2021 day7/python: add faster solution

2021/day7/day7_fast.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python
+
+import sys
+
+if __name__ == '__main__':
+    positions = [int(i) for i in sys.stdin.readline().split(',')]
+
+    def calc(starting_guess, distance_cost):
+        def target_cost(target):
+            return sum(distance_cost(abs(position-target)) for position in positions)
+
+        target = starting_guess(positions)
+        current_cost = target_cost(target)
+
+        for i in (-1, 1):
+            while (next_cost := target_cost(target+i)) < current_cost:
+                current_cost = next_cost
+                target += i
+
+        return current_cost
+
+    positions.sort()
+    print(calc(
+        lambda positions: positions[len(positions)//2],
+        lambda n: n
+    ))
+    print(calc(
+        lambda positions: int(sum(positions)/len(positions)),
+        lambda n: n * (n+1)//2
+    ))