minimal_character_positions.R

# Level 2
minimal_character_positions <- \(s) {
  max_len <- max(nchar(s))
  for (i in seq_len(max_len)) {
    for (indices in combn(seq_len(max_len), i, simplify = FALSE)) {      
      char_vectors <-
        purrr::map(s, \(str, idx = indices) {
          purrr::map_chr(idx, \(ii) {
            if (ii <= nchar(str)) substr(str, ii, ii) else NA
          })
        })            
      if (length(unique(char_vectors)) == length(char_vectors)) {
        return(indices)
      }
    }
  }
  return(NULL)
}

# Is it working?
test_cases <- rlang::list2(
  list(input = c("apple", "app", "applies"), expected = c(5L)),
  list(input = c("cat", "car", "bat"), expected = c(1L, 3L)),
  list(input = c("apple", "banana", "cherry"), expected = c(1L)),
  list(input = c("ape", "bat", "cap", "zap"), expected = c(1L)),
  list(input = c("cat", "car", "bat", "pat"), expected = c(1L, 3L)),
  list(input = c("apple"), expected = c(1L)),
  list(input = c("cat", "car", "bat", "batman"), expected = c(1L, 3L, 4L)),
  list(input = c("a", "b", "c", "d"), expected = c(1L)),
  list(input = c("apple", "", "banana"), expected = c(1L)),
  list(input = c("apple", "app", "appl"), expected = c(4L, 5L))
)
for (i in seq_along(test_cases)) {
  result <- minimal_character_positions(test_cases[[i]]$input)
  expected <- test_cases[[i]]$expected    
  if (!identical(result, expected)) {
    stop(paste("Test case", i, "failed. Expected:", toString(expected), "but got:", toString(result)))
  } else {
    cat(paste("Test case", i, "passed.", '\n'))
  }
}

minimal_prefix_length.R

minimal_prefix_length <- function(strings) {
  max_len <- max(nchar(strings))
  for (prefix_len in seq_len(max_len)) {
    env <- new.env(hash = TRUE)
    collision <- FALSE  
    for (string in strings) {
      prefix <- substr(string, 1, prefix_len)
      if (exists(prefix, envir = env)) {
        collision <- TRUE
        break
      } else {
        assign(prefix, TRUE, envir = env)
      }
    }  
    if (!collision) {
      return(prefix_len)
    }
  }
  return(max_len)
}

s <- c("apple", "app", "aptitude")
minimal_prefix_length(s)