feat: aoc 2025 day 7 solution + tests

Author: mbrickerd

_2025/solutions/day07.py

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+"""Day 7: Laser Lab
+
+This module provides the solution for Advent of Code 2025 - Day 7.
+
+It simulates beams traveling downward through a grid containing a start
+position ('S'), empty cells ('.'), and splitters ('^') that split beams
+diagonally.
+
+The module contains a Solution class that inherits from SolutionBase to
+count how many splitters activate (Part 1) and how many distinct beam
+paths reach the bottom row (Part 2).
+"""
+
+from aoc.models.base import SolutionBase
+
+
+class Solution(SolutionBase):
+    """Simulate vertical beam propagation and splitting in a grid.
+
+    This solution models beams starting from a single source cell 'S' and
+    moving one row downward at a time. When a beam enters a splitter '^',
+    it splits into two beams that continue diagonally down-left and
+    down-right.
+
+    Part 1 tracks a single beam front and counts how many splitters are
+    activated at least once. Part 2 uses dynamic programming to count how
+    many distinct paths reach the bottom of the grid after all splitting.
+    """
+
+    def find_start(self, grid: list[list[str]]) -> tuple[int, int]:
+        """Locate the starting cell 'S' in the grid.
+
+        Args:
+            grid: 2D character grid of the laser lab
+
+        Returns
+        -------
+            tuple[int, int]: (x, y) coordinates of the start cell
+
+        Raises
+        ------
+            ValueError: If no 'S' cell is found in the grid
+        """
+        for y, row in enumerate(grid):
+            for x, cell in enumerate(row):
+                if cell == "S":
+                    return (x, y)
+
+        raise ValueError("No 'S' start cell found in grid!")
+
+    def part1(self, data: list[str]) -> int:
+        """Count how many splitters are activated along a single beam front.
+
+        Simulates a single front of beams starting from 'S' and moving
+        row-by-row downward. Each time a beam enters a '^' splitter, that
+        splitter is counted and the beam splits into two beams diagonally
+        down-left and down-right for the next row.
+
+        Args:
+            data: List of strings representing the lab grid
+
+        Returns
+        -------
+            int: Number of times splitters are activated across all rows
+        """
+        grid = [list(row) for row in data]
+        start_col, start_row = self.find_start(grid)
+
+        beams: set[int] = {start_col}
+        count = 0
+        last_row_idx = len(grid) - 1
+
+        current_row = start_row
+        while current_row < last_row_idx:
+            current_row += 1
+            next_beams: set[int] = set()
+
+            for col in beams:
+                cell = grid[current_row][col]
+                if cell == "^":
+                    next_beams.add(col - 1)
+                    next_beams.add(col + 1)
+                    count += 1
+                else:
+                    next_beams.add(col)
+
+            beams = next_beams
+
+        return count
+
+    def part2(self, data: list[str]) -> int:
+        """Count distinct beam paths that reach the bottom row.
+
+        Uses dynamic programming where dp[r][c] stores how many distinct
+        paths can reach cell (r, c). Paths start from the cell below 'S'
+        and propagate row-by-row, splitting at '^' cells into diagonal
+        positions.
+
+        Args:
+            data: List of strings representing the lab grid
+
+        Returns
+        -------
+            int: Total number of distinct paths that reach the bottom row
+        """
+        grid = [list(row) for row in data]
+        rows, cols = len(grid), len(grid[0])
+        start_col, start_row = self.find_start(grid)
+
+        dp = [[0] * cols for _ in range(rows)]
+
+        if start_row + 1 < rows:
+            dp[start_row + 1][start_col] = 1
+
+        for r in range(start_row + 1, rows - 1):
+            for c in range(cols):
+                if dp[r][c] == 0:
+                    continue
+
+                cell = grid[r][c]
+                if cell == ".":
+                    dp[r + 1][c] += dp[r][c]
+
+                elif cell == "^":
+                    if c - 1 >= 0:
+                        dp[r + 1][c - 1] += dp[r][c]
+                    if c + 1 < cols:
+                        dp[r + 1][c + 1] += dp[r][c]
+
+                else:
+                    dp[r + 1][c] += dp[r][c]
+
+        return sum(dp[rows - 1])

_2025/tests/data/day07/test_01_input.txt

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+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............

_2025/tests/data/day07/test_02_input.txt

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+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............

_2025/tests/test_07.py

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+"""Test suite for Day 7: Laser Lab
+
+This module contains tests for the Day 7 solution, which simulates beam
+propagation and splitting in a laser lab grid. The tests verify:
+
+1. Part 1: Counting how many splitters are activated by the beam front
+2. Part 2: Counting how many distinct beam paths reach the bottom row
+"""
+from aoc.models.tester import TestSolutionUtility
+
+
+def test_day07_part1() -> None:
+    """Test counting activated splitters along the beam front.
+
+    This test runs the solution for Part 1 of the puzzle against the
+    provided test input and compares the result with the expected output.
+    """
+    TestSolutionUtility.run_test(
+        year=2025,
+        day=7,
+        is_raw=False,
+        part_num=1,
+        expected=21,
+    )
+
+
+def test_day07_part2() -> None:
+    """Test counting distinct beam paths reaching the bottom row.
+
+    This test runs the solution for Part 2 of the puzzle against the
+    provided test input and compares the result with the expected output.
+    """
+    TestSolutionUtility.run_test(
+        year=2025,
+        day=7,
+        is_raw=False,
+        part_num=2,
+        expected=40,
+    )