Finding pixels that make an image unique within a list, can you improve on brute force?

Question

Suppose I have a list of strings where each string is

• exactly 4 characters long and
• unique within the list.

For each of these strings I wan

Answer 1

If your goal is to identify images later, you could create a very fast hash of the image by picking predefined points to serve as identity pixels.

for example, you could have a structure (class, struct, doesn't matter what language) as follows:

structure ImageHash {
    int x_pixels, y_pixels;
    u_long hash;
    void createHash(Image img) {
        x_pixels = img.x_pixels;
        y_pixels = img.y_pixels;
        for(int i = 1; i < 5; i++) {
            int x = x_pixels / i;
            for(int j = 1; j < 5; j++) {
                int y = y_pixels / j;
                int r = img.getPixelRed(x,y);
                int g = img.getPixelGreen(x,y);
                int b = img.getPixelBlue(x,y);
                hash = (hash * 31) ^ (r^g^b);
            }
        }
    }
}

This sort of "incomplete hash" will allow you identify possible identities, and then you can do the expensive, full comparison sparingly as required.

Expand the incomplete hash as necessary.