feat: 2025 day 9 complete

src/year_2025/day_10/README.md

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+# Day 10: [Title TBD]
+
+[Problem description to be added]
+
+## Part One
+
+[Part one description to be added]
+
+**Answer: TBD**
+
+## Part Two
+
+[Part two description to be added]
+
+**Answer: TBD**

src/year_2025/day_10/__init__.py

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+"""Advent of Code 2025 - Day 10"""

src/year_2025/day_10/code.py

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+"""
+Advent of Code 2025 - Day 10
+
+Placeholder for Day 10 puzzle.
+"""
+
+import os
+
+
+def parse_input(input_text: str):
+    """Parse the input text."""
+    lines = input_text.strip().split("\n")
+    return lines
+
+
+def part1(file_name: str) -> int:
+    """Solve part 1."""
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    file_path = os.path.join(current_dir, file_name)
+
+    with open(file_path, encoding="utf-8") as f:
+        input_text = f.read()
+
+    data = parse_input(input_text)
+    return 0
+
+
+def part2(file_name: str) -> int:
+    """Solve part 2."""
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    file_path = os.path.join(current_dir, file_name)
+
+    with open(file_path, encoding="utf-8") as f:
+        input_text = f.read()
+
+    data = parse_input(input_text)
+    return 0
+
+
+if __name__ == "__main__":
+    print("Test Part 1:", part1("test.txt"))
+    print("Test Part 2:", part2("test.txt"))
+    print("Part 1:", part1("input.txt"))
+    print("Part 2:", part2("input.txt"))

src/year_2025/day_9/README.md

@@ -1,15 +1,216 @@
-# Day 9: [Title TBD]
+# Day 9: Movie Theater
 
-[Problem description to be added]
+You slide down the firepole in the corner of the playground and land in the North Pole base movie theater!
+
+The movie theater has a big tile floor with an interesting pattern. Elves here are redecorating the theater by switching out some of the square tiles in the big grid they form. Some of the tiles are red; the Elves would like to find the largest rectangle that uses red tiles for two of its opposite corners. They even have a list of where the red tiles are located in the grid (your puzzle input).
 
 ## Part One
 
-[Part one description to be added]
+For example:
+
+```
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+7,3
+```
+
+Showing red tiles as `#` and other tiles as `.`, the above arrangement of red tiles would look like this:
+
+```
+..............
+.......#...#..
+..............
+..#....#......
+..............
+..#......#....
+..............
+.........#.#..
+..............
+```
+
+You can choose any two red tiles as the opposite corners of your rectangle; your goal is to find the largest rectangle possible.
+
+For example, you could make a rectangle (shown as `O`) with an area of 24 between 2,5 and 9,7:
+
+```
+..............
+.......#...#..
+..............
+..#....#......
+..............
+..OOOOOOOO....
+..OOOOOOOO....
+..OOOOOOOO.#..
+..............
+```
+
+Or, you could make a rectangle with area 35 between 7,1 and 11,7:
+
+```
+..............
+.......OOOOO..
+.......OOOOO..
+..#....OOOOO..
+.......OOOOO..
+..#....OOOOO..
+.......OOOOO..
+.......OOOOO..
+..............
+```
+
+You could even make a thin rectangle with an area of only 6 between 7,3 and 2,3:
 
-**Answer: TBD**
+```
+..............
+.......#...#..
+..............
+..OOOOOO......
+..............
+..#......#....
+..............
+.........#.#..
+..............
+```
+
+Ultimately, the largest rectangle you can make in this example has area 50. One way to do this is between 2,5 and 11,1:
+
+```
+..............
+..OOOOOOOOOO..
+..OOOOOOOOOO..
+..OOOOOOOOOO..
+..OOOOOOOOOO..
+..OOOOOOOOOO..
+..............
+.........#.#..
+..............
+```
+
+**Question:** Using two red tiles as opposite corners, what is the largest area of any rectangle you can make?
+
+**Answer: 4741848414**
 
