test: add test for day4

lib/advent_code2024/solutions/day03/input.txt

@@ -1 +0,0 @@
-# Add your puzzle input here

lib/advent_code2024/solutions/day04/READme.md

@@ -0,0 +1,71 @@
+# Coding Challenge: Ceres Search
+
+## Problem Summary
+
+You are presented with a word search puzzle where you must find all occurrences of
+the word "XMAS". The word can appear in various directions and orientations, including:
+
+- Horizontal (left-to-right and right-to-left)
+- Vertical (top-to-bottom and bottom-to-top)
+- Diagonal (both directions)
+- Overlapping with other words
+
+## Example Puzzle
+
+Below is a simplified word search grid where irrelevant characters are replaced
+with `.`:
+
+```txt
+..X...
+.SAMX.
+.A..A.
+XMAS.S
+.X....
+```
+
+In this grid, "XMAS" can appear in multiple ways.
+
+## Larger Example
+
+For the following complete grid:
+
+```txt
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX
+```
+
+The word "XMAS" occurs **18 times**. Visualizing the grid with only the letters
+in "XMAS" highlighted:
+
+```txt
+....XXMAS.
+.SAMXMS...
+...S..A...
+..A.A.MS.X
+XMASAMX.MM
+X.....XA.A
+S.S.S.S.SS
+.A.A.A.A.A
+..M.M.M.MM
+.X.X.XMASX
+```
+
+## Objective
+
+For the given puzzle input, calculate and return the total number of times the word
+"XMAS" appears in all possible orientations.
+
+## Notes
+
+- Words can overlap and share characters.
+- Input will be a rectangular grid of letters.
+
+Good luck helping the little Elf! 🌟

test/advent_code2024/solutions/day04/day4_test.exs

@@ -0,0 +1,23 @@
+defmodule AdventCode2024.Solutions.Day04Test do
+  use ExUnit.Case
+  alias AdventCode2024.Solutions.Day04
+
+  @example_grid [
+    "MMMSXXMASM",
+    "MSAMXMSMSA",
+    "AMXSXMAAMM",
+    "MSAMASMSMX",
+    "XMASAMXAMM",
+    "XXAMMXXAMA",
+    "SMSMSASXSS",
+    "SAXAMASAAA",
+    "MAMMMXMMMM",
+    "MXMXAXMASX"
+  ]
+
+  test "count occurrences of XMAS in word search" do
+    expected_count = 18
+    actual_count = Day04.count_word_occurrences(@example_grid, "XMAS")
+    assert actual_count == expected_count
+  end
+end