From 6669ebfb4c19d19ddcba05dc224a85d17c8abf02 Mon Sep 17 00:00:00 2001
From: Armin Friedl <dev@friedl.net>
Date: Mon, 6 Jan 2025 23:50:24 +0100
Subject: [PATCH] Day 10

---
 day10.py | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 day10.py

diff --git a/day10.py b/day10.py
new file mode 100644
index 0000000..2108e45
--- /dev/null
+++ b/day10.py
@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+
+trail_map = []
+
+with open("day10") as f:
+    for l in f:
+        trail_map.append(list(map(int, l.strip())))
+
+def trail_heads(trail_map):
+    heads = []
+    for y,_ in enumerate(trail_map):
+        for x,_ in enumerate(trail_map[y]):
+            if trail_map[y][x] == 0:
+                heads.append((y,x))
+    return heads
+
+from copy import copy
+
+def check_path_dfs(head, trail_map, part2):
+    visited = set()
+    stack = [(head, [head])]
+
+    def is_height(y,x,h):
+        if x<0 or x>=len(trail_map[0]):
+            return False
+        if y<0 or y>=len(trail_map[0]):
+            return False
+        else:
+            return trail_map[y][x] == h
+
+    trails = []
+    visited_trails = []
+    while stack:
+        el_tr = stack.pop()
+        el = el_tr[0]
+        tr = el_tr[1]
+
+        if part2 and tr in visited_trails:
+            continue
+
+        if part2: visited_trails.append(tr)
+        if not part2: visited.add(el)
+
+        el_height = trail_map[el[0]][el[1]]
+
+        if el_height == 9:
+            trails.append(tr)
+            continue
+
+        up =    (el[0]-1 , el[1])
+        down =  (el[0]+1 , el[1])
+        left =  (el[0]   , el[1]-1)
+        right = (el[0]   , el[1]+1)
+
+        up_ok = up not in visited and is_height(*up, el_height+1)
+        down_ok = down not in visited and is_height(*down, el_height+1)
+        left_ok = left not in visited and is_height(*left, el_height+1)
+        right_ok = right not in visited and is_height(*right, el_height+1)
+
+        if up_ok: stack.append( (up, copy(tr)+[up]) )
+        if down_ok: stack.append( (down, copy(tr)+[down]) )
+        if left_ok: stack.append( (left, copy(tr)+[left]) )
+        if right_ok: stack.append( (right, copy(tr)+[right]) )
+
+    return trails
+
+
+heads = trail_heads(trail_map)
+
+res = 0
+for head in heads:
+    paths = check_path_dfs(head, trail_map, part2=False)
+    res += len(paths)
+print(res)
+
+res = 0
+for head in heads:
+    paths = check_path_dfs(head, trail_map, part2=True)
+    res += len(paths)
+print(res)
+