TomeNET

(5.17a) Digging - exact calculation                                             
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Your digging power is calculated as follows:

For actual digging tools
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An equipped item that provides (+X) to digging will add 20*X.
If you have a digging tool equipped, its weight is added (eg 25.0 lb -> +25).
If the digging tool is enchanted to hit or to damage, those values are added.

The power value so far is now multiplied by a value between 1 and 2, depending
on your 'Digging' skill (see (7.2)). If the skill is 0, the multiplier is 1, if
it's 50, the multiplier is 2. Otherwise the value is linearly inbetween.
After that, a bonus is added depending on your strength:
Ranging from +0 at 3 over +10 at 18/10, +50 at 18/110 up to +100 at 18/***.

If you don't have a digging tool equipped, your total digging power is halved.

If your resulting final digging power is zero, then it is set to 1, so you
always have at least a tiny chance of digging through the weaker materials
that have a +0 difficulty (see table below).

For weapons
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The calculation is simpler than for diggin tools, as it doesn't take your
digging skill into account.
Swords:
    vs wood:    30 + weight/2 + (tohit+todam)/3
    vs fibre:   60 + weight/4 + (tohit+todam)/2
Axes:
    vs wood:    60 + weight   + (tohit+todam)/2
    vs fibre:   30 + weight/2 + (tohit+todam)/3
Scythes and sickles:
    vs fibre:   60 + weight   + (tohit+todam)/2

'Wood' encompasses trees, dead trees and bushes.
'Fibre' encompasses bushes too, ivy, and actually spider webs.
Against other types of 'terrain', weapons have no digging power at all.

Now a bonus is added to the fibre/wood power, depending on your strength:
Ranging from +0 at 3 over +9 at 18/10, +19 at 18/110 up to +30 at 18/***.
(Internals: For this bonus we actually borrow the 'CON-MHP' bonus category,
which we adjust to our purpose by subtracting 128 and re-adding 5 (net -123),
so it's a different bonus scale than the 'STR-DIG' one used for digging tools.)

Now if the weapon is a 1.5-handed weapon but it's not wielded with both hands
because you also have a shield equipped (aka awkward_wield encumberment) the
resulting fibre/wood digging powers are halved.


Now we continue for both, weapons and real digging tools:

To successfully dig through certain features, your digging power must be
greater than the result of a random number roll "random(0..x-1) + y":
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Spider web           100 +  0
Rubble/Ivy           200 +  0
Bush                 300 +  0
Dead tree            300 +  0
Tree                 400 +  0
Sandwall             300 +  0
Magma intrusion      400 + 10
Quartz vein          800 + 20
Granite wall        1600 + 40
Tunneling any other wall types, such as ice walls, and also unknown secret
doors (as long as they're not embedded in permanent wall types) has the same
difficulty as granite walls.

The +y is a fixed value that gets added to the random roll of (0..x-1), the
sum of this is what you have to overcome with your digging power value.
For example you cannot successfully tunnel through quartz if you don't
have at least 21 digging power, since you have to overcome the +20.

Example: If you have 21 digging power and try to dig into quartz you'll have to
be very lucky and hope for the random(0..799) roll for quartz to actually be a
0 so that the result will be 0+20 which is 20 and therefore smaller than your
digging power of 21. So on each turn of tunnelling your chance to succeed would
be 1 in 800 in that case.

If you try to use a digging tool that is too heavy for you, you will see it in
your encumberment bar and there will be a warning message on equipping it,
 "You have trouble wielding such a heavy tool"
which means your digging power will be cut down to 1/3 when using it to dig.