Emulating Exploding Dice With Different Sizes

I want to create or simulate an in-line roll that rolls 1d8, and on a roll of 8 additionally rolls 1d4. This is similar to an Exploding Die roll, with the exception that the exploded die is of a different size than the exploding die. 

Currently, I have simulated this with a rollable table that has 4 values for every number 1-7, as well as 1 value for each of 9, 10, 11, and 12. While this is sufficient for my current needs, if I need to generalize this behavior in the future, I do not want to have to create a rollable table for every combination of dice (i.e. d12+d10, d12+d8, d12+d6...)

I am the game owner of the game where this is relevant and have access to scripts if they are absolutely necessary, but I would like to stick with solutions using dice roll syntax if such solutions exist, regardless of complexity. 

You can mimic a single d8 into a d4 using

[[ { {d8=8}*(8+d4),d(8-1)}k1 ]]

You can replace the 8's and 4's to change it into any dice combo. However if you need to roll more than a single die you have to add the groups together such as:

[[ { {d8=8}*(8+d4),d(8-1)}k1 + { {d8=8}*(8+d4),d(8-1)}k1 ]]

There is an open suggestion for Custom Dice Syntax that could handle rolling a group multiple times with ease, so consider voting for it.

As an alternative if you don't mind being restricted to /roll and /gmroll commands is

/roll (?{Dice Count|1}-[[?{Dice Count}d8=8]])d(8-1) + 8*$[[0]] + $[[0]]d4

again just replace the 8's and 4's to provide other dice combos

Also, as a pro user you have access to scripts, and using something like ScriptCards you could do this in a way that handles any die type simply.

While RainbowEncoder's excellent code shows it can be done without using a script, the dice system isn't really designed for this and you have to force it to comply.

RainbowEncoder said:

You can mimic a single d8 into a d4 using

[[ { {d8=8}*(8+d4),d(8-1)}k1 ]]

Oh man, this is gorgeous and pretty much exactly what I was hoping was possible. 

I've got the dice reference up as I type this, wanting to make sure I understand this query. I believe:

• Boolean expression of a Compare Point
• returns 1 if the d8 is equal to 8
• if not, returns something less than 1 (whether 0 or -1)
•  multiply this value by 8+1d4
• Resulting in 8+1d4 if the Compare Point was met
• Or something less than 1 (by negating the value or multiplying it by 0, depending on the implemented behavior of the Compare Point)
• Separately roll a d8 that cannot land on 8 (handled by making the die size 1 less than the number used as Compare Point)
• Keep the highest value between Bullet 2 and Bullet 3

This is... Genius, and you have both my thanks and that of an ecstatic player!

So, for posterity I believe a general form would be 

X=initial die size
Y=additional die size

{ {dX=X}*(X+dY),d(X-1)}k1

I've also thrown my voice at the suggested idea you pointed me towards - that would not only have provided another avenue for solving this, but it would also have solved a problem I had last week as well. Thank you, again, for this as well!

Cl B. said:

Oh man, this is gorgeous and pretty much exactly what I was hoping was possible.

I've got the dice reference up as I type this, wanting to make sure I understand this query. I believe:

• Boolean expression of a Compare Point
• returns 1 if the d8 is equal to 8
• if not, returns something less than 1 (whether 0 or -1)

It's not a boolean expresion but a target number, it counts the number of dice that hit that target, it does return 1 on a rolled 8 otherwise it's 0

•  multiply this value by 8+1d4
• Resulting in 8+1d4 if the Compare Point was met
• Or something less than 1 (by negating the value or multiplying it by 0, depending on the implemented behavior of the Compare Point)

Yep, this results in the either a value of an exploded die roll or 0 if roll doesn't explode

• Separately roll a d8 that cannot land on 8 (handled by making the die size 1 less than the number used as Compare Point)
• Keep the highest value between Bullet 2 and Bullet 3

Also correct