Try to figure out an O(n) algorithm to find all unique substrings

Substring.R

str <- "asdfasdf"
char <- data_frame(
    letters = unlist(strsplit(str, "")),
    skip = FALSE)
uniques <- c()
for (q in nchar(str):1) {
    for (i in 1:(nchar(str) - q + 1)) {
        start <- i
        end   <- i + q - 1
        if (all( char$skip[start:end])) {
            next
        }
        sub <- substr(str, start, end)
        if (sub %in% uniques) {
            char$skip[start:end] <- TRUE
        } else {
            uniques <- c(uniques, sub)
        }
    }
}

The basic idea here is that if a substring is a duplicate, then all of its substrings will also be duplicates, so you don't have to check them. I feel like this is about half of the answer to the problem.