Advent of Code 2024, Day 4

I lost time because of two blunders:

• In part 1, I thought an “XMAS” can change direction along the way instead of only going straight.
• In part 2, I thought any kind of crossing was allowed (including horizontal and vertical lines) instead of only two diagonals.

Insights for simpler code:

• Don’t filter out illegal coordinates before recursing — let the function reject them instead.
• Don’t hard-code directions — iterate over them.

Part 1

import sys

def find(s, lines, i, j, diri, dirj):
    if s == "": return True
    if not 0 <= i < len(lines): return False
    if not 0 <= j < len(lines[i]): return False
    if not lines[i][j] == s[0]: return False
    return find(s[1:], lines, i+diri, j+dirj, diri, dirj)

with open(sys.argv[1]) as f:
    lines = f.readlines()

s = 0
for i in range(len(lines)):
    for j in range(len(lines[i])):
        for diri in [-1, 0, 1]:
            for dirj in [-1, 0, 1]:
                s += find("XMAS", lines, i, j, diri, dirj)
print(s)

Part 2

import sys

def find(s, lines, i, j, diri, dirj):
    if s == "": return True
    if not 0 <= i < len(lines): return False
    if not 0 <= j < len(lines[i]): return False
    if not lines[i][j] == s[0]: return False
    return find(s[1:], lines, i+diri, j+dirj, diri, dirj)

with open(sys.argv[1]) as f:
    lines = f.readlines()

s = 0
for i in range(len(lines)):
    for j in range(len(lines[i])):
        n = 0
        for diri in [-1, 1]:
            for dirj in [-1, 1]:
                n += find("MAS", lines, i-diri, j-dirj, diri, dirj)
        if n >= 2:
            s += 1
print(s)

2024-12-11