open Util
open Constants

type t = {
  expo     : float;              (* alpha + beta *)
  expo_inv : float;              (* 1/(alpha + beta) *)
  center   : Coordinate.t;       (* P = (alpha * A + beta B)/(alpha+beta) *)
  center_a : Coordinate.t;       (* P - A *)
  center_ab: Coordinate.t;       (* A - B *)
  norm_sq  : float;              (* |A-B|^2 *)
  norm_coef: float;              (* norm_coef_a * norm_coef_b *)
  coef     : float;              (* norm_coef * coef_a * coef_b * g, with
                                    g = (pi/(alpha+beta))^(3/2) exp (-|A-B|^2 * alpha*beta/(alpha+beta)) *)
  norm_coef_scale : float array; (* norm_coef.(i) / norm_coef.(0) *)
  i        : int;
  j        : int;
  shell_a  : Contracted_shell.t;
  shell_b  : Contracted_shell.t;
  monocentric : bool
}


(** Returns an integer characteristic of a primitive shell pair *)
let hash a = 
  Hashtbl.hash (a.expo, a.center_a, a.center_ab, a.coef) 


(** Comparison function, used for sorting *)
let cmp a b =
  hash a - hash b