2024 day 4

.gitignore

@@ -1,3 +1,6 @@
+# ignore aoc input file
+**/input.txt
+
 # ---> Python
 # Byte-compiled / optimized / DLL files
 __pycache__/

2024/day4/day4.py

@@ -0,0 +1,72 @@
+DIRECTIONS = (
+    (0, -1),   # N
+    (-1, -1),  # NW
+    (1, -1),   # NE
+    (-1, 0),   # W
+    (1, 0),    # E
+    (0, 1),    # S
+    (-1, 1),   # SW
+    (1, 1),    # SE
+)
+
+def get_grid_pos(grid, pos):
+    x, y = pos
+    if 0 <= x < len(grid):
+        if 0 <= y < len(grid[0]):
+            return grid[y][x]
+    return None
+
+
+def part1(grid):
+    count = 0
+    for y, row in enumerate(grid):
+        for x, letter in enumerate(row):
+            if letter != "X":
+                continue
+            for direction in DIRECTIONS:
+                acc = letter
+                for i in range(1, 4):
+                    dx, dy = direction
+                    pos = (x + i*dx, y + i*dy)
+                    next_letter = get_grid_pos(grid, pos)
+                    if next_letter is not None:
+                        acc += next_letter
+                    else:
+                        break  # out-of-bounds, go to next direction
+                if acc == "XMAS":
+                    count += 1
+    print("Part 1: ", count)
+
+
+def part2(grid):
+    count = 0
+    for y, row in enumerate(grid):
+        for x, _ in enumerate(row):
+            if y + 2 >= len(grid) or x + 2 >= len(grid[0]):
+                continue
+            if grid[y+1][x+1] != "A":  # center letter is always "A"
+                continue
+            # M.S / .A. / M.S
+            if grid[y][x] == "M" and grid[y][x+2] == "S" and grid[y+2][x] == "M" and grid[y+2][x+2] == "S":
+                count += 1
+            # M.M / .A. / S.S
+            if grid[y][x] == "M" and grid[y][x+2] == "M" and grid[y+2][x] == "S" and grid[y+2][x+2] == "S":
+                count += 1
+            # S.M / .A. / S.M
+            if grid[y][x] == "S" and grid[y][x+2] == "M" and grid[y+2][x] == "S" and grid[y+2][x+2] == "M":
+                count += 1
+            # S.S / .A. / M.M
+            if grid[y][x] == "S" and grid[y][x+2] == "S" and grid[y+2][x] == "M" and grid[y+2][x+2] == "M":
+                count += 1
+    print("Part 2: ", count)
+
+
+
+if __name__ == "__main__":
+    import sys
+    infile = sys.argv[1] if 1 < len(sys.argv) else "example.txt"
+    with open(infile) as f:
+        grid = [l.rstrip() for l in f.readlines()]
+        part1(grid)
+        part2(grid)
+

2024/day4/example.txt

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX