From 79c8fe048a7043b77b8ca4ce2be0f6d4f736bb48 Mon Sep 17 00:00:00 2001
From: FaultyBranches <contact@faultybranches.net>
Date: Fri, 5 Dec 2025 17:06:59 -0600
Subject: [PATCH] feat: Actually day 4 of '25

---
 25/3/main.py      |  3 +-
 25/4/main.py      | 78 +++++++++++++++++++++++++++++++++++++++++++++++
 25/4/test_main.py | 40 ++++++++++++++++++++++++
 3 files changed, 119 insertions(+), 2 deletions(-)
 create mode 100644 25/4/main.py
 create mode 100644 25/4/test_main.py

diff --git a/25/3/main.py b/25/3/main.py
index 6f2a777..bb0a66c 100644
--- a/25/3/main.py
+++ b/25/3/main.py
@@ -26,9 +26,8 @@ def get_highest_dynamic(bank_size: int, input: str) -> int:
     for i in range(bank_size, 0, -1):
         curr_highest = 0
         highest_index = 0
-        limit = len(input) - i + 1
 
-        for j in range(curr_index, limit):
+        for j in range(curr_index, len(input) - i + 1):
             found = int(input[j])
 
             if found > curr_highest:
diff --git a/25/4/main.py b/25/4/main.py
new file mode 100644
index 0000000..010ca52
--- /dev/null
+++ b/25/4/main.py
@@ -0,0 +1,78 @@
+def num_neighbours(lines: [], line_index: int, pos_index: int, num_lines: int, line_length: int) -> int:
+    '''Check surrounding and current row neighbours of a given position.
+    Does not operate on 2d coords directly, but on index and current versus prev/next lines.
+    '''
+    total = 0
+
+    for ii in range(max(0, line_index - 1), min(num_lines, line_index + 2)):
+        for jj in range(max(0, pos_index - 1), min(line_length, pos_index + 2)):
+            if ii == line_index and jj == pos_index:
+                continue
+
+            if lines[ii][jj] == '@':
+                total += 1
+
+    return total
+
+def process_line(lines: []) -> int:
+    '''Iterate through all lines in the map and determine which can be removed this time around'''
+    total = 0
+
+    for line_index, line in enumerate(lines):
+        for index, char in enumerate(line):
+            if char == '@':
+                total_neighbours = num_neighbours(lines, line_index, index, len(lines), len(line))
+
+                if total_neighbours < 4:
+                    total += 1
+
+    return total
+
+
+def find_all_accessible_rolls(lines: []) -> int:
+    '''Recursively iterate through removing paper rolls and see how many are opened up for removal after each pass until none can be removed'''
+    total = 0
+
+    # Copy the map to note which have been removed without affecting the current checks
+    lines_copy = lines
+
+    for line_index, line in enumerate(lines):
+        for index, char in enumerate(line):
+            if char == '@':
+                total_neighbours = 0
+
+                for ii in range(max(0, line_index - 1), min(len(lines), line_index + 2)):
+                    for jj in range(max(0, index - 1), min(len(line), index + 2)):
+                        if ii == line_index and jj == index:
+                            continue
+
+                        if lines[ii][jj] == '@':
+                            total_neighbours += 1
+
+                if total_neighbours < 4:
+                    total += 1
+                    temp_line = list(lines_copy[line_index])
+                    temp_line[index] = '.'
+                    lines_copy[line_index] = ''.join(temp_line)
+
+    if total != 0:
+        return total + find_all_accessible_rolls(lines_copy)
+
+    return total
+
+
+def main():
+    '''Entrypoint'''
+    with open('input.txt', encoding='utf-8') as fh:
+        all_lines = fh.readlines()
+        part1_total = process_line(all_lines)
+
+        print(f'Part 1: {part1_total}')
+
+        part2_total = find_all_accessible_rolls(all_lines)
+
+        print(f'Part 2: {part2_total}')
+
+
+if __name__ == "__main__":
+    main()
diff --git a/25/4/test_main.py b/25/4/test_main.py
new file mode 100644
index 0000000..c44c7af
--- /dev/null
+++ b/25/4/test_main.py
@@ -0,0 +1,40 @@
+import pytest
+
+from main import process_line, find_all_accessible_rolls
+
+@pytest.mark.parametrize('lines, expected',
+                         [
+                            ([
+                                '..@@.@@@@.',
+                                '@@@.@.@.@@',
+                                '@@@@@.@.@@',
+                                '@.@@@@..@.',
+                                '@@.@@@@.@@',
+                                '.@@@@@@@.@',
+                                '.@.@.@.@@@',
+                                '@.@@@.@@@@',
+                                '.@@@@@@@@.',
+                                '@.@.@@@.@.'
+                            ], 13),
+                         ])
+def test_process_line(lines, expected):
+    assert process_line(lines) == expected
+
+
+@pytest.mark.parametrize('lines, expected',
+                         [
+                            ([
+                                '..@@.@@@@.',
+                                '@@@.@.@.@@',
+                                '@@@@@.@.@@',
+                                '@.@@@@..@.',
+                                '@@.@@@@.@@',
+                                '.@@@@@@@.@',
+                                '.@.@.@.@@@',
+                                '@.@@@.@@@@',
+                                '.@@@@@@@@.',
+                                '@.@.@@@.@.'
+                            ], 43),
+                         ])
+def test_iterative_processing(lines, expected):
+    assert find_all_accessible_rolls(lines) == expected