Game Boy Color fade script

fade.c

#include <gb/gb.h>
#include "fade.h"

UWORD FadePalette[32] = {
    RGB(30,0,0), RGB(0,30,0), RGB(0,0,30), RGB(0,0,0)
};

void Fade(UWORD *org_p, FadeDirection direction, FadeColor color, UWORD speed)
{
    UBYTE i,x;
    UWORD bit_mask;

    bit_mask = (direction == FadeDirectionFrom)
        ? 0x0421    /* 0000010000100001 */
        : 0x4210;   /* 0100001000010000 */

    for (i = 0; i < 5; i++) { /* 5 bits per color */
        for (x = 0; x < 32; x++) {
            if (direction == FadeDirectionFrom && color == FadeColorBlack) {
                /* fade from black */
                FadePalette[x] = org_p[x] & bit_mask;
            } else if (direction == FadeDirectionFrom && color == FadeColorWhite) {
                /* fade from white */
                FadePalette[x] = org_p[x] | (0x7FFF & ~bit_mask);
            } else if (direction == FadeDirectionTo && color == FadeColorBlack) {
                /* fade to black */
                FadePalette[x] = org_p[x] & ~bit_mask;
            } else if (direction == FadeDirectionTo && color == FadeColorWhite) {
                /* fade to white */
                FadePalette[x] = org_p[x] | bit_mask;
            }
        }

        bit_mask |= (direction == FadeDirectionFrom)
            ? (bit_mask << 1)
            : (bit_mask >> 1);

        set_bkg_palette(0,8, &FadePalette[0]);
        delay(speed);
    }
}

fade.h

#ifndef _GBX_FADE_H
#define _GBX_FADE_H

typedef enum {
    FadeDirectionFrom,
    FadeDirectionTo
} FadeDirection;

typedef enum {
    FadeColorBlack,
    FadeColorWhite
} FadeColor;

void Fade(UWORD *org_p, FadeDirection direction, FadeColor color, UWORD speed);

#endif

Example usage:

#include <gb/gb.h>
#include "fade.h"

void main() {
    // Define your target palette
    const UWORD TargetPalette[] = {
        RGB(30, 30, 30), RGB(20, 20, 20), RGB(10, 10, 10), RGB(0 , 0 , 0 ),
        RGB(0 , 0 , 0 ), RGB(10, 10, 10), RGB(20, 20, 20), RGB(30, 30, 30),
    };

    // Fade from Black to Palette
    Fade(TargetPalette, FadeDirectionFrom, FadeColorBlack, 300);

    // Fade from White to Palette
    Fade(TargetPalette, FadeDirectionFrom, FadeColorWhite, 300);

    // Fade from Palette to White
    Fade(TargetPalette, FadeDirectionTo, FadeColorWhite, 300);

    // Fade from Palette to Black
    Fade(TargetPalette, FadeDirectionTo, FadeColorBlack, 300);
}