Since I'm sure you're all aware of the "Real Donut Shop Problem" (https://math.stackexchange.com/questions/223345/counting-donuts). So I just start.

I have 3 Integers, all three are entered by a user. With them I need to calculate how many possible permutations they are. I already got some code, it works fine for small integers, if they get bigger, my tool runs for literally days/hours?

**Recursive function to calculate possible permutations:** 

    def T(n, k, K):
    if k==0: return n==0
    return sum(T(n-i, k-1, K) for i in xrange(0, K[k-1]+1))

**Explanation:**

 - n = Number of Bottles
 - k = Number of crates, 
 - K = Maximum Number of possible Bottles one crate can fit

K is different for each crate, and doesn't need to be full, it can even be empty.

I'm calculating how many possibilities there are, to fit X given Bottles inside X given Crates, where one crate can fit a maximum of X Bottles.

**Example for better Understanding:**
Lets say, we have:

 - 7 Bottles *(n)*  
 - 2 Crates *(k) -> [k1, k2]* 
 - k1 fits 3 Bottles *(K1)*, k2 fits 5 Bottles *(K2)* *[k1 -> 3, k2 -> 5]*

So they are **2** possibilities to fit the bottles inside the crates.

Another one:

 - 7 Bottles *(n)*
 - 3 Crates *(k) -> [k1, k2, k3]*
 - k1 fits 2 Bottles, K2 fits 3 Bottles, K3 fits 4 Bottles

**6** possibilities

Above code calculates that flawless, but when I try it with like:

**Problem:**

- 30 Bottles (n)
- 20 Crates (k)
- k1 -> 1 Bottle *(K1)*,  k2 -> 2 Bottles *(K2)*, k3 -> 3 Bottles *(K3)*, k4 -> 4 Bottles *(K4)*.. and so on until k20 -> 20 Bottles *(K20)*, im sure you get the idea..

It takes FOREVER, so I'm asking you; 

**Question:**

How could I improve above code/function?