Solve 2017 day 19.

2016/d19.py

@@ -1,18 +1,87 @@
-data = open(0).read().strip()
-elf_count = int(data)
+from dataclasses import dataclass
+from typing import Optional
 
-elfs = [i for i in range(1, elf_count + 1)]
-while len(elfs) > 1:
-    is_odd = len(elfs) % 2 == 1
-    elfs = [elfs[i] for i in range(0, len(elfs), 2)]
-    if is_odd and len(elfs) > 1:
-        elfs = elfs[1:]
-print(elfs[0])
 
-# elfs = [i for i in range(1, elf_count + 1)]
-# current_i = 0
-# while len(elfs) > 1:
-#     remove_i = (current_i + len(elfs) // 2) % len(elfs)
-#     current_i = (current_i + 1) % len(elfs)
-#     elfs.pop(remove_i)
-# print(elfs[0])
+def part_1(data):
+    elf_count = int(data)
+
+    elves = [i for i in range(1, elf_count + 1)]
+    while len(elves) > 1:
+        is_odd = len(elves) % 2 == 1
+        elves = [elves[i] for i in range(0, len(elves), 2)]
+        if is_odd and len(elves) > 1:
+            elves = elves[1:]
+    print(elves[0])
+
+
+def part_2(data):
+    elf_count = int(data)
+    power_3 = 3
+    while power_3 * 3 < elf_count:
+        power_3 *= 3
+    elf_count -= power_3
+    print(elf_count)
+
+
+def main():
+    data = open(0).read().strip()
+    part_1(data)
+    part_2(data)
+
+
+if __name__ == "__main__":
+    main()
+
+
+def part_2_naiv(data):
+    @dataclass
+    class Elf:
+        id: int
+        prev: Optional["Elf"]
+        next: Optional["Elf"]
+
+        def delete(self):
+            if self.prev is not None:
+                self.prev.next = self.next
+            if self.next is not None:
+                self.next.prev = self.prev
+            self.prev = None
+            self.next = None
+
+        def all(self):
+            elf = self
+            elves = [elf]
+            while elf.next is not None and elf.next.id != self.id:
+                elf = elf.next
+                elves.append(elf)
+            return elves
+
+        def forward(self, n):
+            current = self
+            for _ in range(n):
+                assert current.next is not None
+                current = current.next
+                assert current.id != self.id
+            return current
+
+        def __repr__(self):
+            assert self.prev is not None and self.next is not None
+            return f"{self.prev.id} <- {self.id} -> {self.next.id}"
+
+    elf_count = int(data)
+    # elf_count = 5
+    elves = [Elf(i, None, None) for i in range(1, elf_count + 1)]
+    for i in range(len(elves)):
+        elves[i].prev = elves[(i - 1) % len(elves)]
+        elves[i].next = elves[(i + 1) % len(elves)]
+
+    remaining_elves = elf_count
+    current_elf = elves[0]
+    while remaining_elves > 1:
+        assert current_elf is not None
+        # print(current_elf.all())
+        current_elf.forward(remaining_elves // 2).delete()
+        current_elf = current_elf.next
+        remaining_elves -= 1
+    assert current_elf is not None
+    print(current_elf.id)

README.md

@@ -51,7 +51,8 @@ written in Python.
 - Day 16: 14:11
 - Day 17: 45:00 didn't follow instructions... focus!
 - Day 18: 9:43
-- Day 19: 
+- Day 19: 90:00 that wasn't easy for me
+- Day 20: 
 
 
 # 2017