    return;
                }

                // back out of the action
                if (InputManager.IsActionTriggered(InputManager.Action.Back))
                {
                    highlightedCombatant.CombatAction = null;
                    SetTargets(null, 0);
                    return;
                }

                // start the action
                if (InputManager.IsActionTriggered(InputManager.Action.Ok))
                {
                    highlightedCombatant.CombatAction.Start();
                    return;
                }

                // go to the next target
                if (InputManager.IsActionTriggered(InputManager.Action.TargetUp))
                {
                    // cycle through monsters or party members
                    if (highlightedCombatant.CombatAction.IsOffensive)
                    {
                        // find the index of the current target
                        int newIndex = monsters.FindIndex(
                            delegate(CombatantMonster monster)
                            {
                                return (primaryTargetedCombatant == monster);
                            });
                        // find the next living target
                        do
                        {
                            newIndex = (newIndex + 1) % monsters.Count;
                        }
                        while (monsters[newIndex].IsDeadOrDying);
                        // set the new target
                        highlightedCombatant.CombatAction.Target = monsters[newIndex];
                    }
                    else
                    {
                        // find the index of the current target
                        int newIndex = players.FindIndex(
                            delegate(CombatantPlayer player)
                            {
                                return (primaryTargetedCombatant == player);
                            });
                        // find the next active, living target
                        do
                        {
                            newIndex = (newIndex + 1) % players.Count;
                        }
                        while (players[newIndex].IsDeadOrDying);
                        // set the new target
                        highlightedCombatant.CombatAction.Target = players[newIndex];
                    }
                    return;
                }
                // go to the previous target
                else if (InputManager.IsActionTriggered(InputManager.Action.TargetDown))
                {
                    // cycle through monsters or party members
                    if (highlightedCombatant.CombatAction.IsOffensive)
                    {
                        // find the index of the current target
                        int newIndex = monsters.FindIndex(
                            delegate(CombatantMonster monster)
                            {
                                return (primaryTargetedCombatant == monster);
                            });
                        // find the previous active, living target
                        do
                        {
                            newIndex--;
                            while (newIndex < 0)
                            {
                                newIndex += monsters.Count;
                            }
                        }
                        while (monsters[newIndex].IsDeadOrDying);
                        // set the new target
                        highlightedCombatant.CombatAction.Target = monsters[newIndex];
                    }
                    else
                    {
                        // find the index of the current target
                        int newIndex = players.FindIndex(
                            delegate(CombatantPlayer player)
                            {
                                return (primaryTargetedCombatant == player);
                            });
                        // find the previous living target
                        do
                        {
                            newIndex--;
                            while (newIndex < 0)
                            {
                                newIndex += players.Count;
                            }
                        }
                        while (players[newIndex].IsDeadOrDying);
                        // set the new target
                        highlightedCombatant.CombatAction.Target = players[newIndex];
                    }
                    return;
                }
            }
            else // choosing which character will act
            {
                // move to the previous living character
                if (InputManager.IsActionTriggered(
                    InputManager.Action.ActiveCharacterLeft))
                {
                    int newHighlightedPlayer = highlightedPlayer;
                    do
                    {
                        newHighlightedPlayer--;
                        while (newHighlightedPlayer < 0)
                        {
                            newHighlightedPlayer += players.Count;
                        }
                    }
                    while (players[newHighlightedPlayer].IsDeadOrDying ||
                            players[newHighlightedPlayer].IsTurnTaken);
                    if (newHighlightedPlayer != highlightedPlayer)
                    {
                        highlightedPlayer = newHighlightedPlayer;
                        BeginPlayerTurn(players[highlightedPlayer]);
                    }
                    return;
                }
                // move to the next living character
                else if (InputManager.IsActionTriggered(
                    InputManager.Action.ActiveCharacterRight))
                {
                    int newHighlightedPlayer = highlightedPlayer;
                    do
                    {
                        newHighlightedPlayer = 
                            (newHighlightedPlayer + 1) % players.Count;
                    }
                    while (players[newHighlightedPlayer].IsDeadOrDying ||
                            players[newHighlightedPlayer].IsTurnTaken);
                    if (newHighlightedPlayer != highlightedPlayer)
                    {
                        highlightedPlayer = newHighlightedPlayer;
                        BeginPlayerTurn(players[highlightedPlayer]);
                    }
                    return;
                }
                Session.Hud.UpdateActionsMenu();
            }
        }






        /// <summary>
        /// Draw the combat for this frame.
        /// </summary>
        public static void Draw(GameTime gameTime)
        {
            // if there is no active combat, then there's nothing to draw
            // -- this will be called every frame, so there should be no exception for 
            //    calling this method outside of combat
            if (singleton == null)
            {
                return;
            }

            // update the singleton
            singleton.DrawCombatEngine(gameTime);
        }


        /// <summary>
        /// Draw the combat for this frame.
        /// </summary>
        private void DrawCombatEngine(GameTime gameTime)
        {
            // draw the players
            foreach (CombatantPlayer player in players)
            {
                player.Draw(gameTime);
            }

            // draw the monsters
            foreach (CombatantMonster monster in monsters)
            {
                monster.Draw(gameTime);
            }

            // draw the selection animations
            DrawSelectionSprites(gameTime);

            // draw the combat effects
            DrawCombatEffects(gameTime);
        }


    }
}