2019 day 22 part 1

2019/d22.py

@@ -0,0 +1,53 @@
+from lib import get_data, str_to_ints, mod_inverse
+from math import gcd
+
+data = get_data(__file__)
+deck = list(range(10007))
+
+for line in data.splitlines():
+    if "new stack" in line:
+        deck = list(reversed(deck))
+    elif "cut" in line:
+        n, = str_to_ints(line)
+        deck = deck[n:] + deck[:n]
+    elif "increment" in line:
+        new_deck = [-1] * len(deck)
+        pos = 0
+        n, = str_to_ints(line)
+        deck = list(reversed(deck))
+        while deck:
+            new_deck[pos] = deck.pop()
+            pos = (pos + n) % len(new_deck)
+        deck = new_deck
+    else:
+        assert False
+
+print(deck.index(2019))
+
+# idea: just follow the index of the target card
+len = 119315717514047
+len = 10007
+index = 2019
+
+
+for i in range(101741582076661):
+    for line in data.splitlines():
+        if "new stack" in line:
+            index = len - (index + 1)
+        elif "cut" in line:
+            cut, = str_to_ints(line)
+            cut = (len + cut) % len
+            if index >= cut:
+                index = index - cut
+            else:
+                index = (len - cut) + index
+        elif "increment" in line:
+            n, = str_to_ints(line)
+            assert gcd(n, len) == 1
+            new_index = (n * index) % len
+            m = mod_inverse(n, len)
+            assert (new_index * m) % len == index
+            index = new_index
+    break
+
+print(index)