generate random sequences without consecutive repeats

generate_sequences.R

f <- function(x)
{
    n <- sum(x)
    v <- numeric(n)
    prev <- NULL
    for (i in 1:n)
    {
        idx <- x > floor(sum(x)/2) # how many counts are > n/2
        if (sum(idx) >= 2) # error if more than 2 (impossible?)
        {
            stop("multiple counts > floor(n/2); x = (", paste(x, collapse = ","), ")")
        } else if (any(idx)) {
            j <- which(idx)
            if (!is.null(prev) && prev == j) # error if one count is > n/2, but was prev
            {
                stop("j == prev; prev = ", prev, ", x = (", paste(x, collapse = ","), ")")
            } else { # if one count is > n/2, it must be chosen
                v[i] <- j
                x[j] <- x[j] - 1
            }
        } else {
            p <- x # store counts
            if (!is.null(prev)) { p[prev] <- 0 } # modify counts to not select prev
            j <- sample(seq_along(x), size = 1, prob = p) # sample, weighted by counts
            v[i] <- j
            x[j] <- x[j] - 1
        }
        prev <- j # remember last selection
    }
    v
}

f(c(10, 10))
f(c(4, 4, 5))
f(c(1, 1, 1, 1, 4))
f(c(1, 1, 1, 1, 5))
f(c(1, 1, 1, 1, 6)) # should error out