(** Source locations. Every form carries one: error messages, the step debugger
    and nREPL's find-definition all need them, and retrofitting locations onto a
    reader is far worse than carrying them from the start.

    A location is a *span*, not a point. The start is what [file:line:col]
    prints and what every consumer that wants one place uses; the end is what
    lets an error underline the thing it is about. A column number cannot draw
    a squiggle and a span can, which is the whole reason the two extra fields
    are here.

    The end is *exclusive* and defaults to the start, so a location nobody
    widened is a zero-width span at a point and every old call site keeps its
    old meaning. Only the reader knows where a form ends, so only the reader
    fills these in; a location the checker invents for a node with no syntax
    stays a point. *)

type t = {
  file  : string;
  line  : int;  (* 1-based *)
  col   : int;  (* 1-based *)
  eline : int;  (* 1-based, exclusive end *)
  ecol  : int;
}

let make file line col = { file; line; col; eline = line; ecol = col }
let unknown = make "<unknown>" 0 0

(** [upto start stop] is [start] widened to end where [stop] begins. A [stop]
    that is not after [start], or is in another file, leaves it alone: a span
    that runs backwards would draw nonsense. *)
let upto (start : t) (stop : t) =
  if
    stop.file = start.file
    && (stop.line > start.line
        || (stop.line = start.line && stop.col > start.col))
  then { start with eline = stop.line; ecol = stop.col }
  else start

(** True when the span covers more than its first line. The underline is drawn
    on the start line either way — a form that spans twenty lines is pointed
    at, not boxed — so this is what tells the renderer to run the underline to
    the end of that line rather than to [ecol]. *)
let multiline (t : t) = t.eline > t.line

(** How many columns to underline on the start line, or [None] when the span
    was never widened and there is nothing but a point to draw. *)
let width (t : t) =
  if t.eline = t.line && t.ecol > t.col then Some (t.ecol - t.col) else None

let to_string t = Printf.sprintf "%s:%d:%d" t.file t.line t.col

(** Raised by every stage of the frontend. *)
exception Error of t * string

let fail loc fmt = Printf.ksprintf (fun msg -> raise (Error (loc, msg))) fmt