Day06

day06.py

@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+
+m = []
+
+obstacles = set()
+start = (-1,-1)
+
+lx = ly = 0
+with open('day06') as f:
+    for y,l in enumerate(f):
+        ly += 1
+        lx = lx or len(l)
+        for x,ll in enumerate(l):
+            match ll:
+                case '#': obstacles.add((y,x))
+                case '^': start = (y,x)
+
+lookup = {
+    (-1, 0): ( 0, 1),
+    ( 0, 1): ( 1, 0),
+    ( 1, 0): ( 0,-1),
+    ( 0,-1): (-1, 0)
+}
+
+# 1
+def walk(pos, dir):
+    visited = []
+    visited.append((pos, dir))
+    npos = (pos[0]+dir[0], pos[1]+dir[1])
+
+    while True:
+        while npos not in obstacles:
+            pos = npos
+            visited.append((pos,dir))
+            npos = (pos[0]+dir[0], pos[1]+dir[1])
+
+            if npos[0]==ly or npos[0]==-1: return visited
+            if npos[1]==lx or npos[1]==-1: return visited
+
+        dir = lookup[dir]
+        npos = (pos[0]+dir[0], pos[1]+dir[1])
+
+visited = walk(start, (-1,0))
+vpos = set([v[0] for v in visited])
+res1 = len(vpos)
+
+# 2
+def loop(pos, dir, sv, obstacles):
+    while True:
+        npos = (pos[0] + dir[0], pos[1] + dir[1])
+        if npos[0]==ly or npos[0]==-1: return False
+        if npos[1]==lx or npos[1]==-1: return False
+
+        while npos not in obstacles:
+            sv.add((pos,dir))
+
+            pos = npos
+            npos = (pos[0]+dir[0], pos[1]+dir[1])
+
+            if (pos, dir) in sv:
+                return True
+
+            if npos[0]==ly or npos[0]==-1: return False
+            if npos[1]==lx or npos[1]==-1: return False
+
+        dir = lookup[dir]
+
+done = set()
+res2 = 0
+for k,(pos,_) in enumerate(visited):
+    if pos == start: continue
+    if pos in obstacles: continue
+    if pos in done: continue
+
+    done.add(pos)
+
+    start = visited[k-1][0]
+    dir = visited[k-1][1]
+    sv = set(visited[:k])
+    if loop(start, dir, sv, obstacles | {pos}):
+        res2 += 1