1 files changed, 76 insertions, 0 deletions
diff --git a/day15.py b/day15.py
new file mode 100644
index 0000000..90aaed3
--- /dev/null
+++ b/day15.py
@@ -0,0 +1,76 @@
+#!/usr/bin/env python
+
+import numpy as np
+import math
+from heapq import *
+from collections import defaultdict
+
+with open('day15.txt') as data:
+    costs = []
+    for line in data:
+        costs.append(list(line.strip()))
+    costs = np.array(costs).astype(int)
+
+def a_star(costs) -> int:
+    def h(point):
+        m, n = point
+        # approximate with straight line distance between points
+        return math.sqrt((costs.shape[0]-m)**2 + (costs.shape[1]-n)**2)
+
+    def neighbors(point):
+        m, n = point
+        neighboring = {(0, 1), (1, 0), (0, -1), (-1, 0)}
+        return {(m+dx, n+dy) for dx, dy in neighboring
+                if 0<=m+dx<costs.shape[0] and 0<=n+dy<costs.shape[1]}
+
+    def path_sum(parents, current):
+        complete = [current]
+        while current in parents:
+            current = parents[current]
+            if current != (0, 0):
+                complete.insert(0, current)
+        return sum(map(lambda x: costs[x], complete))
+
+    discovered = [(0,0)]
+    parents = {}
+
+    gScore = defaultdict(lambda : math.inf)
+    gScore[(0,0)] = 0
+
+    fScore = defaultdict(lambda : math.inf)
+    fScore[(0,0)] = h((0,0))
+
+    while discovered:
+        if discovered[0] == (goal := (costs.shape[0]-1, costs.shape[1]-1)):
+            return path_sum(parents, goal)
+
+        current = heappop(discovered)
+        for neighbor in neighbors(current):
+            tentative = gScore[current] + costs[neighbor]
+            if tentative < gScore[neighbor]:
+                parents[neighbor] = current
+                gScore[neighbor] = tentative
+                fScore[neighbor] = tentative + h(neighbor)
+                if neighbor not in discovered:
+                    heappush(discovered, neighbor)
+    else:
+        return None
+# part 1
+print(a_star(costs))
+
+# part 2
+new_costs = np.zeros((costs.shape[0] * 5, costs.shape[1] * 5)).astype(int)
+
+def increment(arr):
+    arr += 1
+    arr[arr > 9] = 1
+
+xDim, yDim = costs.shape
+for i in range(5):
+    for j in range(5):
+        copied = np.copy(costs)
+        for _ in range(i+j):
+            increment(copied)
+        new_costs[xDim*i:xDim*(i+1), yDim*j:yDim*(j+1)] = copied
+
+print(a_star(new_costs))