how to modulo two uint64_t array?

[Dr88]

hi how to mod this two array in c
   //a=0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f8179
const uint64_t  a[4] =  {0x59f2815b16f81798, 0x029bfcdb2dce28d9, 0x55a06295ce870b07, 0x79be667ef9dcbbac};
  // b=0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8

const uint64_t  b[4] =  {0x9c47d08ffb10d4b8, 0xfd17b448a6855419, 0x5da4fbfc0e1108a8, 0x483ada7726a3c465};

// result = a%b;
uint64_t  result[4];

[NotATether]

Use the GMP library which allows for arbitrary-precision integers. It's much easier than trying to modulo multiple uint64_t parts - although you kinda miss out on performance if your end goal is to run it on CUDA.

You can even import these numbers in hexadecimal base - as well as any other base.

I'm going to assume your numbers are in big-endian, because manually flipping the bytes is too tedious for me.

Code:

const char* a = "59f2815b16f81798029bfcdb2dce28d955a06295ce870b0779be667ef9dcbbac";
const char* b = "483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8";
char result[256];

mpz_t ma, mb, mresult;
mpz_init(ma);
mpz_init(mb);
mpz_init(mresult);
mpz_set_str(ma, a, 16);
mpz_set_str(mb, b, 16);
mpz_fdiv_r(mresult, ma, mb); // <--- modulus

mpz_get_str(result, 16, mresult); // returns a hex string without a prefix

// ...

mpz_clear(ma);
mpz_clear(mb);
mpz_clear(mresult);

EDIT: forgot to add bases to mpz_set_str

[digaran]

Generator point on OP is invalid,  the last digit of //a = is missing, and NotATether, can you tell me why you used the correct y but mixed the x? So if your code is used the result would be unrelated to secp256k1.

What I like to know, what is the purpose of using Gx mod Gy?
Edit, thanks for the reply, I thought there is something new regarding the change of G in your script that I didn't know of.

[NotATether]

Generator point on OP is invalid,  the last digit of //a = is missing, and NotATether, can you tell me why you used the correct y but mixed the x? So if your code is used the result would be unrelated to secp256k1.

What I like to know, what is the purpose of using Gx mod Gy?

I just copied OP's pseudocode and changed it to use GMP - I had no idea that this was the generator point, but you can easily fix that.