### Author

#### sleepyone

• Newbie
• Posts: 3
• Reputation Power: 1
• New to Elements
##### Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166478#msg1166478
« on: November 16, 2014, 10:36:21 pm »
Note: all this is contingent on the correctness of Xenocidius's simulator.

I'm a bit surprised this hasn't come up, since it's actually not that difficult a problem to solve directly. Why bother? One, it's much faster than running a high number of trials, and two, it has zero variance. Essentially, what the simulator does is select 4 cards from the deck, rejecting pillars in the first eight iterations, then spin using that smaller pool.

Let's make a first approximation by ignoring the chance of getting pillars altogether. To make the math that follows more concise, I'm going to define the following:
P(N, p, k) = (N choose k) * pk * (1-p)N-k = Prob(X=k) for X ~ Binom(N,p)

The notation that follows could use some work, but meh.

The probability of a given card appearing k times in the 4 slots is P(4, n/N, k), for n = number of given card in deck, N = total cards in deck.

If said card shows up k times in the pool, then we have a probability (k/4)3 of getting that card on any given spin (since the card needs to come up exactly 3 times). It should be intuitive that the total probability of getting a card from a given spin is the sum from k=1 to 4 (technically 0 to 4, but that first term is 0) of the product of these to terms, i.e.
A := \sumk=04 P(4, n/N, k) * (k/4)3

Repeat for each card in the deck.

Simply applying this formula to the first deck in the AI3 Droprate Tests thread in the Farming Studies / Statistics subforum [nofollow], we get the following results:

 Card Expected Simulator results Lightning 0.0432 0.042 Parallel Universe 0.0236 0.023 Immortal 0.0236 0.023 Dimensional Shield 0.0997 0.097 Lobotomizer 0.0236 0.024 Phase Dragon 0.0432 0.043 Phase Spider 0.0685 0.067 Mindgate 0.0685 0.067 Silence 0.0236 0.023
As a first approximation, this is very good, although you'll notice that it's a bit optimistic overall, since I've assumed that pillars won't appear in the pool.

So what happens if we add back p pillars? This part gets a bit more complicated, although we can still explicitly solve it without even resorting to more complicated mathematical constructs like Markov chains.

Let's start by considering 4 additional events E5 through E8, corresponding to 5-8 pillars out of first 8 cards chosen.

Event Ei simply appears with probability P(8, p/(N+p), i).

Given event Ei, the probability that a given non-pillar card appears k times looks like:
\sumj=0k P(8-i, n/N, j) * P(i-4, n/(N+p), k-j)
(win j copies of the card in the first 8, then k-j copies in the remaining trials)

Similarly, the probability that a given pillar card appears k times is:
P(i-4, n/(N+p), k)

Let's assemble all these parts now. The probability of winning a card that appears n times in a deck with N non-pillars and p pillars is
(1 - \sumi=58 P(8, p/(N+p))) * A + \sumi=58 P(8, p/(N+p), i) \sumk=04  (k/4)3 \sumj=0kP(8-i, n/N, j) * P(i-4, n/(N+P), k-j)
if the card is not a pillar, and
\sumi=58 \sumk=04 P(8, p/(N+p), i) * P(i-4, n/(N+P), k)
if it is.

This final formula (as evaluated by the below javascript code) yields the following results on that same ai3 deck:

 Card Expected Simulator results Aether Pillar 0.0038 0.004 Lightning 0.0423 0.042 Parallel Universe 0.0232 0.023 Immortal 0.0232 0.023 Dimensional Shield 0.0973 0.097 Lobotomizer 0.0232 0.024 Phase Dragon 0.0423 0.043 Phase Spider 0.0669 0.067 Mindgate 0.0669 0.067 Silence 0.0223 0.023
Hope someone finds this at least entertaining.

Code: [Select]
function choose(n,k) {    var ret = 1;    for (var i = 0; i < k; i++) {        ret = ret * (n - i) / (k - i);    }    return ret;}function prob(N, p, k) {    // binomial pdf with parameters N, p    return choose(N,k) * Math.pow(p, k) * Math.pow(1 - p, N-k);}function pillarless_yields(n, N) {    var sum = 0;    for (var i = 0; i <= 4; i++) {        sum += Math.pow(i/4, 3) * prob(4, n/N, i);    }    return sum;}function yields(n, N, P, pillar) {    // pillar is true if this item is a pillar.    var psum = 0;    var sum = 0;    for (var i = 5; i <= 8; i++) {        var p = prob(8, P/(N+P), i);        psum += p;        for (var k = 0; k < 4; k++) {            if (!pillar) {                for (var j = 0; j <= k; j++) {                    sum += p * Math.pow(k/4, 3) * prob(8-i, n/N, j) * prob(i-4, n/(N+P), k-j);                }            } else {                sum += p * Math.pow(k/4, 3) * prob(i-4, n/(N+P), k);            }        }    }    if (!pillar) {        sum += pillarless_yields(n, N) * (1-psum);    }    return sum;}// These are the distribution of cards in the ai3 aether deck.var arr = [3,2,2,5,2,3,4,4,2];console.log(arr.map(function (x) {return 3*yields(x, 27, 15, false);}).join('\n'));;console.log(3*yields(15, 27, 15, true));
« Last Edit: November 18, 2014, 01:47:19 am by sleepyone »

