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diff --git a/__pycache__/test_list_ipv4_listed_in_logfiles.cpython-312-pytest-8.4.2.pyc b/__pycache__/test_list_ipv4_listed_in_logfiles.cpython-312-pytest-8.4.2.pyc
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diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..31a72ac
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1764522689,
+        "narHash": "sha256-SqUuBFjhl/kpDiVaKLQBoD8TLD+/cTUzzgVFoaHrkqY=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "8bb5646e0bed5dbd3ab08c7a7cc15b75ab4e1d0f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..9c7af49
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,31 @@
+{
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
+  outputs =
+    { nixpkgs, ... }:
+    {
+      devShells.x86_64-linux =
+        let
+          python = pkgs.python312;
+          pythonEnv = python.withPackages (
+            ps: with ps; [
+              pip
+              ipython
+              pylint
+              black
+
+              ruff
+              jedi-language-server
+              pytest
+            ]
+          );
+          pkgs = import nixpkgs { system = "x86_64-linux"; };
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [
+              pythonEnv
+            ];
+          };
+        };
+    };
+}
diff --git a/main.py b/main.py
index d0445c1..28269ac 100644
--- a/main.py
+++ b/main.py
@@ -3,6 +3,11 @@ This module has no real meaning.
 Simply implemented for evaluating students.
 """
 
+from pathlib import Path
+from itertools import chain
+import re
+import string
+
 def list_filenames_of_log_files(*foldernames) -> list:
     """
     List all the `*.log` files stored in the folders specified by foldernames.
@@ -14,10 +19,21 @@ def list_filenames_of_log_files(*foldernames) -> list:
 
     :raises FileNotFoundError: If one of the speficied folders does not exist
     """
-    return foldernames
+    def walk(folder):
+        if not folder.exists():
+            raise FileNotFoundError(f"{folder} does not exist")
+        for subfold, childfolds, files in folder.walk():
+            for f in files:
+                if ".log" in f:
+                    yield subfold / f
+
+    # final cast as list would generally be omited but due to the test expecting
+    # an indexable object, this cast is need. It would be more idiomatic to return
+    # a generator and iterate over it tho.
+    return list(chain(*[walk(Path(foldername)) for foldername in foldernames]))
 
 
-def list_ipv4_listed_in_logfiles(*foldernames) -> list :
+def list_ipv4_listed_in_logfiles(*foldernames) -> list:
     """
     Parse all the `*.log` files stored in the folders specified by foldernames.
     This method returns a list of ip adresses stored in those files.
@@ -30,10 +46,18 @@ def list_ipv4_listed_in_logfiles(*foldernames) -> list :
 
     :raises FileNotFoundError: If one of the speficied folders does not exist
     """
-    return foldernames
+    def get_ip_from_file(file):
+        matcher = re.compile(r"(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})")
+        with open(file) as f:
+            for line in f:
+                if match := matcher.search(line):
+                    yield match.group()
+    # cast to a set to delete duplicates
+    # cast to a list 'caus the test is hardcoded to get a list
+    return list(set(chain(*[get_ip_from_file(file) for file in list_filenames_of_log_files(*foldernames)])))
 
 
-def cryptarithme(message:str = "SEND + MORE = MONEY") -> list:
+def cryptarithme(message: str = "SEND + MORE = MONEY") -> list:
     """
     The `message` is a formatted mathematical operation.
     Each letter represents a number between 0 and 9.
@@ -64,10 +88,19 @@ def cryptarithme(message:str = "SEND + MORE = MONEY") -> list:
     :raises ValueError: If the string cannot be parsed properly.
     :raises CryptarithmeError : If the string is not a possible Cryptarithme
     """
+
+    # check for syntax
+    for i, character in enumerate(message):
+        if character not in [*string.ascii_uppercase, "+", "=", " "]:
+            raise ValueError(f"illegal character {character} at {i}")
+    if len([i for i in message if i == "="]) >= 2 or len([i for i in message if i == "+"]) >= 2:
+        raise ValueError("too much =")
     return message
 
+class CryptarithmeError(Exception):
+    pass
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     # list_filenames_of_log_files('data')
     # list_ipv4_listed_in_logfiles('data')
     # cryptarithme('A + A +A= B')
diff --git a/test_list_ipv4_listed_in_logfiles.py b/test_list_ipv4_listed_in_logfiles.py
index c619470..7631fc7 100644
--- a/test_list_ipv4_listed_in_logfiles.py
+++ b/test_list_ipv4_listed_in_logfiles.py
@@ -43,4 +43,4 @@ def test_refactoring_has_brought_a_generator():
     with open("main.py") as f:
         lines : str = f.read()
         found = re.search("yield", lines)
-        assert found, "No bonus linked to refactoring to a generator"
\ No newline at end of file
+        assert found, "No bonus linked to refactoring to a generator"