From c45449b1a1cd0eee0c481e1a4f784686e44bb49d Mon Sep 17 00:00:00 2001
From: Tomasz Kramkowski <tk@the-tk.com>
Date: Fri, 3 Dec 2021 09:05:58 +0000
Subject: day 3

---
 3.py | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 3.py

(limited to '3.py')

diff --git a/3.py b/3.py
new file mode 100644
index 0000000..32596ef
--- /dev/null
+++ b/3.py
@@ -0,0 +1,48 @@
+from typing import Generator
+from collections.abc import Iterable
+from collections import Counter
+
+Bits = tuple[bool, ...]
+Input = list[Bits]
+
+def column(rows: Iterable[Bits], n: int) -> Generator[bool, None, None]:
+    for row in rows:
+        yield row[n]
+
+def most_common(bits: Iterable[bool]) -> bool:
+    freqs: list[tuple[bool, int]] = Counter(bits).most_common()
+    if freqs[0][1] == freqs[1][1]:
+        return True
+    return freqs[0][0]
+
+def bits_to_int(bits: Bits) -> int:
+    ret: int = 0
+    for i, b in enumerate(reversed(bits)):
+        if b:
+            ret += 2 ** i
+    return ret
+
+def part1(nums: Input) -> int:
+    nbits: int = len(nums[0])
+    gamma: Bits = tuple(most_common(column(inp, i)) for i in range(nbits))
+    epsilon: Bits = tuple(not bit for bit in gamma)
+    return bits_to_int(gamma) * bits_to_int(epsilon)
+
+def part2_impl(nums: Input, complement: bool, bit: int = 0) -> Bits:
+    if len(nums) == 1:
+        return nums[0]
+    target: bool = most_common(column(nums, bit))
+    if complement:
+        target = not target
+    nums = list(filter(lambda n: n[bit] == target, nums))
+    return part2_impl(nums, complement, bit + 1)
+
+def part2(nums: Input) -> int:
+    oxygen: Bits = part2_impl(nums, False)
+    co2: Bits = part2_impl(nums, True)
+    return bits_to_int(oxygen) * bits_to_int(co2)
+
+inp: Input = [tuple(bool(int(c)) for c in l.strip()) for l in open('3.in')]
+
+print(part1(inp))
+print(part2(inp))
-- 
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