SetMapper.cs

//Copyright 2016 Joe Cooper
//Find the most up to date version on Gist: https://gist.github.com/JoeCooper/032b1337b13bd9fb0eb8c0cae35c5d67

using System;
using System.Collections.Generic;

namespace NobleMuffins
{
    public class SetMapper<TSubject, TRepresentation>: SetMapper<TSubject, TSubject, TRepresentation>
    {
        public SetMapper(Func<TSubject, TRepresentation> createFunction, Action<TRepresentation> destroyFunction): base((o) => o, createFunction, destroyFunction)
        {
        }
    }

    public class SetMapper<TSubject, TKey, TRepresentation>
    {
        readonly Func<TSubject, TKey> keyFunction;
        readonly Func<TSubject, TRepresentation> createFunction;
        readonly Action<TRepresentation> destroyFunction;

        readonly IDictionary<TKey, TRepresentation> extantSet;

        readonly HashSet<TKey> keysToDropBuffer;
        
        public SetMapper(Func<TSubject, TKey> keyFunction, Func<TSubject, TRepresentation> createFunction, Action<TRepresentation> destroyFunction)
        {
            this.keyFunction = keyFunction;
            this.createFunction = createFunction;
            this.destroyFunction = destroyFunction;
            this.extantSet = new Dictionary<TKey, TRepresentation>();
            this.keysToDropBuffer = new HashSet<TKey>();
        }

        public bool Map(IEnumerable<TSubject> subjects)
        {
            var changed = false;
            keysToDropBuffer.Clear();
            foreach (var kvp in extantSet)
            {
                keysToDropBuffer.Add(kvp.Key);
            }
            if (subjects != null)
            {
                foreach (var subject in subjects)
                {
                    var key = keyFunction(subject);
                    keysToDropBuffer.Remove(key);
                    if (extantSet.ContainsKey(key) == false)
                    {
                        var representation = createFunction(subject);
                        extantSet[key] = representation;
                        changed = true;
                    }
                }
            }
            foreach (var key in keysToDropBuffer)
            {
                var representation = extantSet[key];
                destroyFunction(representation);
                extantSet.Remove(key);
            }
            changed |= keysToDropBuffer.Count > 0;
            return changed;
        }

        public TRepresentation this[TKey key]
        {
            get
            {
                return extantSet[key];
            }
        }
    }
}