How can I write custom attribute value rules for the GURPS Character Sheet tool?

Question

When using the tool GURPS Character Sheet, is there a way to change the attribute value when calculating skill levels?

In my next GURPS campaign I'm planning to introduce this rule from Pyramid #3/65:

Instead of the usual default attribute values for Easy, Average, and Hard skills, convert these defaults to half-stat by changing the basis for all defaults to Attribute/2 (round fractions down) plus a modifier. The modifier equals the normal default penalty (expressed as a negative number) plus 5.

I'll also plan to modify the cost of attributes, or apply this rule when buying skill levels as I want to reduce the impact of high attributes on skills.

I have some experience with coding, so feel free to point to any solution involving a snippet or something like that.

Answer 1

(Edit: I wrote that I hadn't actually tried building a new version of the software, but I did actually study the files in all of the directories installed by the software, and I did study the source code of the program, and I am a professional software developer with decades of experience, and a GURPS GM with decades of experience, and I am confident that this is actually most likely the correct place to make the requested change (for the side-question too), and probably a syntactically-correct code change to make the requested change. Please (all ye rpg.se critics) take pity and don't downvote my answer even further just because I didn't spend hours setting up a working development environment (I don't even have a Mac and the dev environment is set up on Mac) and implement and test the changes, for an rpg.se question that just asks for code pointers.)

I haven't actually tried doing (the hours of work it would probably take to get a non-Mac dev environment working and testing) this (I don't have a Java dev environment set up, nor a Mac), but I see a very likely-looking candidate for where to edit the code to do this:

In the file: https://github.com/richardwilkes/gcs/blob/master/src/com/trollworks/gcs/skill/SkillAttribute.java

There are sections for each attribute like:

/** The dexterity attribute. */
DX {
    @Override
    public String toString() {
        return DX_TITLE;
    }

    @Override
    public int getBaseSkillLevel(GURPSCharacter character) {
        return character != null ? character.getDexterity() : Integer.MIN_VALUE;
    }
}

And I would start by changing the

character.getDexterity() : Integer.MIN_VALUE;

part to be more like:

(character.getDexterity() / 2 + 5: Integer.MIN_VALUE);

As for how to modify attribute point costs, it looks like you'd modify the getPointsForAttribute() function (and/or the individual methods for each attribute that call it (e.g. updateStrengthInfo()), and/or look at how SheetPreferences.areOptionalStrengthRulesUsed() is used.

i.e. in https://github.com/richardwilkes/gcs/blob/master/src/com/trollworks/gcs/character/GURPSCharacter.java

private static int getPointsForAttribute(int delta, int ptsPerLevel, int reduction) {
    int amt = delta * ptsPerLevel;

    if (reduction > 0 && delta > 0) {
        int rounder = delta < 0 ? -99 : 99;

        if (reduction > 80) {
            reduction = 80;
        }
        amt = (rounder + amt * (100 - reduction)) / 100;
    }

    return amt;
}