Reorganize code and document functions

lib/state.ml

@@ -0,0 +1,18 @@
+type t = { lines : string array; cursor : int }
+(** Invariant: cursor >= 0 cursor < Array.length lines *)
+
+let size s = Array.length s.lines
+let cursor s = s.cursor
+
+let create (fname : string) : t =
+  let lines =
+    In_channel.with_open_text fname In_channel.input_lines |> Array.of_list
+  in
+  { lines; cursor = 0 }
+
+let show_line s : string = s.lines.(s.cursor)
+
+let next s =
+  if s.cursor < size s - 1 then { s with cursor = s.cursor + 1 } else s
+
+let prev s = if s.cursor > 0 then { s with cursor = s.cursor - 1 } else s