## Tuesday 4 April 2017

### Two game mechanics

Great game follows from small details and smart mechanics.

As always, game mechanics are great source of giving interesting mathematical questions. Here are another two short examples on games that I've been following.

Fire Emblem Heroes

As a hardcore Nintendo fans this is probably a must, especially when the game is now full of GBA-era characters replenished. Without much money and effort I managed to score 4200~4400 per week in the arena, but this is not what I want to talk today. It is the summoning mechanism that caught my eye --- to be honest the summoning mechanism is quite special as a Japanese game because that can be treated as an attempt to adopt western consumption behavior on gaming, rather than the eastern elitism style, but such qualitative topic does not fit this entry as well.

Let's talk about simple probability. This is the (simplified version of) FEH's summoning mechanism:

- You spend 20 orbs to summon 5 heroes/cards (a complete summoning).
- By default, there will be a 6% chance of getting a 5-star (5*) hero.
- If you get no 5* heroes after a complete summoning, the chance for 5* is raised by 0.5%.
- The chance restores to 6% if a 5* hero is summoned.
- The chance only change after a complete summoning.
- Assume everything else to be independent.

Now the interesting special rule: if you get no 5* in 24 complete summonings, you will automatically get 5 5* heroes in the 25th summoning (i.e. the chance become 100% rather than 18%). The question is, why 24? A simple calculation reveals the truth:

Chance of getting 5 5*s under default chance = 0.06^5 = 0.0000007776 = 7.77*10^-7
Chance of getting 5 5*s via the special way = product (i=0..23) (1-(6+i/2)/100)^5 = 2.79*10^-7

That says, both ways getting 5 5*s in a single summoning process are of equal magnitude in terms of probability! It is quite clear that the chance varies exponentially if we change 24 to something else, so that is quite a slick design that gives mathematician a smile on his face.

A round robin league

I am recently sticking to another Japanese baseball android game, in which has an event where each team is assigned into a group of 5 every day. At the end of the day the rank is calculated and points are given to the team accordingly. At the end of the event (10 days, i.e. 10 rounds) the overall team rank is decided by the total points.

That reminds me of the guild battles from Fantasica -- the old good days where a team of 9 players around the world commit in 35 real time battles, each lasting 1 hr in 7 days that takes numerous hours of preparation work and probably thousands of cash -- that has a very similar system. Each round you fight with another guild and the total victory count determines your rank. The common feature between the two is that, your next opponent depends on your current standing and will not be someone that you have fought before.

Depending on your goals, it has been practically shown that in Fantasica there is a range of standing (typically the mid-high range) that can be achieved by lying dead in the first few battles despite random-ish weak opponents, because you can meet fewer top teams (hence less guaranteed defeat) by doing so.

The core principle in guild battle events -- rise fast, sink slow. You quick edge towards the top by defeating consecutive weak opponents then get crushed by top opponents, but your rank drop slowly because everyone of the same tier is taking defeats at the same time as well.

The question here is, does it work for such system where you are assigned into groups of 5 instead of 2? It is foreseeable that if you always group all top teams together you will be running out of top teams very quickly. Another question is if we alter the matching algorithm will it be cheat-free? At the end of the day, we want to distinguish those heavy spenders (top 5-10), light spenders (top tier) and heavy free players (mid-top) while minimal players are not too important to game revenue. With 35 turns, Fanta somehow (but not quite) managed to put mid-top players to where they should have been, but the sinking phase is still a bit too short.

Here the dev solved this problem by matching 5 teams, each with some distance on the rank but not too far away, ranging from top teams to mid table teams. Each round can be deemed as battling with 4 other teams at the same time: finishing second in the day implies you won against the 3 lower teams and lost against the top team. By such the converging can be done even quicker than the Fanta system. It's also cheat-free in the sense that you are battling with teams of the same tier every round that you don't have much space to lose.

But that's only my prediction -- no one knows how the system exactly works and I doubt if this is systematically studied in this game given its competitiveness, but I look forward very much to those post-event statistics.

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3 April, 2017