16-2

.gitignore

@@ -1,4 +1,5 @@
-.venv/
-.vscode/
+.venv*
+.vscode*
 2023
-*screenshots/
+*\screenshots
+*\input.txt

2024/16/16-2.py

@@ -0,0 +1,49 @@
+import networkx as nx
+
+directions = (1, -1, 1j, -1j) # use imaginary numbers for east<->west
+
+with open('2024/16/input.txt') as f:
+    g = nx.DiGraph()
+    for i, line in enumerate(f):
+        line = line.strip()
+        for j, char in enumerate(line):
+            if char == '#':
+                continue
+            node = i + 1j * j
+
+            if char == 'E':
+                e = node
+            if char == 'S':
+                s = (node, 1j) # reindeer starts pointing west
+
+            for nodeDir in directions:
+                g.add_node((node, nodeDir))
+
+    for node, nodeDir in g.nodes:
+        if (node + nodeDir, nodeDir) in g.nodes:
+            # orthogonally connected in the same direction as previous node
+            # eg, connect node A's north direction with node B's north
+            # no need to connect A's north to B's south - it'd be redundant
+            g.add_edge((node, nodeDir), (node + nodeDir, nodeDir), weight=1)
+        for rotation in -1j, 1j:
+            # connect 90 degree corners
+            # using i, since i**2 = 1 so it'll respect a turn
+            g.add_edge((node, nodeDir), (node, nodeDir * rotation), weight=1000)
+
+    # need to make a new endpoint otherwise the exit will not be found
+    for direction in directions:
+        # we appear to have an off-by-one problem, remove the last move to the exit
+        g.add_edge((e, direction), 'end', weight=0)
+
+    # very thankful for NetworkX
+    allShortestPaths = nx.all_shortest_paths(g, s, 'end', weight='weight')
+
+    seats = {
+        node
+        for path in allShortestPaths
+        for node, _ in path[:-1]
+    }
+
+    print(seats)
+    print(len(seats))
+