frame.R

frame <- function(from = -Inf, to = 0) {
  if (from >= to) stop("from must be less than to", call. = FALSE)
  
  dir <- function(x) if (x < 0) "PRECEDING" else "FOLLOWING"
  val <- function(x) if (is.finite(x)) as.integer(x) else "UNBOUNDED"
  bound <- function(x) {
    if (x == 0) return("CURRENT ROW")
    paste(val(x), dir(x))
  }

  if (to == 0) {
    bound(from)  
  } else {
    paste0("BETWEEN ", bound(from), " AND ", bound(to))
  }
}

in JS just for fun:

function frame(from, to) {
  if (!from) from = -Infinity
  if (!to) to = 0
  if (to === 0) console.log(bound(from))
  else console.log("BETWEEN", bound(from), "AND", bound(to))
}

function dir(x) {
  return x < 0 ? "PRECEDING" : "FOLLOWING"
}

function val(x) {
  return isFinite(x) ? x : "UNBOUNDED"
}

function bound(x) {
  if (x === 0) return "CURRENT ROW"
  return val(x) + ' ' + dir(x)
}