Start solving 2018 problems

I have also updated get.py to download the problems as
`d<day>.txt` instead of `i<day>.txt`. That allows me
to get the day input via `__input__.replace('.py', '.txt')`
which is a little more concise. I don't know why
I didn't do this earlier.

2018/d4.py

@@ -0,0 +1,67 @@
+from lib import str_to_ints
+from collections import defaultdict
+
+
+def part_1(data):
+    shifts = sorted(data.splitlines())
+    guards = defaultdict(list)
+    guard = None
+    for s in shifts:
+        try:
+            _, _, _, h, m, guard = str_to_ints(s)
+        except ValueError:
+            _, _, _, h, m = str_to_ints(s)
+            assert guard is not None
+            guards[guard].append((h, m))
+
+    guard_sleep_times = {}
+    guard_heat_maps = {}
+    for guard, times in guards.items():
+        heat_map = [0 for _ in range(60)]
+        total_time = 0
+        start = None
+        for h, m in times:
+            if start is None:
+                start = (h, m)
+            else:
+                delta = (h - start[0]) * 60 + (m - start[1])
+                total_time += delta
+                for t in range(start[1], m):
+                    heat_map[t] += 1
+                start = None
+        guard_sleep_times[guard] = total_time
+        guard_heat_maps[guard] = heat_map
+
+    max_guard, max_time = None, 0
+    for guard, time in guard_sleep_times.items():
+        if time > max_time:
+            max_guard = guard
+            max_time = time
+
+    assert type(max_guard) is int
+    max_minute, max_asleep = 0, 0
+    for minute, asleep in enumerate(guard_heat_maps[max_guard]):
+        if asleep > max_asleep:
+            max_minute = minute
+            max_asleep = asleep
+    print(max_guard * max_minute)
+
+    max_guard, max_minute, max_asleep = 0, 0, 0
+    for guard in guard_sleep_times.keys():
+        for minute, asleep in enumerate(guard_heat_maps[guard]):
+            if asleep > max_asleep:
+                max_minute = minute
+                max_asleep = asleep
+                max_guard = guard
+    print(max_guard * max_minute)
+
+
+def main():
+    input_file = __file__.replace(".py", ".txt")
+    with open(input_file) as f:
+        data = f.read()
+    part_1(data)
+
+
+if __name__ == "__main__":
+    main()