Day 15, probably not optimised at all...

2021/day15.py

@@ -0,0 +1,53 @@
+from collections import defaultdict
+from math import inf
+
+with open("input15.txt") as f:
+    lines = [line.rstrip() for line in f]
+plan = [list(map(int, line)) for line in lines]
+dim = len(plan)
+ndim = dim * 5
+
+
+# wanted to try something other than the yearly dijkstra, so overkill and
+# incomplete bellman-ford, precedents skipped for laughable gains.
+def solve(dimension, risk_f):
+    dists = defaultdict(lambda: inf)
+    dists[(0, 0)] = 0
+    changed = True
+    while changed:
+        changed = False
+        for x in range(dimension):
+            for y in range(dimension):
+                p = (x, y)
+                dist = dists[p]
+                near = []
+                if x > 0:
+                    near.append((x - 1, y))
+                if x < dimension - 1:
+                    near.append((x + 1, y))
+                if y > 0:
+                    near.append((x, y - 1))
+                if y < dimension - 1:
+                    near.append((x, y + 1))
+                for n in near:
+                    nx, ny = n
+                    risk = risk_f(nx, ny)
+                    if dist + risk < dists[n]:
+                        changed = True
+                        dists[n] = dist + risk
+    return dists
+
+
+def risk_f1(nx, ny):
+    return plan[nx][ny]
+
+
+def risk_f2(nx, ny):
+    risk = (plan[nx % dim][ny % dim] + nx // dim + ny // dim)
+    while risk >= 10:
+        risk -= 9
+    return risk
+
+
+print(solve(dim, risk_f1)[(dim - 1, dim - 1)])
+print(solve(ndim, risk_f2)[(ndim - 1, ndim - 1)])