Day 16

16.py

@@ -0,0 +1,46 @@
+import string
+from functools import partial
+
+
+def spin(amount, line):
+    return line[-amount:] + line[:-amount]
+
+
+def exchange(p1, p2, line):
+    line[p1], line[p2] = line[p2], line[p1]
+    return line
+
+
+def partner(n1, n2, line):
+    p1, p2 = line.index(n1), line.index(n2)
+    line[p1], line[p2] = line[p2], line[p1]
+    return line
+
+
+def dance(reps, moves):
+    line = list(string.ascii_lowercase[:16])
+    # Maintain a history to try and find cycles. If we ever generate the same line twice
+    # then all of the remainder will just cycle between those two lines.
+    history = []
+    for i in range(reps):
+        for move in moves:
+            line = move(line)
+        if line in history:
+            start = history.index(line)
+            cycle = len(history) - start
+            return ''.join(history[start + (reps - i - 1) % cycle])
+        history.append(line[:])  # NB: Copy of the line, so it doesn't get mutated next cycle...
+    return ''.join(line)
+
+
+instructions = {
+    's': lambda r: partial(spin, int(r)),
+    'x': lambda r: partial(exchange, *map(int, r.split('/'))),
+    'p': lambda r: partial(partner, *r.split('/'))
+}
+
+with open('data/16.txt', 'r') as file:
+    # Pre-parse the list of moves into function calls to save string-fiddling lots of times
+    moves = list(map(lambda line: instructions[line[0]](line[1:]), file.readline().strip().split(',')))
+    print(f'Part one: {dance(1, moves)}')
+    print(f'Part two: {dance(1000000000, moves)}')