From 0dcf0cb22cd4ac210677cae7ea16df88c2b878bb Mon Sep 17 00:00:00 2001
From: Gabriel Augendre <gabriel@augendre.info>
Date: Sun, 13 Dec 2020 09:41:21 +0100
Subject: [PATCH] Solve day 13 part 1 and nearly 2

---
 2020/day13_bus.py       | 69 +++++++++++++++++++++++++++++++++++++++++
 2020/inputs/day13       |  2 ++
 2020/inputs/day13-test1 |  2 ++
 2020/inputs/day13-test2 |  2 ++
 2020/inputs/day13-test3 |  2 ++
 2020/inputs/day13-test4 |  2 ++
 2020/inputs/day13-test5 |  2 ++
 2020/inputs/day13-test6 |  2 ++
 8 files changed, 83 insertions(+)
 create mode 100644 2020/day13_bus.py
 create mode 100644 2020/inputs/day13
 create mode 100644 2020/inputs/day13-test1
 create mode 100644 2020/inputs/day13-test2
 create mode 100644 2020/inputs/day13-test3
 create mode 100644 2020/inputs/day13-test4
 create mode 100644 2020/inputs/day13-test5
 create mode 100644 2020/inputs/day13-test6

diff --git a/2020/day13_bus.py b/2020/day13_bus.py
new file mode 100644
index 0000000..d1a4877
--- /dev/null
+++ b/2020/day13_bus.py
@@ -0,0 +1,69 @@
+import enum
+import functools
+import math
+from typing import List, Dict
+
+
+def main(filename: str, expected_part_1: int = None, expected_part_2: int = None):
+    print(f"\n+ Running on {filename}")
+    with open(filename) as f:
+        notes = f.read().strip().split("\n")
+
+    counter_part_1 = solve_part_1(notes)
+
+    print(f"1. Found {counter_part_1}")
+    if expected_part_1:
+        assert expected_part_1 == counter_part_1
+
+    counter_part_2 = solve_part_2(notes)
+    print(f"2. Found {counter_part_2}")
+    if expected_part_2:
+        assert expected_part_2 == counter_part_2
+
+
+def solve_part_1(notes) -> int:
+    earliest = int(notes[0])
+    buses = map(int, filter(lambda bus: bus != "x", notes[1].split(",")))
+    min_delta = math.inf
+    min_bus = None
+    for bus in buses:
+        delta = math.ceil(earliest / bus) * bus - earliest
+        if delta < min_delta:
+            min_delta = delta
+            min_bus = bus
+
+    return min_bus * min_delta
+
+
+def solve_part_2(notes) -> int:
+    """Works but far too slow for the real deal."""
+    split = notes[1].split(",")
+    buses = list(map(int, filter(lambda bus: bus != "x", split)))
+    minutes = {}  # type: Dict[int, int]
+    biggest_bus = max(buses)
+    biggest_bus_index = split.index(str(biggest_bus))
+
+    for bus in buses:
+        minutes[split.index(str(bus)) - biggest_bus_index] = bus
+
+    found = False
+    timestamp = 99999999999628
+    while not found:
+        timestamp += biggest_bus
+        found = True
+        for delta, bus in minutes.items():
+            if (timestamp + delta) % bus != 0:
+                found = False
+                break
+
+    return timestamp + min(minutes.keys())
+
+
+if __name__ == "__main__":
+    main("inputs/day13-test1", 295, 1068781)
+    main("inputs/day13-test2", None, 3417)
+    main("inputs/day13-test3", None, 754018)
+    main("inputs/day13-test4", None, 779210)
+    main("inputs/day13-test5", None, 1261476)
+    main("inputs/day13-test6", None, 1202161486)
+    main("inputs/day13", 4315)
diff --git a/2020/inputs/day13 b/2020/inputs/day13
new file mode 100644
index 0000000..6d4ea89
--- /dev/null
+++ b/2020/inputs/day13
@@ -0,0 +1,2 @@
+1001938
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,431,x,x,x,x,x,x,x,23,x,x,x,x,13,x,x,x,17,x,19,x,x,x,x,x,x,x,x,x,x,x,863,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,29
diff --git a/2020/inputs/day13-test1 b/2020/inputs/day13-test1
new file mode 100644
index 0000000..e473080
--- /dev/null
+++ b/2020/inputs/day13-test1
@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19
\ No newline at end of file
diff --git a/2020/inputs/day13-test2 b/2020/inputs/day13-test2
new file mode 100644
index 0000000..7bcbba0
--- /dev/null
+++ b/2020/inputs/day13-test2
@@ -0,0 +1,2 @@
+939
+17,x,13,19
\ No newline at end of file
diff --git a/2020/inputs/day13-test3 b/2020/inputs/day13-test3
new file mode 100644
index 0000000..eb10fd9
--- /dev/null
+++ b/2020/inputs/day13-test3
@@ -0,0 +1,2 @@
+939
+67,7,59,61
\ No newline at end of file
diff --git a/2020/inputs/day13-test4 b/2020/inputs/day13-test4
new file mode 100644
index 0000000..9e3ef37
--- /dev/null
+++ b/2020/inputs/day13-test4
@@ -0,0 +1,2 @@
+939
+67,x,7,59,61
\ No newline at end of file
diff --git a/2020/inputs/day13-test5 b/2020/inputs/day13-test5
new file mode 100644
index 0000000..355d02c
--- /dev/null
+++ b/2020/inputs/day13-test5
@@ -0,0 +1,2 @@
+939
+67,7,x,59,61
\ No newline at end of file
diff --git a/2020/inputs/day13-test6 b/2020/inputs/day13-test6
new file mode 100644
index 0000000..fd0a4d0
--- /dev/null
+++ b/2020/inputs/day13-test6
@@ -0,0 +1,2 @@
+939
+1789,37,47,1889
\ No newline at end of file