takes a sprite frame, from stretched frames, and returns the image index for it

sprite_get_index_from_frame.gml

/*
 * takes a sprite frame, from stretched frames, and returns the image index for it
 * @param {Asset.GMSprite} sprite the sprite we're looking up
 * @param {Real} frame the frame to check
 * @return {Real}
 */
function sprite_get_index_from_frame(sprite, frame) {
	
	var all_frames_information = sprite_get_info(sprite).frame_info;
	
	if (is_undefined(all_frames_information) == true) {
		return frame;
	}
	
	var index_count = sprite_get_number(sprite);
	
	var this_frame_information = undefined;
	var this_frame_number = 0;
	var this_index_frame_length = 0;
	
	for (var lookup_index = 0; lookup_index < index_count; lookup_index = lookup_index + 1) {
		
		this_frame_information = all_frames_information[lookup_index];
		this_frame_number = this_frame_information.frame;
		this_index_frame_length = this_frame_number + this_frame_information.duration;
		
		if (clamp(frame, this_frame_number, this_index_frame_length) == frame) {
			break;
		}		
	}
	
	return this_frame_information.image_index + (frame - this_frame_number) / (this_index_frame_length - this_frame_number);

}