Fury Swipes Mechanics

I bet that you have heard this question a lot before,
What’s the best spell in DotA?

Now of course, this question makes you instantly think that the asker is a noob.
So you can’t really find an answer to that question, but the next question indeed has an answer:

What’s the best DMG spell in DotA?
Now, such spell really exists, and the best DMG spell (orb) is:

Fury Swipes
Roshan dead in 26 hits, level 7. No damage items, at ALL!

But of course you’ll say, “Bah, what’s this? Everybody knows Ursa is a very dangerous hero!”.
That’s true, but this article will show you why it really is such a great hero,
and will also show you the difference between normal attacks and Fury Swipes attacks.

Ursa Warrior attack

Before I start, please do not mix this with Necrolyte’s ultimate for instance.
That’s a powerful spell of course, but we will be discussing strictly about normal hits here..

Now begins the mathematics.
Please note that as we go through this documentation (or call it whatever you want),
I will ignore all armors which decrease your damage.

I’ll first define what average damage means. Consider you’re playing Mirana,
and your damage stats look like 20 – 50 + 73. Then the average damage is computed this way:
(20+50)/2 + 73 = 35 + 73 = 108.

Why is this so? Because your hero has a chance to output damage in the range (20, 50) plus the
bonus damage you receive from items (73 in this case).

The average (which is a measure of the “middle” of some data set) usually gives you the “final”
number after n output of damage, then divided by n. This means that, no matter if your damage is 2 to 4,
in the real long run it will just equal to (2+4)/2 = 3.

Late game Mirana

Now consider you’re in some game, 40th minute, you got all these imba
items which make your damage stats look something like this: 93 – 104 + 130. Computing the
average, it gives us 228.5 damage (but that’s in the long run, remember?).
Okay, 1 hit equals 228.5 damage, 2 hits equal 2 * 228.5 damage, and so on…
We get that n hits equal n * 228.5 damage (linear)

I will not define what a mathematical function means, you’ll have to go through your high
(or even basic) school books to understand this. So we have this function:

f(n) = n * 269, n = positive integer.

This function gives us the total damage after n hits. So consider you hit your enemy
30 times, and we calculate this using our function: f(30) = 30*269 = 8070 total damage.

Now, if you spent enough time in school, you’d instantly notice that this function
is a linear function. And it’s monotone increasing –
meaning that, the more hits we do, the more damage we do which is logical.

But now, enough examples, time to go back to Fury Swipes to see what’s it all about.
So the definition of this spell is pretty clear:
“Each attack deepends the wound in the target, causing each attack
to deal 25 more damage than the previous attack.”

Let a = average dmg, now we have:
I = a + 25 (This is the first attack, average dmg + 25 [Fury Swipes dmg])
II = I + 25 (This is the second attack, first attack + 25 [Fury Swipes dmg])
III = II + 25 (This is the third attack, second attack + 25 [Fury Swipes dmg])
… and so on. The train is long, believe me 🙂

Let’s simplify these equations a bit to see if we get something better
so we can make a function out of it.

I = a + 25
II = I + 25 = (a + 25) + 25
III = II + 25 = (a + 25) + 25 + 25

Now we have all three attacks (separate), but to get the total damage,
we sum all 3 equations and we get:

Total = I + II + III = a + 25 + (a + 25 + 25) + (a + 25 + 25 + 25)

So for three attacks, we have something like:
a + 1*25 + (a + 2*25) + (a + 3*25) = a + a + a + 25(1 + 2 + 3) = 3a + 25(Sum 0 to 3).

This sum (0 to n) given by the general formula n(n+1)/2 which is also learnt in high school.

So we have 3a + 25/2 3(3+1). Consider Ursa’s average dmg is 30,
we get total dmg done: 3 * 30 + 25/2 3*4 = 240 in just three hits! With only 30 dmg!

Now is the time to get into the general formula. As you proceed with expansion of
more and more hits, you will eventually get to this formula:

f(n) = na + 25/2 n(n+1) – where a = average damage, n = number of hits.

The special thing about this formula is that it’s quadratic!, and the others were just linear!
And what do we know about quadratic functions?
That they are much much bigger than the linear ones!

Difference between quadratic (red) and linear (green)

As x rises (our hits), the quadratic function (our total damage) rises faster!

