We use Cairo’s common math library, specifically unsigned_div_rem (unsigned division with remainder) to calculate the amount of tokens to be received.

// Calculate swap amount.
    let (local amount_to, _) = unsigned_div_rem(
        amm_to_balance * amount_from,
        amm_from_balance + amount_from,
    );
uint256_unsigned_div_rem takes Uint256 arguments.

Recall that Uint256 integers are encoded as two felts representing the 256 bits as two 128 bits numbers low and high.

Thus, uint256_unsigned_div_rem needs to do more thorough checks than the felt version to make sure the encoding is safe and sound. Namely, it calls uint256_check on return values and needs to verify the result (gotten via a hint). Those checks can be seen here in the source, while the felt counterpart is more straightforward.

In general, functions using the Uint256 struct will be less efficient than their felt counterpart.

Additional resources can be found here and here.

Can someone explain the difference `between unsigned_div_rem` and `uint256_unsigned_div_rem`?

Why unit version process so many extra steps, but overall number of steps for computing are different for these methods?



Can someone explain the difference between unsigned_div_rem and uint256_unsigned_div_rem?

Initializing the pool is a simple function that accepts two balances for the tokens (A,B), and sets them using the set_pool_token_balance function we defined above: The POOL_UPPER_BOUND is a constant defined to prevent overflows.

Having this function defined, we proceed to add demo tokens to an account:
@external
func add_demo_token{
    syscall_ptr: felt*,
    pedersen_ptr: HashBuiltin*,
    range_check_ptr,
}(token_a_amount: felt, token_b_amount: felt) {
    let (account_id) = get_caller_address();

    // Make sure the account's balance is much smaller than
    // the pool init balance.
    assert_nn_le(token_a_amount, ACCOUNT_BALANCE_BOUND - 1);
    assert_nn_le(token_b_amount, ACCOUNT_BALANCE_BOUND - 1);

    modify_account_balance(
        account_id=account_id,
        token_type=TOKEN_TYPE_A,
        amount=token_a_amount,
    );
    modify_account_balance(
        account_id=account_id,
        token_type=TOKEN_TYPE_B,
        amount=token_b_amount,
    );

    return ();
}

Hey!
I had a question about what would be the cleanest way to imitate a `uint32` or `uint64` in cairo. Two approaches came to mind:
1. Simply use a felt and check that is less than `2**32 - 1`
2. Pass a Uint256 and check that var.high == 0 and var.low is less than `2 ** 32 -1`

First approach seems straightforward, but I wanted to know if there was a convention for this or a better way.

How to check for smaller Uints in Cairo?

In order to obtain the address of the account contract (or any other contract, in the case that the function was invoked by a contract) that invoked our function, we can use the get_caller_address() library function:

from starkware.starknet.common.syscalls import get_caller_address

// ...

let (caller_address) = get_caller_address();
get_caller_address() returns the address of the source contract that called this contract. It can be the address of the account contract or the address of another contract (if the function was invoked by another contract). When the contract is called directly (rather than through a contract), the function returns 0.

Note that if you use get_caller_address() in a function foo() that was called by another function bar() within your contract, it will still return the address of the contract that invoked bar() (or 0 if it was invoked directly).



Hello! I was wondering whether it was possible to check if a certain address has code in Cairo.
In Solidity, we do something like:

```
assembly {
  codeSize := extcodesize(contractAddress)
}
or just:
uint256 codeSize = contractAddress.code;
```
I was wondering if there was a way to check something like this in cairo? I checked the syscalls file but didn't find anything promising there.

Thank you in advance!

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