type t = { text : string; pos : int; right_boundary : int }

let overran cursor = cursor.pos >= cursor.right_boundary
let next_char cursor = { cursor with pos = cursor.pos + 1 }
let char_at cur offset = String.get cur.text (cur.pos + offset)
let char cur = String.get cur.text cur.pos
let distance a b = b.pos - a.pos

let sub ?left ?right cur = { cur with
  pos = Option.value left ~default:cur.pos;
  right_boundary = Option.value right ~default:cur.right_boundary }

let unwrap num cur = sub ~left:(cur.pos+num) ~right:(cur.right_boundary-num) cur

let segment_string cur = String.sub cur.text cur.pos (cur.right_boundary - cur.pos)

(*todo: reconsider +1 result and type cursor*)
let rec find_end e = function
  | cur when cur.pos + 1 = String.length cur.text -> Some cur.pos
  | cur when overran cur -> None
  | cur when e cur (char cur) -> Some (cur.pos + 1)
  | cur -> find_end e (next_char cur)