day6

6/6.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+
+import sys
+
+# run with '2' argument for part2 result.
+part2 = (len(sys.argv) == 2 and sys.argv[1] == '2')
+
+objects = {}
+orbits = {}
+
+for line in sys.stdin:
+  line = line.rstrip()
+  bodies = line.split(')')
+  objects[bodies[0]] = 1
+  objects[bodies[1]] = 1
+  orbits[bodies[1]] = bodies[0]
+
+traces = {}
+
+count = 0
+for object in objects:
+  trace = []
+  if object == "COM":
+    continue
+  n = orbits[object]
+  ocount = 1
+  trace.append(object)
+  trace.append(n)
+  while n != "COM":
+    n = orbits[n]
+    trace.append(n)
+    ocount += 1
+  count += ocount
+  traces[object] = trace
+
+print(count)
+
+want1='YOU'
+want2='SAN'
+if want1 in traces and want2 in traces:
+  pos = -1
+  t1 = traces[want1][pos]
+  t2 = traces[want2][pos]
+  # work from COM (array[-1]) towards each planet, until we find the point they diverge
+  while t1 == t2:
+    pos -= 1
+    t1 = traces[want1][pos]
+    t2 = traces[want2][pos]
+  # then return the remaining length of each branch
+  len1 = len(traces[want1]) + pos
+  len2 = len(traces[want2]) + pos
+  print(len1+len2)