Day 4 2025

3 files changed, 54 insertions, 1 deletions

diff --git a/2025/04/a.py b/2025/04/a.py
index 5fdb11b..a693841 100644
--- a/2025/04/a.py
+++ b/2025/04/a.py
@@ -1,6 +1,21 @@
 import fileinput
 
+def neigh(a, i, j):
+        return [
+                a[i-1][j-1], a[i-1][j], a[i-1][j+1],
+                a[i][j-1],              a[i][j+1],
+                a[i+1][j-1], a[i+1][j], a[i+1][j+1],
+        ].count('@')
+
+s = 0
 with fileinput.input() as lines:
+        a = [['.'] * 200]
         for line in lines:
-                ...
+                a.append(['.'] + list(line.replace('\n', '.')))
+        a.append(['.'] * 200)
 
+for i in range(1,len(a)-1):
+        for j in range(1,len(a[1])-1):
+                if a[i][j] == '@' and neigh(a, i, j) < 4:
+                        s += 1
+print(s)
diff --git a/2025/04/b.py b/2025/04/b.py
new file mode 100644
index 0000000..93b1fff
--- /dev/null
+++ b/2025/04/b.py
@@ -0,0 +1,26 @@
+import fileinput
+
+def neigh(a, i, j):
+        return [
+                a[i-1][j-1], a[i-1][j], a[i-1][j+1],
+                a[i][j-1],              a[i][j+1],
+                a[i+1][j-1], a[i+1][j], a[i+1][j+1],
+        ].count('@')
+
+s = 0
+with fileinput.input() as lines:
+        a = [['.'] * 200]
+        for line in lines:
+                a.append(['.'] + list(line.replace('\n', '.')))
+        a.append(['.'] * 200)
+
+rem = True
+while rem:
+        rem = False
+        for i in range(1,len(a)-1):
+                for j in range(1,len(a[1])-1):
+                        if a[i][j] == '@' and neigh(a, i, j) < 4:
+                                s += 1
+                                rem = True
+                                a[i][j] = '.'
+print(s)
diff --git a/2025/README.md b/2025/README.md
index c72c8a5..8fccef4 100644
--- a/2025/README.md
+++ b/2025/README.md
@@ -11,6 +11,7 @@ Example
 
 ```
 Day   -Part 1-   -Part 2-
+  4   00:25:36   00:27:49
   3   00:05:02   00:16:09
   2   00:10:36   00:11:55
   1   00:06:20   00:50:41
@@ -49,3 +50,14 @@ For part 2 I used recursion with memorization (using Python's
 `functools.cache`), which is fast enough. But later Chiara pointed
 out to me that actually the solution is quite trivially greedy; I
 implemented the greedy version in `b-alt.py`.
+
+### Day 4: Printing Department
+
+The first "map" problem of the year! This one was easy, but I
+made a lot of mistakes in part 1. I decided to use the trick that
+[Jared](https://guissmo.com) suggested a couple of years ago: extend the
+map by 1 cell in all directions so you don't have to deal with indices
+out of bounds.
+
+For part 2 I decided to quickly code the dumb "repeat part 1 until no
+rolls are removed" strategy and it worked.