Swapped outcome mapping in `SAMPLE_FROM_PROBS`

[gvegayon]

Summary

In ModelMeaslesSchool, the m_update_rash and m_update_isolated functions incorrectly map the outcomes of the SAMPLE_FROM_PROBS macro, causing the hospitalization probability to differ from the expected competing-risks formula.

Expected behavior

The probability of hospitalization (conditional on leaving the rash state) should be:

$$P(\text{hosp}) = \frac{h}{h + r}$$

where $h$ is the hospitalization rate and $r = 1/\text{rash period}$ is the recovery rate.

Actual behavior

Due to the swapped outcome mapping, agents recover with daily probability $1 - r - h$ (the "nothing happens" leftover) instead of $r$. This yields:

$$P(\text{hosp}) = \frac{h}{1 - r}$$

Root cause

The SAMPLE_FROM_PROBS(2, which) macro (defined at the top of measlesschool.hpp) draws from a cumulative roulette and assigns outcomes as:

which	Probability
0	array_double_tmp[0]
1	array_double_tmp[1]
2	remainder: 1 - [0] - [1]

In both m_update_rash and m_update_isolated, the arrays are set to:

m->array_double_tmp[0] = 1.0/m->par("Rash period");       // recovery rate r
m->array_double_tmp[1] = m->par("Hospitalization rate");    // hosp rate h

But the outcome handling code had which == 0 → "stay in rash" (or just isolate if detected) and which == 2 → "recover", which is backwards. which == 0 fires with probability r (the recovery rate) but was mapped to "nothing happens", while which == 2 fires with probability 1 - r - h (the true "nothing happens" remainder) but was mapped to "recover".

Fix

Swap the outcome handling so that:

• which == 0 (probability r = 1/rash_period) → recover
• which == 2 (probability 1 - r - h) → stay in current state (no transition)

This was applied to both m_update_rash and m_update_isolated. Test expectations in 14a-measles.cpp and 14b-measles.cpp were also updated: p_recovered changed from 1 - (1/rash_period + hosp_rate) to 1/rash_period.

Affected files

• include/epiworld/models/measlesschool.hpp — two functions: m_update_rash, m_update_isolated
• tests/14a-measles.cpp — test expectation for p_recovered
• tests/14b-measles.cpp — test expectation for p_recovered