From 46a84ac4766beeb70d3003d10e3653f4145ef9f6 Mon Sep 17 00:00:00 2001
From: Holly McFarland <me@hollymcfarland.com>
Date: Wed, 4 Dec 2024 19:34:19 -0500
Subject: [PATCH] day 4

---
 day_04/part_a.py | 79 ++++++++++++++++++++++++++++++++++++++++
 day_04/part_b.py | 93 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 172 insertions(+)
 create mode 100644 day_04/part_a.py
 create mode 100644 day_04/part_b.py

diff --git a/day_04/part_a.py b/day_04/part_a.py
new file mode 100644
index 0000000..98c51c5
--- /dev/null
+++ b/day_04/part_a.py
@@ -0,0 +1,79 @@
+import aocd
+
+from enum import Enum
+import collections
+
+class Direction(Enum):
+    NORTH = (0, -1)
+    NORTHEAST = (1, -1)
+    EAST = (1, 0)
+    SOUTHEAST = (1, 1)
+    SOUTH = (0, 1)
+    SOUTHWEST = (-1, 1)
+    WEST = (-1, 0)
+    NORTHWEST = (-1, -1)
+
+class Letter:
+    target = 'XMAS'
+
+    def __init__(self, char, x, y, grid):
+        self.char = char
+
+        self.x = x
+        self.y = y
+        self.grid = grid
+    
+    def neighbour_coords(self, direction):
+        return (self.x + direction.value[0], self.y + direction.value[1])
+
+    def neighbour(self, direction):
+        return self.grid[self.neighbour_coords(direction)]
+
+    def num_starts(self):
+        """Return number of times this letter is the start of the target word"""
+        return sum(self._is_nth_position(0, d) for d in Direction)
+
+    def _is_nth_position(self, n, direction):
+        """
+        Return number of times this letter is the nth letter of the target word 
+        in the given direction
+        """
+        # Must be the correct letter
+        if self.char != self.target[n]:
+            return False
+        
+        # Return True if it's the last letter
+        if n == len(self.target) - 1:
+            return True
+
+        # Return False if there are no more letter in this direction
+        if (next_letter := self.neighbour(direction)) is None:
+            return False
+        
+        # Ask the next letter
+        return next_letter._is_nth_position(n+1, direction)
+
+def parse(data):
+    """
+    Parse string into grid, return all possible first letters of XMAS
+    """
+    grid = collections.defaultdict(lambda: None)
+    x_list = []
+
+    for y, r in enumerate(data.split('\n')):
+        for x, c in enumerate(r):
+            grid[(x, y)] = Letter(c, x, y, grid)
+            if c == 'X':
+                x_list.append(grid[(x, y)])
+    
+    return x_list
+
+def solve(data):
+    x_list = parse(data)
+    return sum(x.num_starts() for x in x_list)
+
+if __name__ == '__main__':
+    solution = solve(aocd.get_data(year=2024, day=4))
+
+    print(solution)
+    aocd.submit(solution, year=2024, day=4, part='a')
\ No newline at end of file
diff --git a/day_04/part_b.py b/day_04/part_b.py
new file mode 100644
index 0000000..1201da2
--- /dev/null
+++ b/day_04/part_b.py
@@ -0,0 +1,93 @@
+import aocd
+
+from enum import Enum, nonmember
+import collections
+
+class Direction(Enum):
+    NORTH = (0, -1)
+    NORTHEAST = (1, -1)
+    EAST = (1, 0)
+    SOUTHEAST = (1, 1)
+    SOUTH = (0, 1)
+    SOUTHWEST = (-1, 1)
+    WEST = (-1, 0)
+    NORTHWEST = (-1, -1)
+
+    @nonmember
+    def opposite(d):
+        return Direction((d.value[0]*-1, d.value[1]*-1))
+
+class Letter:
+    target = 'MAS'
+
+    def __init__(self, char, x, y, grid):
+        self.char = char
+
+        self.x = x
+        self.y = y
+        self.grid = grid
+    
+    def neighbour_coords(self, direction):
+        return (self.x + direction.value[0], self.y + direction.value[1])
+
+    def neighbour(self, direction):
+        return self.grid[self.neighbour_coords(direction)]
+
+    def is_center(self):
+        """Return whether this letter is in the center of the target pattern"""
+        if self.char != self.target[1]:
+            return False
+        
+        corners = (
+            Direction.NORTHWEST,
+            Direction.NORTHEAST,
+            Direction.SOUTHWEST,
+            Direction.SOUTHEAST,
+        )
+
+        return sum(self.neighbour(d)._is_nth_position(0, Direction.opposite(d)) for d in corners) == 2
+
+    def _is_nth_position(self, n, direction):
+        """
+        Return number of times this letter is the nth letter of the target word 
+        in the given direction
+        """
+        # Must be the correct letter
+        if self.char != self.target[n]:
+            return False
+        
+        # Return True if it's the last letter
+        if n == len(self.target) - 1:
+            return True
+
+        # Return False if there are no more letter in this direction
+        if (next_letter := self.neighbour(direction)) is None:
+            return False
+        
+        # Ask the next letter
+        return next_letter._is_nth_position(n+1, direction)
+
+def parse(data):
+    """
+    Parse string into grid, return all possible first letters of XMAS
+    """
+    grid = collections.defaultdict(lambda: Letter(None, None, None, None)) # Null letters
+    a_list = []
+
+    for y, r in enumerate(data.split('\n')):
+        for x, c in enumerate(r):
+            grid[(x, y)] = Letter(c, x, y, grid)
+            if c == 'A':
+                a_list.append(grid[(x, y)])
+    
+    return a_list
+
+def solve(data):
+    a_list = parse(data)
+    return sum(a.is_center() for a in a_list)
+
+if __name__ == '__main__':
+    solution = solve(aocd.get_data(year=2024, day=4))
+
+    print(solution)
+    aocd.submit(solution, year=2024, day=4, part='b')
\ No newline at end of file