## Thanks to https://github.com/helmingstay for the test case

pars <- list(npair = 20, q = 0.05, prob=list(S=0.2, I=0.8), 
             deltat=5, tmax=20)
sim.grid <- with(pars,
                 expand.grid(time=seq(0,tmax,by=deltat), subject=1:npair))

## initial conditions / observation model
config <- list(
    qmat = rbind(
        c(0, pars$q),
        c(0, 0)
    ),
    hmod = list(
        S=hmmBinom(size=1, prob=0.2),
        I=hmmBinom(size=1, prob=0.8)
    )
)

sim.dat <- simmulti.msm(sim.grid, 
    config$qmat, drop.absorb=F,
    hmodel=list(
        S=hmmBinom(size=1, prob=0.2),
        ## sim uses larger size
        I=hmmBinom(size=2, prob=0.8)
    )
)

panel <- subset(sim.dat, select=c(subject, time, obs))
panel <- within(panel, {
    ## force state 2
    obstrue <- ifelse(obs==2, 2, NA)
    obs <- ifelse(obs>1, 1, 0)
})

result <- msm(data=panel,
              obs ~ time, subject=subject, obstrue=obstrue, qmatrix=config$qmat, hmodel=config$hmod,
    fixedpars=T
)

pred <- merge(panel,
              viterbi.msm(result),
              by=c('subject', 'time'), sort=F)

test_that("Viterbi with obstrue",{
    predtrue <- subset(pred, obstrue==2)
    expect_equal(predtrue$pstate[1,1], 0)
    expect_equal(predtrue$pstate[1,2], 1)
    expect_true(isTRUE(all.equal(rowSums(predtrue$pstate), rep(1,nrow(predtrue)))))
})