minx.md

code golf for https://fosstodon.org/@coolbutuseless/113668013712145841

x <- c("apple", "app", "aptitude")
mx <- max(nchar(x))

pool <- c(letters, LETTERS)
flat <- function(x) {
  c(x, rep(0L, mx - length(x)))
}
## match letter to number and flatten to mx, then process for uniqueness by column (possibly could use rowSums to avoid apply loop)
which(apply(do.call(rbind, lapply(strsplit(x, ""), \(.x) flat(match(.x, pool)))), 2, \(.x) length(unique(.x))) > 1)[1L] + 1

[1] 4

is it faster if we avoid apply()?

(none of this is really tested, fwiw)

## to use rowSums we need the longest string as index
index <- match(unlist(strsplit(x[which.max(nchar(x))], "")), pool)

which(!rowSums(do.call(cbind, lapply(strsplit(x, ""), \(.x) flat(match(.x, pool))))) == (index * length(x)))[1] + 1

ok, no need for pool or strsplit or match

x <- c("apple", "app", "aptitude")
mx <- max(nchar(x))

## a bad name for a function to pad zeros to the end of a vector shorter than 'mx'
flat <- function(x) {
  c(x, rep(0L, mx - length(x)))
}

## minimum number of first letters needed to determine non-uniqueness
minnl <- function(x) {
  ## to use rowSums we need the longest string as index
  index <- as.integer(charToRaw(x[which.max(nchar(x))]))
  which(!rowSums(do.call(cbind, lapply(x, \(.x) flat(as.integer(charToRaw(.x)))))) == (index * length(x)))[1] + 1
}

minnl(x)