2021 day3/python: add solution

2021/data/day3.expected

@@ -0,0 +1,2 @@
+3895776
+7928162

2021/day3/day3.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+
+import sys
+from collections import Counter
+
+def most_common(ns, pos):
+    c = Counter(n[pos] for n in ns)
+    num_zeros = c['0']
+    num_ones = c['1']
+    if num_ones >= num_zeros:
+        return '1'
+    else:
+        return '0'
+
+def least_common(ns, pos):
+    return '0' if most_common(ns, pos) == '1' else '1'
+
+def find_criteria(ns, criteria):
+    ns = ns
+    i = 0
+    while len(ns) > 1:
+        x = criteria(ns, i)
+        ns = [n for n in ns if n[i] == x]
+        i += 1
+    return ns[0]
+
+if __name__ == '__main__':
+    lines = [line.strip() for line in sys.stdin.readlines()]
+
+    line_len = len(lines[0])
+    gamma = ''.join(most_common(lines, n) for n in range(line_len))
+    epsilon = ''.join(least_common(lines, n) for n in range(line_len))
+    print(int(gamma, 2) * int(epsilon, 2))
+
+    oxygen_rating = find_criteria(lines, most_common)
+    co2_rating = find_criteria(lines, least_common)
+
+    print(int(oxygen_rating, 2) * int(co2_rating, 2))