add better day 7

7/b_better.py

@@ -0,0 +1,37 @@
+from collections import defaultdict
+
+with open("input.txt") as f:
+    all_crabs = [int(i) for i in f.read().strip().split(",")]
+
+
+# group all crabs by starting position
+crabs = defaultdict(int)
+for crab in all_crabs:
+    crabs[crab] += 1
+
+
+crabs = dict(crabs)
+tries = []
+
+# pre-calcuate crab movement costs
+crab_cost = []
+cost = 0
+for i in range(0, max(all_crabs) + 1):
+    cost = cost + i
+    crab_cost.append(cost)
+
+# apparently the equation `v = k * (k+1) / 2` is equivalent to the list
+# generated above, however, this approach using a pre-computed list is 10-15%
+# faster in my testing.
+
+for i in range(min(all_crabs), max(all_crabs) + 1):
+    moves = 0
+    for crab, count in crabs.items():
+        more = crab_cost[abs(i - crab)] * count
+        moves += more
+
+    tries.append((i, moves, ))
+
+
+tries.sort(key=lambda x: x[1])
+print(tries[0][1])