#### 10 men

• Sr. Member
• Posts: 987
• Country:
• Reputation Power: 0
• Honesty is the privilege of the infallible
• Awards:
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166483#msg1166483
« Reply #1 on: November 16, 2014, 11:31:39 pm »
You are absolutely right that Xeno's Simulator is kinda inelegant for determining all the droprates. Back when we investigated drop rates in the wake of the FG farming study, we used the exact numbers as well.
« Last Edit: November 16, 2014, 11:39:58 pm by 10 men »
"My big fear is that one day I may become so vain that I will quote myself in my own signature."  ---  10 men

#### sleepyone

• Newbie
• Posts: 3
• Reputation Power: 1
• New to Elements
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166491#msg1166491
« Reply #2 on: November 17, 2014, 12:33:15 am »
Interesting, I guess I completely missed that thread. I just played a game in the trainer and confirmed in the actual game that more than four unique cards can come up in the same set of spins. If I had to guess, this means that either the spins are independent but the model is correct (best-case scenario), or the model is simply incorrect (e.g. pool is larger than four cards).

edit: if the model still needs confirming, obviously the bonus electrum (first two slots are the same) is more frequent than bonus card, which yields a lower relative variance (so fewer trials needed to be more certain). The problem is actually gathering the data in the first case, I suppose.
« Last Edit: November 17, 2014, 12:45:06 am by sleepyone »

#### 10 men

• Sr. Member
• Posts: 987
• Country:
• Reputation Power: 0
• Honesty is the privilege of the infallible
• Awards:
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166492#msg1166492
« Reply #3 on: November 17, 2014, 12:44:03 am »
If I had to guess, this means that either the spins are independent but the model is correct

Yes, that is the case. For each spin the algorithm is performed seperately. Fwiw the algorithm was pulled directly from the game code by Dark Weaver. Of course it's possible that the algorithm changed in the last 3 years, but I'm pretty sure it didn't - there weren't many updates since then and I also didn't notice any differences in spin behavior.
"My big fear is that one day I may become so vain that I will quote myself in my own signature."  ---  10 men

#### CuCN

• Hero Member
• Posts: 1756
• Reputation Power: 25
• Toxic
• Awards:
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166497#msg1166497
« Reply #4 on: November 17, 2014, 01:42:39 am »
Overall this looks correct.

Your javascript code does this correctly, but there's a mistake in the formula posted here. The fourth sum, across j, is missing.
Let's assemble all these parts now. The probability of winning a card that appears n times in a deck with N non-pillars and p pillars is
(1 - \sumi=58 P(8, p/(N+p))) * A + \sumi=58 P(8, p/(N+p), i) \sumk=04 (k/4)3 * P(8-i, n/N, k) * P(i-4, n/(N+P), k)
if the card is not a pillar

#### sleepyone

• Newbie
• Posts: 3
• Reputation Power: 1
• New to Elements
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1166634#msg1166634
« Reply #5 on: November 18, 2014, 01:47:38 am »
Good catch, I guess I just got a bit overzealous with the copy-pasting. Fixed in the original post.

#### CuCN

• Hero Member
• Posts: 1756
• Reputation Power: 25
• Toxic
• Awards:
##### Re: Spin Chance Analysishttps://elementscommunity.org/forum/index.php?topic=56657.msg1167262#msg1167262
« Reply #6 on: November 21, 2014, 02:15:35 am »
I've found a minor mistake in your program that affects the numbers by about 0.0001. In the "yields" function, the loop for k should have "k <= 4", not "k < 4".

The summations can be reduced to polynomials in two variables.
Let x=n/(N+p) and let y=p/(N+p). Then the probability of winning a card is
(1/16)(x(1+y+y2+y3+y4+y5-13y6+15y7-5y8)+x2(9+18y+27y2+36y3+45y4+54y5-189y6+72y7+15y8)+x3(6+18y+36y2+60y3+90y4+126y5-84y6-36y7-6y8))
if the card is not a pillar, and otherwise:
(1/16)(x(14y5-28y6+20y7-5y8)+x2(42y6-48y7+15y8)+x3(12y7-6y8))

The polynomials in y only need to be calculated once for each deck, so it comes down to a cubic equation in x.

For example, when y=15/42 (as for the ai3 deck with 15 pillars in 42 cards), the probability of a card that isn't a pillar is
(1/23612624896)(2265548601x+31321258539x2+32094593346x3)
and for a pillar
(1/23612624896)(54221875x+81984375x2+10781250x3)

giving the results:

 Card Copies Calculated (x3) Aether Pillar 15 0.00385132 Lightning 3 0.042349 Parallel Universe 2 0.0231705 Immortal 2 0.0231705 Dimensional Shield 5 0.0975435 Lobotomizer 2 0.0231705 Phase Dragon 3 0.042349 Phase Spider 4 0.0670299 Mindgate 4 0.0670299 Silence 2 0.0231705
« Last Edit: November 21, 2014, 04:15:00 am by CuCN »

blarg: