From 165159cba946591a91f1835bd0cd96c34e23b62f Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Mon, 1 Feb 2021 22:39:09 +0100
Subject: [PATCH] Implement 2019 day 15

---
 2019/aoc2019/day15.py   | 102 ++++++++++++++++++++++++++++++++++++++++
 2019/aoc2019/intcode.py |   3 ++
 2 files changed, 105 insertions(+)
 create mode 100644 2019/aoc2019/day15.py

diff --git a/2019/aoc2019/day15.py b/2019/aoc2019/day15.py
new file mode 100644
index 0000000..2373d25
--- /dev/null
+++ b/2019/aoc2019/day15.py
@@ -0,0 +1,102 @@
+import sys
+from collections import defaultdict
+from typing import TextIO
+
+import networkx  # type: ignore
+
+from aoc2019.intcode import Computer, read_program
+
+
+def step(computer: Computer, direction: int) -> int:
+    computer.input.append(direction)
+
+    try:
+        computer.run()
+    except IndexError:
+        return computer.get_output()
+
+    sys.exit("computer terminated unexpectedly")
+
+
+def inverse(direction: int):
+    if direction % 2 == 1:
+        return direction + 1
+    else:
+        return direction - 1
+
+
+def read_graph(data: TextIO) -> networkx.Graph:
+    computer = Computer(read_program(data))
+
+    pos = (0, 0)
+    tiles = defaultdict(int)
+    tiles[0, 0] = 1
+
+    prev = [((0, 0), 1)]
+
+    while prev:
+        x, y = pos
+
+        if (x, y + 1) not in tiles:
+            movement = 1
+            next_pos = (x, y + 1)
+        elif (x, y - 1) not in tiles:
+            movement = 2
+            next_pos = (x, y - 1)
+        elif (x - 1, y) not in tiles:
+            movement = 3
+            next_pos = (x - 1, y)
+        elif (x + 1, y) not in tiles:
+            movement = 4
+            next_pos = (x + 1, y)
+        else:
+            # No movement available, backtrack
+            prev_pos, prev_dir = prev.pop()
+            step(computer, inverse(prev_dir))
+            pos = prev_pos
+            continue
+
+        result = step(computer, movement)
+        tiles[next_pos] = result
+
+        if result != 0:
+            # Movement was successful
+            prev.append((pos, movement))
+            pos = next_pos
+
+    graph = networkx.Graph()
+
+    for pos, value in tiles.items():
+        if value == 0:
+            continue
+
+        if value == 2:
+            # Create an imaginary edge to the oxygen
+            graph.add_edge('O', pos, weight=0)
+
+        x, y = pos
+
+        neighbours = [
+            (x - 1, y),
+            (x + 1, y),
+            (x, y - 1),
+            (x, y + 1),
+        ]
+
+        for neighbour in neighbours:
+            if tiles[neighbour] != 0:
+                graph.add_edge(pos, neighbour)
+
+    return graph
+
+
+def part1(data: TextIO) -> int:
+    graph = read_graph(data)
+
+    return networkx.shortest_path_length(graph, (0, 0), 'O') - 1
+
+
+def part2(data: TextIO) -> int:
+    graph = read_graph(data)
+
+    return networkx.eccentricity(graph, 'O') - 1
diff --git a/2019/aoc2019/intcode.py b/2019/aoc2019/intcode.py
index d00801d..d90fac9 100644
--- a/2019/aoc2019/intcode.py
+++ b/2019/aoc2019/intcode.py
@@ -73,6 +73,9 @@ class Computer:
         while self._execute_current():
             pass
 
+    def get_output(self) -> int:
+        return self.output.popleft()
+
     def _execute_current(self) -> bool:
         """
         Execute a single instruction