Solution to problem 6 in Python

pyproject.toml

@@ -30,6 +30,11 @@ force_grid_wrap = 0
 use_parentheses = true
 line_length = 79
 
+[tool.mypy]
+pretty = true
+show_traceback = true
+allow_redefinition = true
+
 [tool.black]
 line-length = 79
 target-version = ['py38']
@@ -53,8 +58,3 @@ exclude = '''
                      # the root of the project
 )
 '''
-
-[tool.mypy]
-pretty = true
-show-traceback = true
-allow-redefinition = true

src/Year_2019/P6.py

@@ -70,6 +70,7 @@
 
 
 from collections import defaultdict
+from typing import Dict, List, Optional
 
 with open("files/P6.txt") as f:
     orbits = [line for line in f.read().strip().split()]
@@ -90,8 +91,105 @@ def part_1():
 
     total_orbits = count_orbits(universe, "COM", 0, 0)
 
-    print(f"There are {total_orbits} in our universe")
+    print(f"There are {total_orbits} orbits in our universe")
+
+
+# --- Part Two ---
+
+# Now, you just need to figure out how many orbital transfers you (YOU) need to\
+# take to get to Santa (SAN).
+
+# You start at the object YOU are orbiting; your destination is the object SAN
+# is orbiting. An orbital transfer lets you move from any object to an object
+# orbiting or orbited by that object.
+
+# For example, suppose you have the following map:
+
+# COM)B
+# B)C
+# C)D
+# D)E
+# E)F
+# B)G
+# G)H
+# D)I
+# E)J
+# J)K
+# K)L
+# K)YOU
+# I)SAN
+
+# Visually, the above map of orbits looks like this:
+
+#                           YOU
+#                          /
+#         G - H       J - K - L
+#        /           /
+# COM - B - C - D - E - F
+#                \
+#                 I - SAN
+
+# In this example, YOU are in orbit around K, and SAN is in orbit around I.
+# To move from K to I, a minimum of 4 orbital transfers are required:
+
+#     K to J
+#     J to E
+#     E to D
+#     D to I
+
+# Afterward, the map of orbits looks like this:
+
+#         G - H       J - K - L
+#        /           /
+# COM - B - C - D - E - F
+#                \
+#                 I - SAN
+#                  \
+#                   YOU
+
+# What is the minimum number of orbital transfers required to move from the
+# object YOU are orbiting to the object SAN is orbiting? (Between the objects
+# they are orbiting - not between YOU and SAN.)
+
+
+def get_all_orbits(node: str, universe: Dict[str, str]) -> List[str]:
+    orbits = []
+    while node in universe:
+        node = universe[node]
+        orbits.append(node)
+    return orbits
+
+
+def common_orbit(node_a: List[str], node_b: List[str]) -> Optional[str]:
+    for orbit in node_a:
+        if orbit in node_b:
+            return orbit
+    return None
+
+
+def distance_from(node_a: List[str], node_b: Optional[str]) -> int:
+    transfers = 0
+    for v in node_a:
+        if v == node_b:
+            return transfers
+        transfers += 1
+    return transfers
+
+
+def part_2() -> None:
+    universe = {}
+    for orbit in orbits:
+        planet, satellite = orbit.split(")")
+        universe[satellite] = planet
+
+    you = get_all_orbits("YOU", universe)
+    san = get_all_orbits("SAN", universe)
+    _common = common_orbit(you, san)
+    transfers = distance_from(you, _common) + distance_from(san, _common)
+
+    print(f"There will be at least {transfers} orbital transfers")
 
 
 if __name__ == "__main__":
     part_1()
+    part_2()