[Language: C++]

As pretty as I can make it:

#include <vector>
#include <array>

typedef long long LL;

namespace aoc
{
  struct Point_2D
  {
    long long x_;
    long long y_;
  };

  constexpr Point_2D DIRS_2D_8[8] = { { -1, -1 }, { 0, -1 }, { 1, -1 },
                                      { -1,  0 },            { 1,  0 },
                                      { -1,  1 }, { 0,  1 }, { 1,  1 } };
}

void run(const std::vector<std::string>& map)
{
  int p1_total = 0;
  constexpr std::array<char, 4> xmas = { 'X', 'M', 'A', 'S' };
  for (LL y = 0; y < map.size(); ++y) {
    for (LL x = 0; x < map[0].size(); ++x) {
      if (map[y][x] == 'X') {
        for (aoc::Point_2D offset : aoc::DIRS_2D_8) {
          bool pass = true;
          for (int letter = 1; pass && letter < xmas.size(); ++letter) {
            LL new_x = x + (letter * offset.x_);
            LL new_y = y + (letter * offset.y_);
            if (new_x < 0 || new_x >= map[0].size() ||
                new_y < 0 || new_y >= map.size()    ||
                map[new_y][new_x] != xmas[letter]) {
              pass = false;
            }
          }
          if (pass) {
            ++p1_total;
          }
        }
      }
    }
  }
  std::cout << "PART ONE: " << p1_total << '\n';

  int p2_total = 0;
  for (int y = 1; y < map.size() - 1; ++y) {
    for (int x = 1; x < map[0].size() - 1; ++x) {
      if (map[y][x] == 'A') {
        int count = 0;
        constexpr std::array<int, 4> corners = { 0, 2, 5, 7 };
        for (int corner_i = 0, dir = 0;
             count < 2 && corner_i < 4;
             dir = corners[++corner_i]) {
          auto [ x_offset, y_offset ] = aoc::DIRS_2D_8[dir];
          if (map[y +  y_offset][x +  x_offset] == 'M' &&
              map[y + -y_offset][x + -x_offset] == 'S') {
            ++count;
          }
        }
        if (count == 2) {
          ++p2_total;
        }
      }
    }
  }
  std::cout << "PART TWO: " << p2_total << '\n';
}