2021 day 3

2021/3/day3.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+from collections import Counter
+from functools import reduce
+import fileinput
+
+def tonumber(num_list):
+    return reduce(lambda number, digit: (number << 1) + digit, num_list)
+
+matrix = sorted(map(lambda line: list(map(int, line.strip())), fileinput.input()))
+zipped_matrix = list(zip(*matrix))
+gamma = tonumber([numbers.count(1) >= len(numbers) / 2 for numbers in zipped_matrix])
+epsilon = ~gamma & (2 ** len(zipped_matrix) - 1)
+print(gamma * epsilon)
+
+def binary_search(criteria):
+    min, max = 0, len(matrix)
+    i = 0
+    while (max - min) > 1 and i < len(zipped_matrix):
+        slice = zipped_matrix[i][min:max]
+        zeroes, ones = slice.count(0), slice.count(1)
+        if criteria(zeroes, ones):
+            min += zeroes
+        else:
+            max -= ones
+        i += 1
+    assert (max - min) == 1
+    return tonumber(matrix[min])
+
+oxygen = binary_search(lambda zeroes, ones: ones >= zeroes)
+co2 = binary_search(lambda zeroes, ones: ones < zeroes)
+print(oxygen * co2)

README.md

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