Do day 13 and add cleaner solution to day 18.

d18.py

@@ -51,8 +51,7 @@ def solve(i: Input, second=False):
                 to_visit.append(nc)
     return len(set(coords))
 
-
-def calc(ins):
+def area(ins):
     c = (0, 0)
     corners = []
     for i, (count, d) in enumerate(ins):
@@ -86,6 +85,20 @@ def calc(ins):
     return int(shoelace_area(corners))
 
 
+def area2(ins):
+    # Solution based on Shoelace area and the idea that the outside area is
+    # perimate * 1 // 2, and then +1 (for four quarter corners). All other
+    # corners cancel out to zero.
+    c = (0, 0)
+    corners = []
+    perimeter = 0
+    for i, (count, d) in enumerate(ins):
+        c = (c[0] + count * d[0], c[1] + count * d[1])
+        corners.append(c)
+        perimeter += count
+    return int(shoelace_area(corners)) + perimeter // 2 + 1
+
+
 def solve2(i: Input, second=False):
     lines = i.lines()
     ins = []
@@ -97,7 +110,8 @@ def solve2(i: Input, second=False):
         d = int(c[:5], 16)
         m = {"0": Grid2D.E, "1": Grid2D.S, "2": Grid2D.W, "3": Grid2D.N}[c[5]]
         ins.append((d, m))
-    return calc(ins)
+    assert area(ins) == area2(ins)
+    return area(ins)
 
 
 def debug():
@@ -113,7 +127,7 @@ def debug():
     ]
     # Should be 14 but is 2 because it's going counter-clockwise, but mapping
     # only works for clockwise.
-    print(calc(ins))
+    print(area(ins))
 
 
 def main():