adventofcode/2015/day19/problem

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Advent of Code
--- Day 19: Medicine for Rudolph ---
Rudolph the Red-Nosed Reindeer is sick! His nose isn't shining very
brightly, and he needs medicine.
Red-Nosed Reindeer biology isn't similar to regular reindeer biology;
Rudolph is going to need custom-made medicine. Unfortunately, Red-Nosed
Reindeer chemistry isn't similar to regular reindeer chemistry, either.
The North Pole is equipped with a Red-Nosed Reindeer nuclear fusion/fission
plant, capable of constructing any Red-Nosed Reindeer molecule you need. It
works by starting with some input molecule and then doing a series of
replacements, one per step, until it has the right molecule.
However, the machine has to be calibrated before it can be used. Calibration
involves determining the number of molecules that can be generated in one
step from a given starting point.
For example, imagine a simpler machine that supports only the following
replacements:
H => HO
H => OH
O => HH
Given the replacements above and starting with HOH, the following molecules
could be generated:
 HOOH (via H => HO on the first H).
 HOHO (via H => HO on the second H).
 OHOH (via H => OH on the first H).
 HOOH (via H => OH on the second H).
 HHHH (via O => HH).
So, in the example above, there are 4 distinct molecules (not five, because
HOOH appears twice) after one replacement from HOH. Santa's favorite
molecule, HOHOHO, can become 7 distinct molecules (over nine replacements:
six from H, and three from O).
The machine replaces without regard for the surrounding characters. For
example, given the string H2O, the transition H => OO would result in OO2O.
Your puzzle input describes all of the possible replacements and, at the
bottom, the medicine molecule for which you need to calibrate the machine.
How many distinct molecules can be created after all the different ways you
can do one replacement on the medicine molecule?
Your puzzle answer was 518.
--- Part Two ---
Now that the machine is calibrated, you're ready to begin molecule
fabrication.
Molecule fabrication always begins with just a single electron, e, and
applying replacements one at a time, just like the ones during calibration.
For example, suppose you have the following replacements:
e => H
e => O
H => HO
H => OH
O => HH
If you'd like to make HOH, you start with e, and then make the following
replacements:
 e => O to get O
 O => HH to get HH
 H => OH (on the second H) to get HOH
So, you could make HOH after 3 steps. Santa's favorite molecule, HOHOHO, can
be made in 6 steps.
How long will it take to make the medicine? Given the available replacements
and the medicine molecule in your puzzle input, what is the fewest number of
steps to go from e to the medicine molecule?
Your puzzle answer was 200.
Both parts of this puzzle are complete! They provide two gold stars: **
At this point, you should return to your advent calendar and try another
puzzle.
If you still want to see it, you can get your puzzle input.
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