fishy:

#' #' Determine window ranges
#' #'
#' #' This function determines ranges (i.e., from and to positions) for windows of
#' #' size `window`. `n` specifies the number of windows.
#' #'
#' #' @param n Number of windows.
#' #' @param start Starting position of the first window.
#' #' @param window Window size.
#' #' @param na_rm A logical indicating whether to remove invalid ranges (i.e.,
#' #'   whether to keep `from` and `to` as `NA`) or otherwise.
#' #'
#' #' @return A matrix of ranges (`from` and `to`), each row pertains a window.
#' #'
#' #' @examples
#' #' window_ranges(10, start = 2, window = 3)
#' #'
#' #' window_ranges(10, start = 2, window = 3, na_rm = TRUE)
#' #' @md
#' #' @export
#' window_ranges <- function(n, start, window = 1, na_rm = FALSE) {
#'
#'   if(n < 1) stop('`n` must be greater than one.')
#'   if(start < 1) stop('`start` must be greater than one.')
#'   if(start > n) stop('`start` must be less than or equal to `n`.')
#'   if(window < 1) stop('`window` must be equal or greater than one.')
#'
#'   i <- seq_len(n)
#'   from<- i + start - 1
#'   to <- pmin(from + window - 1, n)
#'
#'   # Update first `e` and then `s` because we need the old version of `s`, not
#'   # the updated version.
#'   if(na_rm) {
#'     to <- to[!(from > n)]
#'     from <- from[!(from > n)]
#'   } else {
#'     to[from > n] <- NA_integer_
#'     from[from > n] <- NA_integer_
#'   }
#'
#'   m <- matrix(c(from, to), ncol = 2)
#'   colnames(m) <- c('from', 'to')
#'
#'   return(m)
#' }