#include "trunc.h"

int32_t get_otp(uint8_t* hmac, uint8_t digits){
        uint8_t n;
        uint32_t dt;

        /*Step 1 generate HMAC-SHA1*/
        /*Step 2 get DT(HMAC)      */
        /*  get the lower 4-bits and treat them as a number n*/
        /*  DT is the 32bit integer at HMAC[n]+HMAC[n+1]+HMAC[n+2]+HMAC[n+3]*/
        n=hmac[SHA1_SIZE-1] & 0x0F;
        dt= (uint32_t)(hmac[n]&0x7F)<<(uint32_t)24 |
            (uint32_t)(hmac[n+1])<<(uint32_t)16 | 
            (uint32_t)(hmac[n+2])<<(uint32_t)8  | 
            (uint32_t)(hmac[n+3]);
        /*Step 3 return dt mod 10^digits*/
        switch(digits){
                     case 6: return dt % 1000000;
                     case 7: return dt % 10000000;
            default: case 8: return dt % 100000000;
        }
}

int32_t get_otp_from_cnt(uint64_t cnt, uint8_t digits){
    cnt=TO_LITTLE_ENDIAN64(cnt);
    hmac((uint8_t*)(&cnt),8);
    return get_otp(hmac_result,digits);
}