2019 day 1

2019/day01/README.org

@@ -1,8 +1,4 @@
-* --- Day 1: The Tyranny of the Rocket Equation ---
-   :PROPERTIES:
-   :CUSTOM_ID: day-1-the-tyranny-of-the-rocket-equation----
-   :END:
-
+** --- 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
@@ -38,6 +34,42 @@ it, individually calculate the fuel needed for the mass of each module
 /What is the sum of the fuel requirements/ for all of the modules on
 your spacecraft?
 
-To begin, [[file:1/input][get your puzzle input]].
+Your puzzle answer was =3318604=.
 
-Answer:
+** --- 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? (Calculate the fuel requirements for each module separately, then
+add them all up at the end.)
+
+Your puzzle answer was =4975039=.
+
+Both parts of this puzzle are complete! They provide two gold stars: **