Monster to-hit calculation question

[MattB]

Having had my 8th, and best, Dismas killed by Nick (the Mummy) in the current competition, I figured the game was trying to tell me something and decided to retire (Dis)gracefully and return to my Slapper dynasty (no weapons). But it is a bit soul-destroying so I have decided to alternate with a Streaker dynasty who runs around butt-naked (no armour at all).

I reckon that, with AC of practically zero, I need to avoid monsters that hit rarely, but for massive damage. But how does the monster to-hit calculation work? Do they all have the same chance to hit @ at a given AC? There doesn't seem to be anything in monster.txt or monster_base.txt. Or am I being daft, as usual?

[Timo Pietilä]

I reckon that, with AC of practically zero, I need to avoid monsters that hit rarely, but for massive damage. But how does the monster to-hit calculation work? Do they all have the same chance to hit @ at a given AC? There doesn't seem to be anything in monster.txt or monster_base.txt. Or am I being daft, as usual?

I'm guessing monster depth (level) affects that calc. No idea if that is the case though.

[takkaria]

Having had my 8th, and best, Dismas killed by Nick (the Mummy) in the current competition, I figured the game was trying to tell me something and decided to retire (Dis)gracefully and return to my Slapper dynasty (no weapons). But it is a bit soul-destroying so I have decided to alternate with a Streaker dynasty who runs around butt-naked (no armour at all).

I reckon that, with AC of practically zero, I need to avoid monsters that hit rarely, but for massive damage. But how does the monster to-hit calculation work? Do they all have the same chance to hit @ at a given AC? There doesn't seem to be anything in monster.txt or monster_base.txt. Or am I being daft, as usual?

The calculation is kind of gross.

There’s always 12% chance of hitting, 5% chance of missing.

Otherwise:

Chance = power + (monster race native dungeon level*3)
Chance is never lower than 9

Powers can be found in this list:

https://github.com/angband/angband/blob/restruct/src/list-blow-effects.h

X = roll 1d(chance)

If x-1 >= (ac * 2/3), it hits
If x-1 < (ac * 2/3), it fails

[MattB]

The calculation is kind of gross.

Oh, I don't know.
That actually looks dangerously close to elegant!

So, if I've got it right, Morgoth's shatter attack rolls a d360, which gives him a 5 in 6 chance of hitting @ with AC 90?

Well, apart from the '-1' in the calculation. What's that for?

Thanks for the reply, Takkaria.

[takkaria]

Oh, I don't know.
That actually looks dangerously close to elegant!

So, if I've got it right, Morgoth's shatter attack rolls a d360, which gives him a 5 in 6 chance of hitting @ with AC 90?

Well, apart from the '-1' in the calculation. What's that for?

It seems gross to me because it's hard to work out % or fractional chances in your head. Multiplying AC by 2/3 is annoying, too.

The -1 is because in the code, the final calculation is based on randint0(chance), which makes a random number from 0 to (chance-1). We don't have convenient notation for that, but we do have 1dx, so it made sense to write it like that and take the 1 off later.