01.ss

#!/usr/bin/env scheme --script

(load "utils.ss")

;; https://adventofcode.com/2019/day/1

;; --- Day 1: The Tyranny of the Rocket Equation ---

; Santa has become stranded at the edge of the Solar System while delivering
; presents to other planets! To accurately calculate his position in space,
; safely align his warp drive, and return to Earth in time to save Christmas,
; he needs you to bring him measurements from fifty stars.
; 
; Collect stars by solving puzzles. Two puzzles will be made available on each
; day in the Advent calendar; the second puzzle is unlocked when you complete
; the first. Each puzzle grants one star. Good luck!
; 
; The Elves quickly load you into a spacecraft and prepare to launch.
; 
; At the first Go / No Go poll, every Elf is Go until the Fuel Counter-Upper.
; They haven't determined the amount of fuel required yet.
; 
; Fuel required to launch a given module is based on its mass. Specifically, to
; find the fuel required for a module, take its mass, divide by three, round
; down, and subtract 2.
; 
; For example:
; 
;     For a mass of 12, divide by 3 and round down to get 4, then subtract 2 to
;     get 2.  For a mass of 14, dividing by 3 and rounding down still yields 4,
;     so the fuel required is also 2.  For a mass of 1969, the fuel required is
;     654.  For a mass of 100756, the fuel required is 33583.
; 
; The Fuel Counter-Upper needs to know the total fuel requirement. To find it,
; individually calculate the fuel needed for the mass of each module (your
; puzzle input), then add together all the fuel values.
; 
; What is the sum of the fuel requirements for all of the modules on your
; spacecraft?
 
(define (mass->fuel m)
  (- (floor (/ m 3)) 2))

(assert (= (mass->fuel 12) 2))
(assert (= (mass->fuel 14) 2))
(assert (= (mass->fuel 1969) 654))
(assert (= (mass->fuel 100756) 33583))

;; 3464458
(define module-fuel
  (fold-left + 0
             (map mass->fuel 
                  (map string->number (read-file "inputs/01.txt")))))

(print module-fuel)

;; --- Part Two ---

; During the second Go / No Go poll, the Elf in charge of the Rocket Equation
; Double-Checker stops the launch sequence. Apparently, you forgot to include
; additional fuel for the fuel you just added.
; 
; Fuel itself requires fuel just like a module - take its mass, divide by
; three, round down, and subtract 2. However, that fuel also requires fuel, and
; that fuel requires fuel, and so on. Any mass that would require negative fuel
; should instead be treated as if it requires zero fuel; the remaining mass, if
; any, is instead handled by wishing really hard, which has no mass and is
; outside the scope of this calculation.
; 
; So, for each module mass, calculate its fuel and add it to the total. Then,
; treat the fuel amount you just calculated as the input mass and repeat the
; process, continuing until a fuel requirement is zero or negative. For
; example:
; 
;     A module of mass 14 requires 2 fuel. This fuel requires no further fuel
;     (2 divided by 3 and rounded down is 0, which would call for a negative
;     fuel), so the total fuel required is still just 2.  At first, a module of
;     mass 1969 requires 654 fuel. Then, this fuel requires 216 more fuel (654
;     / 3 - 2). 216 then requires 70 more fuel, which requires 21 fuel, which
;     requires 5 fuel, which requires no further fuel. So, the total fuel
;     required for a module of mass 1969 is 654 + 216 + 70 + 21 + 5 = 966.  The
;     fuel required by a module of mass 100756 and its fuel is: 33583 + 11192 +
;     3728 + 1240 + 411 + 135 + 43 + 12 + 2 = 50346.
; 
; What is the sum of the fuel requirements for all of the modules on your
; spacecraft when also taking into account the mass of the added fuel? 
 
(define (total-fuel f)
  (define (total-fuel total last)
    (cond [(<= last 0) total]
          [else
            (total-fuel (+ total last) (mass->fuel last))]))
  (total-fuel 0 (mass->fuel f)))

(assert (= (total-fuel 14) 2))
(assert (= (total-fuel 1969) 966))
(assert (= (total-fuel 100756) 50346))


;; calculate the fuel requirements for each module separately, then add them up
;; at the end

(define (module+fuel m)
  (let [(fuel (mass->fuel m))]
    (+ fuel (total-fuel fuel))))

(print
  (fold-left + 0
             (map
               (lambda (s)
                 (module+fuel (string->number s)))
               (read-file "inputs/01.txt"))))
;; 5193796