text: #include "Halide.h"
  #include <iostream>
  
  using namespace Halide;
  
  // Given a 3x3 patch, find the middle element
  // We do this by first finding the minimum, maximum, and middle for each row.
  // Then across rows, we find the maximum minimum, the minimum maximum, and the middle middle.
  // Then we take the middle of those three results.
  
  Expr mid3(Expr a, Expr b, Expr c) {
      return max(min(max(a, b), c), min(a, b));
  }
  
  int main(int arch, char **argv) {
      const int W = 256, H = 256;
      Buffer<uint8_t> in(W, H);
      // Set up the input.
      for (int y = 0; y < H; y++) {
          for (int x = 0; x < W; x++) {
              in(x, y) = rand() & 0xff;
          }
      }
  
      Var x("x"), y("y");
  
      // Boundary condition.
      Func input = BoundaryConditions::constant_exterior(in, 0);
      input.compute_root();
  
      // Algorithm.
      Func max_x("max_x"), min_x("min_x"), mid_x("mid_x");
      max_x(x, y) = max(input(x - 1, y), input(x, y), input(x + 1, y));
      min_x(x, y) = min(input(x - 1, y), input(x, y), input(x + 1, y));
      mid_x(x, y) = mid3(input(x - 1, y), input(x, y), input(x + 1, y));
  
      Func min_max("min_max"), max_min("max_min"), mid_mid("mid_mid");
      min_max(x, y) = min(max_x(x, y - 1), max_x(x, y), max_x(x, y + 1));
      max_min(x, y) = max(min_x(x, y - 1), min_x(x, y), min_x(x, y + 1));
      mid_mid(x, y) = mid3(mid_x(x, y - 1), mid_x(x, y), mid_x(x, y + 1));
  
      Func median3x3("median3x3");
      median3x3(x, y) = mid3(min_max(x, y), max_min(x, y), mid_mid(x, y));
  
      // Schedule.
      Target target = get_jit_target_from_environment();
      if (target.has_gpu_feature()) {
          Var xi("xi"), yi("yi");
          median3x3.gpu_tile(x, y, xi, yi, 16, 16);
      } else if (target.has_feature(Target::HVX)) {
          median3x3.hexagon().vectorize(x, 64);
      } else {
          median3x3.vectorize(x, target.natural_vector_size<uint8_t>());
      }
  
      // Run the pipeline and verify the results are correct.
      Buffer<uint8_t> out = median3x3.realize({W, H}, target);
  
      for (int y = 1; y < H - 1; y++) {
          for (int x = 1; x < W - 1; x++) {
              uint8_t inp[9] = {in(x - 1, y - 1), in(x, y - 1), in(x + 1, y - 1),
                                in(x - 1, y), in(x, y), in(x + 1, y),
                                in(x - 1, y + 1), in(x, y + 1), in(x + 1, y + 1)};
              std::nth_element(&inp[0], &inp[4], &inp[9]);
              uint8_t correct = inp[4];
              if (correct != out(x, y)) {
                  std::cout << "out(" << x << ", " << y << ") = " << out(x, y) << " instead of " << correct << "\n";
                  return -1;
              }
          }
      }
      std::cout << "Success!\n";
      return 0;
  }
  