I've been looking over the source code for some of our simulators and checking to see how much the agree on how our mechanics work. In general, the mechanics they use can differ in a lot of small ways from each other, and probably from the game itself.

Thus I'd like to start a discussion on our mechanics, focusing on (but not limited to) the unique things that DKs have. My aim here is to eventually develop a reference that people can use to improve the accuracy of their modelling of DKs. Once we have some agreement on how on a particular mechanic works I'll update this first post with the details.

Current questions of interest

4) How do diseases determine their damage? In particular, how do they interact with the GoD and Pestilence,
I assume that after the most recent GoD change they work like this. The damage multiplier is set when you apply the diseases (and the GoD effect doesn't actually reapply them to the main target). The tick base damage is calculated when you apply or refresh the diseases. The crit rate is set on application (for 4 pc T9). Is wandering plague calculated using your current crit rate or the disease's crit rate? Is there still a 1 second cooldown on wandering plague?

5) How does the Rune grace period work? Current working theory is if you use a rune within a short period of it coming available (after the "10" second refresh time) then it will consider that the next refresh time starts from when the rune was first available rather than when it was used. This grace period you have to use the rune is usually considered to be 2 seconds. Is that the correct duration? Does it change in unholy presence or with the imp unholy presence talent?

6) How do death runes work? In particular is it still as the theorycraft thinktank article suggest? Death Knight: PvE DPS

7) How do attacks choose which runes to use? Attacks use there actual matching runes before using Death Runes. But if two runes of the same type are available how do they determine which one to use? Do they use one within the grace period if it's available, the longest available rune or is it simple random?

8) How does blood tap work? We know that it activates a blood rune, and also puts up a buff such that one of the blood runes is considered a death rune for 20 seconds. They don't have to be the same rune however. And it doesn't always choose the best rune. How does cancelling the buff interact with Reaping/BotN.


Working Knowledge

Stat Multipliers
While base stats and ratings all stack additively, stat boosting talents and buffs usually stack multiplicatively. This rule applies to stats like strength and stamina, derived values like haste and AP, and even damage multipliers for abilities.
To calculate your haste convert your haste rating to a haste value then multiply by the buffs.
The shadow damage part of Scourge Strike is one exception to this rule where the multipliers appear to stack additively.

Examples:
Let's assume i'm a Frost Specced DK with Improved Icy Talons, 200 Haste rating and 2000 base strength. I get 1% melee haste per 25.21 rating, so I have 7.93% haste from that. Assuming I've got frost fever up then I get 20% haste from Icy Talons and 5% from Imp Icy Talons which all stack multiplicatively to give me 1.0793 * 1.2 * 1.05 = 1.36 times quicker attacks or the equivalent of 36% haste from a single source.
If I have the talent Ravanous Dead, then I get 3% more strength so will actually have 2060 unbuffed.
Let's say my Fallen Crusader runeforge procs for 15% more strength, and I hit unbreakable armor for another 10% (soon to be 20%) strength. Then I will have a total of 2000*1.03*1.15*1.1 = 2605.9 strength. which rounds up for display purposes.
Finally, let my offhand have a Razorice runeforge which is fully stacked on the target (+10% frost damage). If I have the Tundra Stalker (+15% damage against targets with frost fever) and Black Ice (+10% frost/shadow damage) and am in Blood Presence (+15% damage). Then my Howling Blast will have an effective damage multiplier of 1.1*1.15*1.1*1.15 = 1.60 or do 60% more damage than the base damage.

Ability misses
If a frost strike fails to hit, then you get back 90% of the runic power cost. If a death coil misses you do not.
If a rune using ability fails to hit you get the rune back. (Still need to check whether the grace period changes)

Runeforges
Runeforges with a PPM appear to have the same PPM when used in the offhand. Using the same Runeforge on both weapons while dual weilding still only applies a single buff.
Cinderglacier is a 1.5 PPM enchant. It puts up 30 second buff with the same name and 2 charges. Blood Boil, Icy Touch, Death Coil and any Scourge Strike will consume a charge of the buff. Death and Decay, diseases and appropriate weapon procs will be buffed by the damage but not consume a charge. Icy Touch using the last charge will still buff the ensuing frost fever.
Razorice is either a high PPM enchant or has a flat rate. In 3.3.3 it will be applied by every attack. It has 2 effects, one called Razor Frost that does 2% of your mainhand weapon damage as frost damage (around 10 damage), the other provides a stacking buff called Frost Vulnerability that buffs your frost damage by up to 10% for 20 seconds. In 3.3.3 it will stack 5 times at 2% which currently it's 10 times at 1%.
Fallen Crusader is a 2.0 PPM enchant. The buff is called Unholy Strength buffs your strength by 15% and lasts 15 seconds.

Regarding the haste from Icy Talons, Blizzard has implied that in 3.3.3 Icy Talons will stack additively with Windfury.

To my knowledge you are correct though, that haste buffs do indeed stack multiplicatively with each other (Slice and Dice/Blade Flurry), while haste rating is additive with its self then multiplicative also with the % based buffs. Has anyone tested this on the PTR to verify if blizzard is changing haste buffs to stack additively?

Yeah I'm pretty sure this is still the case. There isn't any reason for Blizzard. However if someone can easily test it, it's probably still worth doing so.

