From b9bafb8746a19a85713cbee8a20b2ca39eca3fcb Mon Sep 17 00:00:00 2001
From: Lachlan Meyer <lachlanmeyer@gmail.com>
Date: Fri, 20 Dec 2024 17:18:45 +1100
Subject: [PATCH] 20-1

We don't need to calculate subpaths every time, just splice the original shortest path

20-1 optimisations
---
 2024/20/20-1.py       | 55 +++++++++++++++++++++++++++++++++++++++++++
 2024/20/testinput.txt | 15 ++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 2024/20/20-1.py
 create mode 100644 2024/20/testinput.txt

diff --git a/2024/20/20-1.py b/2024/20/20-1.py
new file mode 100644
index 0000000..4f3f418
--- /dev/null
+++ b/2024/20/20-1.py
@@ -0,0 +1,55 @@
+import networkx as nx
+
+map = set()
+directions = [(1, 0), (0, 1), (-1, 0), (0, -1)]
+
+minimumStepsSaved = 100
+picosecondsCheatable = 2
+totalGoodCheats = 0
+
+with open('2024/20/input.txt') as f:
+    g = nx.DiGraph()
+    for j, line in enumerate(f):
+        line = line.strip()
+        for i, char in enumerate(line):
+            if char == '#':
+                continue
+
+            if char == 'E':
+                e = (i, j)
+            if char == 'S':
+                s = (i, j) # reindeer starts pointing west
+
+            map.add((i,j))
+            
+    for node in map:
+        x, y = node
+        for direction in directions:
+            dirX, dirY = direction
+            neighbour = (x + dirX, y + dirY)
+            
+            if neighbour in map: 
+                g.add_edge(node, neighbour, weight=1)
+
+
+    shortestPath = nx.shortest_path(g, s, e, weight='weight')
+
+    print(shortestPath)
+
+    # new idea since brute force blows
+    # try find nodes with manhattan distance of 2, subtract the steps between these two points
+    # the time saved should be this subpath's distance, no?
+    for i, node in enumerate(shortestPath):
+        for j, otherNode in enumerate(shortestPath):
+            if i > j or node == otherNode:
+                continue
+
+            manhattanDistance = abs(node[0] - otherNode[0]) + abs(node[1] - otherNode[1])
+
+            if 2 <= manhattanDistance <= picosecondsCheatable:
+                newPath = shortestPath[:i+1] + shortestPath[j-1:] # gotta keep the start and end in the original path
+                stepsSaved = len(shortestPath) - len(newPath)
+                if stepsSaved >= minimumStepsSaved:
+                    totalGoodCheats += 1
+
+print(totalGoodCheats)
\ No newline at end of file
diff --git a/2024/20/testinput.txt b/2024/20/testinput.txt
new file mode 100644
index 0000000..f107d40
--- /dev/null
+++ b/2024/20/testinput.txt
@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############
\ No newline at end of file