Day 7 2025

4 files changed, 42 insertions, 3 deletions

diff --git a/2025/07/a.py b/2025/07/a.py
index 5fdb11b..0e13b75 100644
--- a/2025/07/a.py
+++ b/2025/07/a.py
@@ -1,6 +1,19 @@
 import fileinput
 
 with fileinput.input() as lines:
-        for line in lines:
-                ...
+        rows = [line[:-1] for line in lines]
+        pos = [rows[0].find('S')]
 
+s = 0
+for row in rows[1:]:
+        newpos = set()
+        for p in pos:
+                if row[p] == '^':
+                        s += 1
+                        newpos.add(p-1)
+                        newpos.add(p+1)
+                else:
+                        newpos.add(p)
+        pos = list(newpos)
+
+print(s)
diff --git a/2025/07/b.py b/2025/07/b.py
new file mode 100644
index 0000000..41c0db0
--- /dev/null
+++ b/2025/07/b.py
@@ -0,0 +1,15 @@
+import fileinput
+
+with fileinput.input() as lines:
+        rows = [line[:-1] for line in lines]
+        pos = {rows[0].find('S'): 1}
+
+for row in rows[1:]:
+        newpos = {}
+        for p, w in pos.items():
+                for pp in [p-1,p+1] if row[p] == '^' else [p]:
+                        t = newpos.get(pp, 0)
+                        newpos[pp] = t + w
+        pos = dict(newpos)
+
+print(sum(pos.values()))
diff --git a/2025/README.md b/2025/README.md
index 548deb3..c859ec8 100644
--- a/2025/README.md
+++ b/2025/README.md
@@ -11,6 +11,7 @@ Example
 
 ```
 Day   -Part 1-   -Part 2-
+  7   00:05:27   00:20:40
   6   00:13:38   01:49:24
   5   00:04:39   00:22:19
   4   00:25:36   00:27:49
@@ -85,3 +86,13 @@ smart, very accomplishment.
 
 This year so far the only problems that took me more than 30 minutes
 are this and the first one, not exactly the hardest problems imaginable.
+
+### Day 7: Laboratories
+
+This was quite fun! For part 1, I iterate over the rows of the diagram
+keeping a list of the position currently occupied by a tachyon. I use a
+Python set to avoid duplicates. Part 2 is very similar, but the set is
+changed to a map where the keys are the positions and the values are the
+number of multiverses where a tachyon is in that position.  To update
+this value for the current row, I sum the values of all tachyons that
+end there from the previous row (that can be one or two tachyons).
diff --git a/README.md b/README.md
index 8b83987..d1ca637 100644
--- a/README.md
+++ b/README.md
@@ -14,4 +14,4 @@ See `year/README.md` for instructions on how to run my code.
 |2022|  50 | Rust   | Done in 2025 to learn Rust |
 |2023|  50 | C      | All solved by December 25, 2023 |
 |2024|  50 | C++    | Each solved within 24h |
-|2025|  12 | Python | Work in progress... |
+|2025|  14 | Python | Work in progress... |