[3.5.1] Random dice in item properties

[Cold_Heart]

3.5.1 object.txt, lines 23-30 (emphasis mine):

# Some fields accept randomized numbers of the form "10+2d3M4" where
# 10 is the non-variable base, 2d3 is a standard die roll, and
# M4 uses the m_bonus function to generate a number between 0 and
# 4 according to a normal distribution.  All three components are
# optional, and the number of dice is optional and assumed to be 1
# when not specified.  10+2d3M4 has the range 12-20.  [B]10+M4 has the[/B]
# [B]range 10-14[/B].  10+2d3 has the range 12-16.  10+d3 has the range 11-13.
# 2d3, d3, M4, 2d3M4, and d3M4 are all acceptable as well.

And in the same file, lines 1213-1218 contain the following (emphasis mine):

N:194:Speed
G:=:d
I:45:5
W:75:0:2:100000
A:40:75 to 100
L:[B]4+M6[/B]:SPEED

But at the same time, my current run http://angband.oook.cz/ladder-show.php?id=17670 has:

d) a Ring of Speed <+15> {!*!*!*}
     Found lying on the floor in a vault at 3000 feet (level 60).
     
     +15 speed.

So my question is, how is +15 generated from 4+M6?

[Derakon]

So my question is, how is +15 generated from 4+M6?

Rings of speed specifically have a superboost that can send their pvals to theoretically unlimited values. They do the 4+m6 bonus, then flip a coin; if it's heads, they add 1 to the pval and flip again, etc., and stop when they get to tails. So your +15 ring had to pass, at bare minimum, 5 of those checks (1 in 32 odds).

I don't know how the code knows to apply a superboost to the item. It certainly doesn't apply to boots of speed, which are capped at a maximum of +10.

[Nick]

I don't know how the code knows to apply a superboost to the item.

It's a special case - it checks the ring name, and if it's "Speed" does the superboost thing.