#include "Arduino.h"
#include "variables.h"

#define SBUS_MIN_OFFSET 173
#define SBUS_MID_OFFSET 992
#define SBUS_MAX_OFFSET 1811
#define SBUS_CHANNEL_NUMBER 16
#define SBUS_PACKET_LENGTH 25
#define SBUS_FRAME_HEADER 0x0f
#define SBUS_FRAME_FOOTER 0x00
#define SBUS_FRAME_FOOTER_V2 0x04
#define SBUS_STATE_FAILSAFE 0x08
#define SBUS_STATE_SIGNALLOSS 0x04

/*
Precomputed mapping from 990-2010 to 173:1811
equivalent to 
map(channels[i], RC_CHANNEL_MIN, RC_CHANNEL_MAX, SBUS_MIN_OFFSET, SBUS_MAX_OFFSET);
*/
int mapChannelToSbus(int in) {
    return (((long) in * 1605l) / 1000l) - 1417;
}

void sbusPreparePacket(uint8_t packet[], int channels[], bool isSignalLoss, bool isFailsafe){
    
    static int output[SBUS_CHANNEL_NUMBER] = {0};
    
    /*
     * Map 1000-2000 with middle at 1500 chanel values to
     * 173-1811 with middle at 992 S.BUS protocol requires
     */
    for (uint8_t i = 0; i < SBUS_CHANNEL_NUMBER; i++) {
        output[i] = mapChannelToSbus(channels[i]);
    }

    uint8_t stateByte = 0x00;
    if (isSignalLoss) {
        stateByte |= SBUS_STATE_SIGNALLOSS;
    }
    if (isFailsafe) {
        stateByte |= SBUS_STATE_FAILSAFE;
    }
    packet[0] = SBUS_FRAME_HEADER; //Header
    
    packet[1] = (uint8_t) (output[0] & 0x07FF);
    packet[2] = (uint8_t) ((output[0] & 0x07FF)>>8 | (output[1] & 0x07FF)<<3);
    packet[3] = (uint8_t) ((output[1] & 0x07FF)>>5 | (output[2] & 0x07FF)<<6);
    packet[4] = (uint8_t) ((output[2] & 0x07FF)>>2);
    packet[5] = (uint8_t) ((output[2] & 0x07FF)>>10 | (output[3] & 0x07FF)<<1);
    packet[6] = (uint8_t) ((output[3] & 0x07FF)>>7 | (output[4] & 0x07FF)<<4);
    packet[7] = (uint8_t) ((output[4] & 0x07FF)>>4 | (output[5] & 0x07FF)<<7);
    packet[8] = (uint8_t) ((output[5] & 0x07FF)>>1);
    packet[9] = (uint8_t) ((output[5] & 0x07FF)>>9 | (output[6] & 0x07FF)<<2);
    packet[10] = (uint8_t) ((output[6] & 0x07FF)>>6 | (output[7] & 0x07FF)<<5);
    packet[11] = (uint8_t) ((output[7] & 0x07FF)>>3);
    packet[12] = (uint8_t) ((output[8] & 0x07FF));
    packet[13] = (uint8_t) ((output[8] & 0x07FF)>>8 | (output[9] & 0x07FF)<<3);
    packet[14] = (uint8_t) ((output[9] & 0x07FF)>>5 | (output[10] & 0x07FF)<<6);  
    packet[15] = (uint8_t) ((output[10] & 0x07FF)>>2);
    packet[16] = (uint8_t) ((output[10] & 0x07FF)>>10 | (output[11] & 0x07FF)<<1);
    packet[17] = (uint8_t) ((output[11] & 0x07FF)>>7 | (output[12] & 0x07FF)<<4);
    packet[18] = (uint8_t) ((output[12] & 0x07FF)>>4 | (output[13] & 0x07FF)<<7);
    packet[19] = (uint8_t) ((output[13] & 0x07FF)>>1);
    packet[20] = (uint8_t) ((output[13] & 0x07FF)>>9 | (output[14] & 0x07FF)<<2);
    packet[21] = (uint8_t) ((output[14] & 0x07FF)>>6 | (output[15] & 0x07FF)<<5);
    packet[22] = (uint8_t) ((output[15] & 0x07FF)>>3);
    
    packet[23] = stateByte; //Flags byte
    packet[24] = SBUS_FRAME_FOOTER; //Footer
}