Do day 18.

lib.py

@@ -31,6 +31,8 @@ class Grid2D:
     NE = (-1, 1)
     SE = (1, 1)
     SW = (1, -1)
+    COORDS_ORTH = (N, E, S, W)
+    COORDS_DIAG = (NW, NE, SE, SW)
 
     def __init__(self, text: str):
         lines = [line for line in text.splitlines() if line.strip() != ""]
@@ -222,3 +224,12 @@ class A_Star(object):
                     g_score[neighbor] = tentative_g_score
                     f_score = g_score[neighbor] + h(neighbor)
                     heapq.heappush(open_set, (f_score, neighbor))
+
+def shoelace_area(corners):
+    n = len(corners)
+    area = 0
+    for i in range(n):
+        x1, y1 = corners[i]
+        x2, y2 = corners[(i + 1) % n]
+        area += (x1 * y2) - (x2 * y1)
+    return abs(area) / 2.0