starting Day 21

Day21/python/solution1.py

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+from collections import deque
+
+def read_grid(file_path):
+    """Reads the grid from a file and returns it along with the starting position."""
+    with open(file_path, 'r') as file:
+        grid = [list(line.strip()) for line in file.readlines()]
+
+    start = None
+    for i, row in enumerate(grid):
+        for j, cell in enumerate(row):
+            if cell == 'S':
+                start = (i, j)
+                break
+        if start:
+            break
+
+    assert start is not None, "Starting position 'S' not found in the grid."
+    print(f"Grid loaded from {file_path}. Start position: {start}")
+    return grid, start
+
+def bfs(grid, start):
+    """Performs BFS on the grid to track the minimum steps to each plot."""
+    printf("Start BFS")
+    steps = 64
+    queue = deque([(start[0], start[1], 0)])
+    distances = {}
+    while queue:
+        x, y, step = queue.popleft()
+        if step > steps:
+            continue  # Skip plots that are more than 64 steps away
+        if step == steps:
+            distances[(x, y)] = step  # Only consider plots at exactly 64 steps
+        else:
+            for dx, dy in [(1, 0), (-1, 0), (0, 1), (0, -1)]:
+                nx, ny = x + dx, y + dy
+                if 0 <= nx < len(grid) and 0 <= ny < len(grid[0]) and grid[nx][ny] == '.' and (nx, ny) not in distances:
+                    queue.append((nx, ny, step + 1))
+
+    print(f"BFS completed. Number of plots visited: {len(distances)}")
+    return distances
+
+def count_reachable_plots(distances, steps):
+    """Counts the number of plots reachable in exactly the given number of steps."""
+    count = sum(1 for d in distances.values() if d == steps)
+    print(f"Number of plots reachable in exactly {steps} steps: {count}")
+    return count
+
+def run_test():
+    """Runs the algorithm with the test data and asserts the result."""
+    print("Running test...")
+    test_grid, test_start = read_grid("../test.txt")
+    distances = bfs(test_grid, test_start)
+    test_result = count_reachable_plots(distances, 64)
+    assert test_result == 16, f"Test failed: Expected 16, got {test_result}"
+    print("Test passed successfully.")
+
+def main():
+    """Main function to run the algorithm with the actual puzzle data."""
+    try:
+        run_test()
+        grid, start = read_grid("../input.txt")
+        distances = bfs(grid, start)
+        result = count_reachable_plots(distances, 64)
+        print(f"Final Result: Number of reachable plots in exactly 64 steps: {result}")
+    except AssertionError as ae:
+        print(f"Assertion Error: {ae}")
+    except FileNotFoundError as fnfe:
+        print(f"File not found error: {fnfe}")
+    except Exception as e:
+        print(f"Unexpected error: {e}")
+
+if __name__ == "__main__":
+    main()