Open this document in the playground · Download source

A later save can depend on earlier damage

Part G · L25 · Prerequisite: L24 · One modelling idea

Situation, question, and prediction

A Cairn Blood Elk hits a PC with HP 4, armour 2, and STR 11. Damage overflow reduces STR before a save. Predict why a single precomputed “save chance at STR 11” cannot model every damage branch.

Build

Pair raw damage with an independent potential d20 save using joint_classify. The callback subtracts armour, checks HP, then computes reduced STR. It ignores the potential save when no save is required. Death at STR zero must be checked before the natural-1 success exception. max(0, raw - armour) takes the larger of zero and the reduced damage: it is the scalar version of L19’s zero floor.

hp = 4
armour = 2
strength = 11
bands = scale().step("No effect").step("HP loss").step("Scar").step("STR loss, save passed").step("Critical damage").step("Death")
def injury(raw, save):
    damage = max(0, raw - armour)
    if damage == 0:
        return "No effect"
    if damage < hp:
        return "HP loss"
    if damage == hp:
        return "Scar"
    remaining = strength - (damage - hp)
    if remaining <= 0:
        return "Death"
    if save == 1:
        return "STR loss, save passed"
    if save == 20:
        return "Critical damage"
    if save <= remaining:
        return "STR loss, save passed"
    return "Critical damage"
output("One Blood Elk hit", joint_classify(d(8), d(20), bands, injury))

One Blood Elk hit · Outcomes

Run, read, and check

The default categories are 1/4,3/8,1/8,19/160,21/160,0. Horn faces 7 and 8 leave STR 10 and 9, so their passed-save pairs total 10+9 out of 160. Low-damage outcomes ignore the save; counting all twenty potential values still gives the right total probability.

Change one thing

Set strength to 1. Both overflowing damage faces now cause death, probability 1/4. A potential save of 1 cannot rescue zero STR.

Try it yourself

At strength 2, which horn faces cause death? Answer: only 8, probability 1/8. Explain why multiplying an independent “STR loss chance” by an unchanged STR-11 save chance would be wrong.

Rules and model notes

Cairn 2e core rules, Critical Damage and Attribute Loss; Blood Elk. Positive integer HP, armour 0–3, STR 1–20. Immediate categories only: no scar resolution, recovery, subsequent attacks, or goring aftermath.

What you now know / where next

A later save can depend on earlier damage is the reusable idea. Follow the generated previous/next links below, or return to the course index. For a complete self-contained application, see cairn-blood-elk.

Content ID: L25 · Review status: pilot

Prerequisites: L24