UnityEffectQueue.cs

[System.Serializable]
public class Effect
{
    public string name;
    public float multiplier = 1f;
    public float additional = 0f;
    public float expiration = 0f;
    public bool stackable = false;

    public GameObject addPrefab = null;
    public GameObject removePrefab = null;

    public Effect():
        this(null, 1f, 0f, 0f)
        { ; }
    public Effect(Effect original):
        this(original.name, original.multiplier,
            original.additional, original.expiration,
            original.stackable,
            original.addPrefab, original.removePrefab)
        { ; }
    public Effect(string name,
        float factor=1f, float bonus=0f,
        float expire=0f, bool stacks=false,
        GameObject addPrefab=null, GameObject removePrefab=null)
    {
        if (expire == 0f)
            expire = float.PositiveInfinity;
        this.name = name;
        this.multiplier = factor;
        this.additional = bonus;
        this.expiration = expire;
        this.stackable = stacks;
        this.addPrefab = addPrefab;
        this.removePrefab = removePrefab;
    }
}

// queue of effects ordered by their expiration
//
public SimplePriorityQueue<Effect,float> effects =
    new SimplePriorityQueue<Effect,float>();

// base value before any effects applied
//
public float baseValue = 0f;

public float GetEffectiveValue()
{
    float value = baseValue;

    // All factors first, all bonuses afterwards.
    // Priority queue iterates in undefined order.
    //
    if (effects != null)
    {
        // remove expired effects if any
        ExpireEffects();
        foreach (Effect effect in effects)
            if (effect.name.Equals(name))
                value *= effect.multiplier;
        foreach (Effect effect in effects)
            if (effect.name.Equals(name))
                value += effect.additional;
    }

    // Within the limits.
    value = ClampToLegalRange(value);

    return value;
}