Using recursive function (divide and conquer) to render ruler with SDL2. Compile with `gcc divide_and_conquer.c -lSDL2`

divide_and_conquer.c

#include <SDL2/SDL.h>
#include <stdio.h>

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 480

#define RULER_HEIGHT_FACTOR 5

static const int base_line = SCREEN_HEIGHT / 2;

SDL_Window *window = NULL;
SDL_Renderer *renderer = NULL;

static void render_ruler(SDL_Renderer* renderer, int l, int r, int h);
static void render_mark(SDL_Renderer* renderer, int x, int h);

void render_ruler(SDL_Renderer* renderer, int l, int r, int h)
{
  int m = (l+r)/2;
  if (h > 0)
  {
    render_mark(renderer, m, h);
    render_ruler(renderer, l, m, h-1);
    render_ruler(renderer, m, r, h-1);
  }
}

void render_mark(SDL_Renderer* renderer, int x, int h)
{
  SDL_RenderDrawLine(renderer, x, base_line, x, base_line - h*RULER_HEIGHT_FACTOR);
}

int main(int argc, char* argv[])
{
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    SDL_Log("failed to init: %s", SDL_GetError());
    return -1;
  }
  window = SDL_CreateWindow(
      "Ruler - Divide and Conquer", 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT,
      SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL
      );
  if (window == NULL) {
    SDL_Log("Failed to create window: %s", SDL_GetError());
    return -1;
  }

  renderer = SDL_CreateRenderer(window, -1, 0);
  if (renderer == NULL)
  {
    SDL_Log("Failed to create renderer: %s", SDL_GetError());
    return -1;
  }

  SDL_bool quit = SDL_FALSE;

  while (!quit)
  {
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
    SDL_RenderClear(renderer);
    SDL_SetRenderDrawColor(renderer, 255, 255, 255, SDL_ALPHA_OPAQUE);
    render_ruler(renderer, 5, SCREEN_WIDTH-5, 5);
    SDL_RenderPresent(renderer);

    SDL_Event e;

    // we need to call SDL_PollEvent to let window rendered, otherwise
    // no window will be shown
    while (SDL_PollEvent(&e) != 0)
    {
      if (e.type == SDL_QUIT)
      {
        quit = SDL_TRUE;
      }
    }
  }

  // free
  if (renderer != NULL)
  {
    SDL_DestroyRenderer(renderer);
    renderer = NULL;
  }
  if (window != NULL)
  {
    SDL_DestroyWindow(window);
    window = NULL;
  }
  SDL_Quit();

  return 0;
}