Simple auto-tiler in C++

autotile.cpp

// Libraries
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <vector>

#include <SDL2/SDL.h>

// SDL
SDL_Window   * window;
SDL_Renderer * renderer;

// Game
struct Tile {
    Tile(char c = ' ') {
        if (c != ' ') this->visible = true;
    }
    
    union {
        struct {
            int top_l : 1;
            int top_m : 1;
            int top_r : 1;
            int mid_l : 1;
            int mid_r : 1;
            int bot_l : 1;
            int bot_m : 1;
            int bot_r : 1;
        };
        char bitmask = 0;
    };
    
    bool visible = false;
};

void draw_pt(int x, int y, SDL_Color col) {
    SDL_SetRenderDrawColor(renderer, col.r, col.g, col.b, col.a);
    SDL_RenderDrawPoint(renderer, x, y);
}

void draw_rec(int x, int y, int w, int h, SDL_Color col) {
    SDL_Rect rect = { x, y, w, h };
    SDL_SetRenderDrawColor(renderer, col.r, col.g, col.b, col.a);
    SDL_RenderFillRect(renderer, &rect);
}

void init() {
    // SDL
    window   = SDL_CreateWindow("doom", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 512, 512, 0);
    renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
    if (!window || !renderer) 
        fprintf(stderr, "[error]: failed to initialize SDL: %s", SDL_GetError());
    SDL_RenderSetScale(renderer, 12, 12);
    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
    
    // Game
    std::vector<Tile> tiles = {
        ' ',' ',' ',' ',' ',' ', 
        ' ',' ',' ',' ',' ',' ', 
        ' ',' ',' ',' ',' ',' ', 
        ' ',' ',' ',' ',' ',' ', 
        ' ',' ',' ',' ',' ',' ', 
        ' ',' ',' ',' ',' ',' ', 
    };
    
    auto get = [&](int x, int y) -> Tile {
        if (x >= 0 && x < 6 && y >= 0 && y < 6) {
            return tiles[x + y * 6];
        }
        return Tile(' ');
    };
    
    auto check_tile = [&](int i) {
        tiles[i].bitmask = 0;

        if (!tiles[i].visible) return;

        int x = i % 6;
        int y = i / 6;

        // Sides
        if (!get(x,     y - 1).visible) tiles[i].top_m = 1;
        if (!get(x,     y + 1).visible) tiles[i].bot_m = 1;
        if (!get(x - 1, y).visible)     tiles[i].mid_l = 1;
        if (!get(x + 1, y).visible)     tiles[i].mid_r = 1;

        // Diagonal
        if (!get(x - 1, y - 1).visible || !get(x, y - 1).visible || !get(x - 1, y).visible)
            tiles[i].top_l = 1;

        if (!get(x + 1, y - 1).visible || !get(x, y - 1).visible || !get(x + 1, y).visible)
            tiles[i].top_r = 1;

        if (!get(x - 1, y + 1).visible || !get(x, y + 1).visible || !get(x - 1, y).visible)
            tiles[i].bot_l = 1;

        if (!get(x + 1, y + 1).visible || !get(x, y + 1).visible || !get(x + 1, y).visible)
            tiles[i].bot_r = 1;
    };
    
    auto check = [&]() {
        for (int i = 0; i < tiles.size(); ++i) {
            check_tile(i);
        }
    };
    
    // Loop
    int mx, my;
    
    bool running = true;
    while (running) {
        bool btn = false;
        
        SDL_Event event;
        while (SDL_PollEvent(&event)) {
            switch (event.type) {
                case SDL_QUIT: {
                    running = false;
                    break;
                }
                case SDL_MOUSEBUTTONDOWN: {
                    btn = true;
                    mx  = event.motion.x / 12;
                    my  = event.motion.y / 12;
                    break;
                }
            }
        }
        
        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
        SDL_RenderClear(renderer);
        
        for (int i = 0; i < tiles.size(); ++i) {
            int x = 8 * (i % 6);
            int y = 8 * (i / 6);
            
            SDL_Rect r1 = { x,  y,  8, 8 };
            SDL_Rect r2 = { mx, my, 1, 1 };
            if (SDL_HasIntersection(&r1, &r2) && btn) {
                tiles[i].visible = !tiles[i].visible;
            }
            
            if (!tiles[i].visible) continue;
            
            draw_rec(x, y, 8, 8, { 0, 255, 0, 255 });
            
            if (tiles[i].top_l) draw_pt(x,     y,     { 255, 0, 255, 255 });
            if (tiles[i].top_m) draw_pt(x + 4, y,     { 255, 0, 255, 255 });
            if (tiles[i].top_r) draw_pt(x + 7, y,     { 255, 0, 255, 255 });
            if (tiles[i].mid_l) draw_pt(x,     y + 4, { 255, 0, 255, 255 });
            if (tiles[i].mid_r) draw_pt(x + 7, y + 4, { 255, 0, 255, 255 });
            if (tiles[i].bot_l) draw_pt(x,     y + 7, { 255, 0, 255, 255 });
            if (tiles[i].bot_m) draw_pt(x + 4, y + 7, { 255, 0, 255, 255 });
            if (tiles[i].bot_r) draw_pt(x + 7, y + 7, { 255, 0, 255, 255 });
        }
        
        if (btn) check();
        
        draw_pt(mx, my, { 255, 255, 0, 255 });
        
        SDL_RenderPresent(renderer);
    }
    
    // Cleanup
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
}

// Main
int main(int argc, char *argv[]) {
    init();
    
    return 0;
}