add helper function to compute pixel indices

src/image.h

@@ -16,4 +16,8 @@ struct image {
 struct image *create_image(size_t w, size_t h);
 void destroy_image(struct image *image);
 
+static inline size_t to_index(size_t h, size_t w, const struct image *image) {
+    return h * image->w + w;
+}
+
 #endif /* !IMAGE_H */

src/main.c

@@ -9,7 +9,7 @@
 static void fill_image(struct image *image) {
     for (size_t i = 0; i < image->h; ++i) {
         for (size_t j = 0; j < image->w; ++j) {
-            struct pixel *p = &image->buf[i * image->w + j];
+            struct pixel *p = &image->buf[to_index(i, j, image)];
             p->r = 255 * i / image->h;
             p->g = 255 * j / image->w;
             p->g = 255 * (i + j) / (image->h + image->w);

src/ppm.c

@@ -14,7 +14,7 @@ void print_ppm(const struct image *image, FILE *f) {
         for (size_t j = 0; j < image->w; ++j) {
             if (j != 0)
                 fputc(' ', f);
-            const struct pixel *p = &image->buf[i * image->w + j];
+            const struct pixel *p = &image->buf[to_index(i, j, image)];
             fprintf(f, "%hhu %hhu %hhu", p->r, p->g, p->b);
         }
         fputc('\n', f);