#define STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image.h"
#include "stb_image_write.h"

#include <math.h>

void doit(
    const char *filenameIn,
    const char *filenameOut,
    stbi_uc r_fg_old,
    stbi_uc g_fg_old,
    stbi_uc b_fg_old,
    stbi_uc r_bg,
    stbi_uc g_bg,
    stbi_uc b_bg,
    stbi_uc r_fg_new,
    stbi_uc g_fg_new,
    stbi_uc b_fg_new)
{
    stbi_uc rDiff = abs(r_bg - r_fg_old);
    stbi_uc gDiff = abs(g_bg - g_fg_old);
    stbi_uc bDiff = abs(b_bg - b_fg_old);

    int x, y, n;
    stbi_uc *img = stbi_load(filenameIn, &x, &y, &n, 0);

    for (int i = 0; i < x; i++) {
        for (int j = 0; j < y; j++) {
            size_t index = (j*x + i) * n;
            stbi_uc r = img[index + 0];
            stbi_uc g = img[index + 1];
            stbi_uc b = img[index + 2];
            stbi_uc a = img[index + 3];

            stbi_uc rAbs = abs(r - r_fg_old);
            double rNorm = (double)rAbs / rDiff;
            stbi_uc gAbs = abs(g - g_fg_old);
            double gNorm = (double)gAbs / gDiff;
            stbi_uc bAbs = abs(b - b_fg_old);
            double bNorm = (double)bAbs / bDiff;
            
            img[index + 0] = r_fg_new;
            img[index + 1] = g_fg_new;
            img[index + 2] = b_fg_new;
            img[index + 3] =
                (stbi_uc)(255.0*(1.0 - ((rNorm + gNorm + bNorm) / 3.0)));
        }
    }

    stbi_write_png(filenameOut, x, y, n, img, 0);

    stbi_image_free(img);
}

stbi_uc hex(char *c) {
    stbi_uc c0 = 0, c1 = 0;
    if (c[0] >= '0' && c[0] <= '9') c0 = c[0] - '0';
    else if (c[0] >= 'a' && c[0] <= 'f') c0 = c[0] - 'a' + 10;
    if (c[1] >= '0' && c[1] <= '9') c1 = c[1] - '0';
    else if (c[1] >= 'a' && c[1] <= 'f') c1 = c[1] - 'a' + 10;
    return c0*16 + c1;
}

int main(int argc, char **argv) {
    if (argc != 6) {
        fprintf(stderr, "Usage: %s filename_old filename_new fgcolor bgcolor fgcolor_new\n", argv[0]);
        return 1;
    }

    // rgb, foreground and background and new foreground
    stbi_uc r_f = hex(&argv[3][0]);
    stbi_uc g_f = hex(&argv[3][2]);
    stbi_uc b_f = hex(&argv[3][4]);
    stbi_uc r_b = hex(&argv[4][0]);
    stbi_uc g_b = hex(&argv[4][2]);
    stbi_uc b_b = hex(&argv[4][4]);
    stbi_uc r_n = hex(&argv[5][0]);
    stbi_uc g_n = hex(&argv[5][2]);
    stbi_uc b_n = hex(&argv[5][4]);
    
    doit(
        argv[1], argv[2],
        r_f, g_f, b_f,
        r_b, g_b, b_b,
        r_n, g_n, b_n);

    return 0;
}