ANSI C: How can I abstract over structure fields?
I have an image rendered as a 2D RGB pixel array.
typedef struct {
char R;
char G;
char B;
} RGB;
and a function that calculates a new color for each pixel for this image:
RGB new_color(RGB image[][], int r, int c){
RGB color;
color.R = image[r][c].R + image[r+1][c].R + image[r-1][c].R + image[r][c+1].R + image[r][c-1].R;
color.G = image[r][c].G + image[r+1][c].G + image[r-1][c].G + image[r][c+1].G + image[r][c-1].G;
color.B = image[r][c].B + image[r+1][c].B + image[r-1][c].B + image[r][c+1].B + image[r][c-1].B;
return color;
}
Is it possible to remove duplicate code in the body of new_color? In other words, abstractly over the field names of the RGB data structure?
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1 answer
Let's ignore now, which is RGB image[][]
not legal in C89.
You can unlock access functions.
char *get_RGB_R(RGB* rgb) { return &rgb->R; }
char *get_RGB_G(RGB* rgb) { return &rgb->G; }
char *get_RGB_B(RGB* rgb) { return &rgb->B; }
And then write a helper function that uses them.
void set_color_channel_from_adjacent(
char *(*accessor)(RGB*), RGB* result, RGB image[][], int r, int c)
{
*accessor(result) = *accessor(&image[r][c]) +
*accessor(&image[r+1][c]) +
*accessor(&image[r-1][c]) +
*accessor(&image[r][c+1]) +
*accessor(&image[r][c-1]);
}
And then call the helper function
RGB new_color(RGB image[][], int r, int c)
{
RGB color;
set_color_channel_from_adjacent(get_RGB_R, &color, image, r, c);
set_color_channel_from_adjacent(get_RGB_G, &color, image, r, c);
set_color_channel_from_adjacent(get_RGB_B, &color, image, r, c);
return color;
}
Modern compilers build short functions and generate equivalent code to the original.
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