2019 day 6 init

2019/day06/README.org

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+** --- Day 6: Universal Orbit Map ---
+You've landed at the Universal Orbit Map facility on Mercury. Because
+navigation in space often involves transferring between orbits, the
+orbit maps here are useful for finding efficient routes between, for
+example, you and Santa. You download a map of the local orbits (your
+puzzle input).
+
+Except for the universal Center of Mass (=COM=), every object in space
+is in orbit around exactly one other object. An
+[[https://en.wikipedia.org/wiki/Orbit][orbit]] looks roughly like this:
+
+#+BEGIN_EXAMPLE
+                    \
+                     \
+                      |
+                      |
+  AAA--> o            o <--BBB
+                      |
+                      |
+                     /
+                    /
+#+END_EXAMPLE
+
+In this diagram, the object =BBB= is in orbit around =AAA=. The path
+that =BBB= takes around =AAA= (drawn with lines) is only partly shown.
+In the map data, this orbital relationship is written =AAA)BBB=, which
+means "=BBB= is in orbit around =AAA=".
+
+Before you use your map data to plot a course, you need to make sure it
+wasn't corrupted during the download. To verify maps, the Universal
+Orbit Map facility uses /orbit count checksums/ - the total number of
+/direct orbits/ (like the one shown above) and /indirect orbits/.
+
+Whenever =A= orbits =B= and =B= orbits =C=, then =A= /indirectly orbits/
+=C=. This chain can be any number of objects long: if =A= orbits =B=,
+=B= orbits =C=, and =C= orbits =D=, then =A= indirectly orbits =D=.
+
+For example, suppose you have the following map:
+
+#+BEGIN_EXAMPLE
+  COM)B
+  B)C
+  C)D
+  D)E
+  E)F
+  B)G
+  G)H
+  D)I
+  E)J
+  J)K
+  K)L
+#+END_EXAMPLE
+
+Visually, the above map of orbits looks like this:
+
+#+BEGIN_EXAMPLE
+          G - H       J - K - L
+         /           /
+  COM - B - C - D - E - F
+                 \
+                  I
+#+END_EXAMPLE
+
+In this visual representation, when two objects are connected by a line,
+the one on the right directly orbits the one on the left.
+
+Here, we can count the total number of orbits as follows:
+
+- =D= directly orbits =C= and indirectly orbits =B= and =COM=, a total
+  of =3= orbits.
+- =L= directly orbits =K= and indirectly orbits =J=, =E=, =D=, =C=, =B=,
+  and =COM=, a total of =7= orbits.
+- =COM= orbits nothing.
+
+The total number of direct and indirect orbits in this example is =42=.
+
+/What is the total number of direct and indirect orbits/ in your map
+data?