At least 2 other cases are also important however.

a) Damage percentage buffs, these should still be multiplicative.
b) Strength boosts. Are all the strength talents additive and all the different buffs multiplicative? That's how I would assume it to be, however the strength talents could also be multiplicative.

Some other nagging questions that I would like to know the answer to:

3) Does the SS magical portion eat Cinderglacier charges? I assume so, but as far as I know no-one has tested this.

4) How do diseases determine their damage? In particular, how do they interact with the GoD and Pestilence, and also Wandering Plague.
I assume that after the most recent GoD change they work like this. The damage multiplier is set when you apply the diseases (and the GoD effect doesn't actually reapply them to the main target). The tick base damage is calculated when you apply or refresh the diseases. The crit rate is set on application (for 4 pc T9). Is wandering plague calculated using your current crit rate or the disease's crit rate? Is there still a 1 second cooldown on wandering plague?

This is absolutely true, I tested it on live recently. Also even if you are Scourge Striking with no diseases (No shadow damage component) it will STILL eat a Cinderglacier charge. It is also the case that the Bryntroll weapon proc gets the increased damage but does NOT consume a charge.

From my experience I think the strength and ap buffs and such are calculated to a flat number at the time of the proc. Lets say you have 2000 strength and FC procs your have your then buffed by 300 strength, during its duration DBW procs and you gain 600 strength, i think you end up with 2800 str instead of 2600*1.15=2990. I think thats at least the case for some seconds, until its updated, if it even is. anyone confirm this?

Yeah I tested this too. Didn't know about the procs though. I also tested applying frost fever, and it appears that even if the icy touch uses up the second charge then the frost fever still gets the damage buff.

I also did some Razor-ice testing, and confirmed the current implementation of a stacking 20 second duration debuff with a high proc rate (5PPM apparently) that does some small amount of MH weapon damage (just the weapon no AP or anything, and 2% seemed to fit) as frost.

I believe they were changing it to a guaranteed proc, and 5 stacks (still for a 10% buff), and now a 30 second duration. Which is definitely a small buff. However I only see it driving DW builds towards the rogue weapon switching situation. The razor-ice strike needs to do 15-20 times as much damage in order for using a weapon with Razor-ice over switching every 30 seconds or so to one with razor-ice to keep the debuff up and using Fallen Crusader the rest of the time for a higher uptime on that.

This was touched upon in the Frost thread, but basically, weapon switching will never be viable for Frost since you would have no free GCDs to spare for repeated weapon swaps. Using a GCD for slightly higher FC uptime in lieu of a Frost Strike will never be a dps increase.

This could in my opinion become a very informative thread (in form of a mechnics compilation) even without any thought about simulators but as a reference for old and new DKs seeking compact information about our mechanics. Larisroth, I would very much appreciate it, if you could edit the opening post to include current and future questions/answers and other findings. I know the mages had such a thread back in TBC when I was playing one and I loved it.

Edit: I see the TTT article touches a bit on this but not in depth and is heavily outdated.

Edit2: I reread the first post and noticed that was kind of already your plan (or I misunderstood something this time. I'm sorry, english is not my first language.). In this case ignore my post.

I have a mechanics question : Say I have 0 haste ... and in 3.3.3 I have IIT and the proc'd IT haste effects... is it 45% of my haste from gear or just a 45% faster strikes?

I tested using a macro, and if you swap weapons after a GCD it will use the same GCD. Also an add-on can swap weapons for you in a clumsy manner using item sets. That said, because it resets the swing timer on both weapons it might not be that much of an advantage. For most builds GCDs are at a premium so it makes sense to swap when the gcd is triggered, thus you'd lose (doing it naively) half a swing every swap.

That said the dps advantage of GoD rolling maxed multiplier diseases for a number of builds kind of means that swapping to CG is useful on at least a few fights.

As for haste, different sources of haste stack multiplicatively although your haste rating stacks additively and your talents might also stack additively. (see Haste - WoWWiki - Your guide to the World of Warcraft) for details. So in the case of 0 haste rating, Your sources of haste are 20% from wind fury / IIT buff, 20% from the icy talons buff and 5% from the IIT talent. They should all stack multiplicatively to give you 1.2 * 1.2 * 1.05 = 1.512 as many attacks as normal.

Haste rating and some talents also affect spell casting, and more importantly for some DKs the spell GCD. The spell description for IIT says it personally affects melee and ranged haste for some reason, although we have no use for ranged haste.

It seems that when an ability which costs runes is used, that it will always use the default rune before death runes, moreover the game is also coded to choose the left most rune of the required type in the standard Blizzard Rune User Interface.

I've done some limited testing on Blood Tap and these are what I have discovered (based on an "If type of runes available/unavailable-Then Blood Tap Used" scenario):

Two Blood Runes available or Two Blood runes unavailable, it will convert one of them to a Death Rune and make it available.

One Blood Rune available and One Blood Rune unavailable, it will convert the available rune to a Death Rune and leave the other unavailable.

Two Death Runes and one or both Death Runes are unavailable, it will make one Death Rune available.

One Blood Rune and one Death Rune and one or both are available, it will make the unavailable rune available and convert the Blood Rune to a Death Rune.

One Blood Rune and one Death rune and both are unavailable, it will make the Blood Rune available and convert it to a Death Rune.