;;
;; FILE:     01-opening-exercise.rkt
;; AUTHOR:   Eugene Wallingford
;; DATE:     2023/02/07
;; COMMENT:  Improving a file originally written in 2020.
;;

#lang racket

;; --------------------------------------------------
;; set-up for exercise
;; --------------------------------------------------

   ; our data: a file of module masses, one per line:
   ;
   ; 12
   ; 14
   ; 1969
   ; 100756
   ;
   ; Racket has a function to read this file into a list of strings:
   ;
   ; > (file->lines "modules.txt")
   ; '("12" "14" "1969" "100756")

(define modules (file->lines "modules.txt"))

   ; our helper function: convert a mass to an amount of fuel:

(define mass->fuel
  (lambda (m)
    (- (quotient m 3) 2)))

;; --------------------------------------------------
;; solution with anonymous lambda
;; --------------------------------------------------

   ; (map (lambda (module)
   ;        (mass->fuel (string->number module)))
   ;      modules)

;; --------------------------------------------------
;; solution with named lambda
;; --------------------------------------------------

(define module->fuel
  (lambda (module)
    (mass->fuel (string->number module))))

   ; (map module->fuel modules)

;; --------------------------------------------------
;; using the result to compute other solutions
;; --------------------------------------------------

   ; (apply + (map module->fuel modules))

(define total-fuel
  (lambda (filename)
    (apply +
      (map module->fuel
           (file->lines filename)))))

   ; We can use other reducing functions, too:
   ; (apply min (map module->fuel modules))

;; --------------------------------------------------
;; A sequence of maps can usually be collapsed into
;; a single map with a more powerful helper function.
;; Consider Problem 5 on Homework 3...
;; --------------------------------------------------