;;; -*- lexical-binding: t -*-
;;
;; this file:
;;   https://dataswamp.org/~incal/emacs-init/all-substance.el

(require 'cl-lib)

(defun half-time-to-day-drop (ht)
  (if (< ht 24)
      (/ ht 24 2.0)
    (- 1 (/ 24 ht 2.0)) ))

;; (half-time-to-day-drop 12) ; 0.25
;; (half-time-to-day-drop 48) ; 0.75

(defun all-substance (q d ht &optional verbose day-q)
  "Q is the quantity, for example 10. This is a number; do not use a unit.
D is the number of days, for example 20.
HT is the half-time in hours, for example 6.
Set VERBOSE to return a string.
Set DAY-Q if Q instead is submitted the daily quantity, for example 0.5."
  (cl-loop
    with daily    = (if day-q q (/ q d 1.0))
    with day-drop = (half-time-to-day-drop ht)
    for i downfrom (1- d) to 0
    sum (* daily (expt day-drop i)) into total
    finally return
      (if verbose
          (message "daily %.2f, days %d, total %.2f" daily d total)
        total) ))

;; (all-substance 300 365 6)       ; 0.94
;; (all-substance 0.5  30 6 nil t) ; 0.57
;; (all-substance 365 365 6 t)     ; daily 1.00, days 365, total 1.14

(defun wash-out (q ht &optional days)
  (or days (setq days 5))
  (cl-loop
    with day-drop = (half-time-to-day-drop ht)
    for d from 1 to days
    collect (* q (expt day-drop d)) into res
    finally return res) )

;; (wash-out 1.14 6) ; 0.1      1 day w/o resupply
                    ;; 0.02     2 ..
                    ;; 0.002    3 ..
                    ;; 0.0003   4 ..
                    ;; 0.00003  5 ..

(provide 'all-substance)