The green graph shows the damage output of a hero with 10 average dmg (10x), the cyan graph shows the same
but for a 80 average dmg hero (80x).. and so on. The red graph is for Ursa’s Fury Swipes (with 59 avg. dmg).

If you have also heard of differentiation, there is a rule:
for f(x) = ax^n we have f'(x) = na*x^(n-1). Differentiation can show us the behaviour of a rise a function.
If you try to differentiate f(x) = ax, you will get f'(x) = a (normal hero).
But if you try to differentiate f(x) = ax^2 you get f'(x) = 2ax (Ursa Fury Swipes).
It is easy to notice that 2ax > a, so clearly x^2 rises faster than linear functions.


Fury Swipes, as we all know, works against Roshan!

But what’s the big deal? How does it work? How many hits do we have to do in order to kill him?

Roshan starts with 7500 hp, and receives +500 hp bonus after every 5 minutes.
So the table looks something like this:

Start: 7500 hp
5th min: 8000 hp
10th min: 8500 hp
15th min: 9000 hp
20th min: 9500 hp
25th min: 10000 hp
30th min: 10500 hp
35th min: 11000 hp
40th min: 11500 hp
45th min: 12000 hp

Now, let’s calculate the average health of Roshan. This will give us a value that is only
between start and 45th min, because after that, Roshan’s HP does not increase,
it stays at 12000 constant.
(7500 + 8000 + 8500 + 9000 + 9500 + 10000 + 10500 + 11000 + 11500 + 12000)/10 = 9750

At level 7, you can max the bonus in Fury Swipes which is 25 damage,
and Ursa has 59 average damage with no damage items at all.
Let’s calculate the damage Ursa does in n hits: f(n) = 59*n + 25/2(n^2 + n) = n(25n + 143)/2

Let’s take a look at some values of this function:
f(22) = 7623 damage. (This is enough to kill Roshan with 7500 hp)
f(23) = 8257 damage. (Enough to kill Roshan with 8000 hp)

f(26) = 10309 damage. Enough to kill Roshan anywhere between start and 45th min.

But we will need more to kill him after 45 minutes have passed, or
f(29) = 12586 damage.
Roshan dead in 29 hits with no items at all at level 7?? Wow! Plus, we didn’t even count Enrage!
(Of course, you’d have to be able to handle the damage he deals)



Now, let’s talk a bit about Enrage. Let’s first see how Enrage increases our damage.


Level 1 is 4%, level 2 is 5%, and level 3 is 6%. Now, let’s make a formula out of this.
From previously, we had f(n) = na + 25/2 n(n+1) – where a = average damage, n = number of hits.

Now to expand this formula to calculate Enrage, we also need one more variable which will be our total health.
Also note that “Enrage doubles the damage dealt by Fury Swipes”. You can read at http://www.playdota.com/heroes/ursa-warrior.

With a little help from Cano from the PlayDotA forums, here is some info about Enrage:
1 damage is added to each attack (by triggers on the attack event), so we have n damage for n attacks;
the bonus damage from Enrage is 6*n*b/100, and then the Fury Swipes damage is dealt. This damage lacks 1 attack
so (n-1)*n*25/2 damage is dealt in n attacks. Now after we sum all this up, we get the next formula:

f(n) = n(a + 6/100b) + 25 n^2 + n – where a = average damage, b = current health, n = number of hits.

Please note that after Enrage effect passes, this formula does not work.
Also, your HP may drop in fights and it’s a bit hard to calculate the correct damage output. There are several
variables that come into play, such as: the damage that the enemy does to you (i.e. the rate of drop of your HP),
the time when you activated Enrage, etc. But I will not trouble you about this, since we already know that Ursa
deals a lot of damage with Fury Swipes alone.


It’s Armor time!

From Blizzard’s official webpage:

For positive Armor, damage reduction =((armor)*0.06)/(1+0.06*(armor))
For negative Armor, it is damage increase = 2-0.94^(-armor) since you take more damage for negative armor scores.

Once you calculate these numbers, just multiply the damage reduction/increase by f(n).
That will give you the damage reduction/increase. Let p = damage reduction/increase. We have 2 cases:

1) If it’s reduction, you have total damage = f(n) – f(n)*p
2) If it’s increase, you have total damage = f(n) + f(n)*p

If you have read the whole tutorial, I thank you for your time! I had much fun writing this,
so I hope that you enjoyed the reading as I did enjoy the writing. Good luck!


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