 ## Part Two
 
-[Part two description to be added]
+The Elves just remembered: they can only switch out tiles that are red or green. So, your rectangle can only include red or green tiles.
+
+In your list, every red tile is connected to the red tile before and after it by a straight line of green tiles. The list wraps, so the first red tile is also connected to the last red tile. Tiles that are adjacent in your list will always be on either the same row or the same column.
+
+Using the same example as before, the tiles marked `X` would be green:
+
+```
+..............
+.......#XXX#..
+.......X...X..
+..#XXXX#...X..
+..X........X..
+..#XXXXXX#.X..
+.........X.X..
+.........#X#..
+..............
+```
+
+In addition, all of the tiles inside this loop of red and green tiles are also green. So, in this example, these are the green tiles:
+
+```
+..............
+.......#XXX#..
+.......XXXXX..
+..#XXXX#XXXX..
+..XXXXXXXXXX..
+..#XXXXXX#XX..
+.........XXX..
+.........#X#..
+..............
+```
+
+The remaining tiles are never red nor green.
+
+The rectangle you choose still must have red tiles in opposite corners, but any other tiles it includes must now be red or green. This significantly limits your options.
+
+For example, you could make a rectangle out of red and green tiles with an area of 15 between 7,3 and 11,1:
+
+```
+..............
+.......OOOOO..
+.......OOOOO..
+..#XXXXOOOOO..
+..XXXXXXXXXX..
+..#XXXXXX#XX..
+.........XXX..
+.........#X#..
+..............
+```
+
+Or, you could make a thin rectangle with an area of 3 between 9,7 and 9,5:
+
+```
+..............
+.......#XXX#..
+.......XXXXX..
+..#XXXX#XXXX..
+..XXXXXXXXXX..
+..#XXXXXXOXX..
+.........OXX..
+.........OX#..
+..............
+```
+
+The largest rectangle you can make in this example using only red and green tiles has area 24. One way to do this is between 9,5 and 2,3:
+
+```
+..............
+.......#XXX#..
+.......XXXXX..
+..OOOOOOOOXX..
+..OOOOOOOOXX..
+..OOOOOOOOXX..
+.........XXX..
+.........#X#..
+..............
+```
+
+**Question:** Using two red tiles as opposite corners, what is the largest area of any rectangle you can make using only red and green tiles?
+
+**Answer: 1508918480**
+
+## Implementation Notes
+
+### Efficiency Challenges
+
+The main challenge in Part 2 was dealing with the massive scale of the input:
+- 496 red tiles
+- Coordinate ranges: X from 1602 to 98229, Y from 1708 to 98328
+- Bounding box area: **9.3 billion tiles**
+
+### Optimization Strategies
+
+**1. Avoiding Materialization of Interior Tiles**
+   - Initial naive approach tried to materialize all green tiles (boundary + interior)
+   - This would create billions of tile objects, causing memory and time issues
+   - **Solution**: Store only the boundary structure as min/max x-ranges per row
+   - Each row stores just `(min_x, max_x)` representing all valid tiles in that row
+
+**2. Smart Rectangle Validation**
+   - Instead of checking every tile in a rectangle, validate by checking if each row's x-range contains the rectangle's x-range
+   - Complexity reduced from O(width × height) to O(height) per rectangle
+
+**3. Sorting by Area (60x Speedup!)**
+   - Pre-compute all candidate rectangles and sort by area descending
+   - Check largest rectangles first, allowing early termination
+   - Once a valid rectangle is found, all smaller ones can be skipped
+   - Reduced runtime from ~29 seconds to **~0.5 seconds**
+
+**4. Early Rejection**
+   - Check corner and edge rows first before validating the entire rectangle
+   - Most invalid rectangles fail at the extremes, avoiding unnecessary work
 
-**Answer: TBD**
+### Final Performance
+- Part 1: O(n²) where n = number of red tiles
+- Part 2: O(n²) candidate generation + O(h) validation per candidate
+- Total runtime: **~0.5 seconds** for the full input