Day 15

day15/part2.py

@@ -1,35 +1,26 @@
 #!/usr/bin/env python3
 import re
+from collections import defaultdict
 
 lines = (x.strip() for x in open("input.txt"))
-lines = [tuple(map(int, re.search(r"(-?\d+).+?(-?\d+).+?(-?\d+).+?(-?\d+)", line).groups())) for line in lines]
-
-for target in range(0, 4000000): #3186981
-    if target % 40000 == 0:
-        print(target/4000000)
-    intervals = []
-    known_targets = set()
+# all lines of possible positions
+# mx+b; tl/br have m=1; tr/bl have m=-1
+tl, tr, bl, br = set(), set(), set(), set()
+stations = []
 
 for line in lines:
-        x, y, a, b = line
-        dist = abs(x-a) + abs(y-b)
-        width = dist-abs(y-target)
-        if width >= 0:
-            intervals.append((x-width, x+width))
-        if b == target:
-            known_targets.add(a)
-        if y == target:
-            known_targets.add(x)
+    x, y, bx, by = map(int, re.search(r"(-?\d+).+?(-?\d+).+?(-?\d+).+?(-?\d+)", line).groups())
+    dist = abs(x-bx) + abs(y-by) + 1
+    tl.add(y-dist+x)
+    br.add(y+dist+x)
+    bl.add(y+dist-x)
+    tr.add(y-dist-x)
+    stations.append((x, y, dist))
 
+for bu, bd in ((a, b) for a in tl & br for b in tr & bl):
+    # -x+bu=x+bd <=> x=(bd-bu)/2
+    x = (bu-bd)//2
+    y = x+bd
 
-    intervals.sort(key=lambda x: x[0])
-    merged = [intervals[0]]
-    for i in intervals[1:]:
-        if merged[-1][1] >= i[0]:
-            merged[-1] = (merged[-1][0], max(merged[-1][1], i[1]))
-        else:
-            merged.append(i)
-
-    if len(merged) > 1:
-        print(target, merged, (merged[0][1]+1)*4000000+target)
-        break
+    if all(abs(x-sx)+abs(y-sy) >= dist for sx, sy, dist in stations):
+        print(x*4000000+y)