How Jettons work on TON with sharding in mind (Part 2)

19 July, 2024

Written by 9oelm

jetton-wallet

Now, we have a look at jetton-wallet.fc, which is the 'child' contract of jetton.

Let us start with the TL-B scheme of storage:

The storage contains balance, owner_address, jetton_master_address, and jetton_wallet_code. Should be self-explanatory. Recall that jetton_wallet_code is the compiled code of jetton-wallet.

load_data() loads all information stored:

save_data does the opposite:

pack_jetton_wallet_data does nothing but to store the data into a cell, which was the same for jetton-minter. Recall that this is because we can only store a cell:

Now, let us start from recv_internal, which is invoked when the contract receives an internal message:

On bounce

From the previous section explaining jetton-minter, we know that the 4th bit is bounced:Bool:

That is why we are checking the 4th bit of the message in the code:

slice cs = in_msg_full.begin_parse();
int flags = cs~load_uint(4);
if (flags & 1) {
  on_bounce(in_msg_body);
  return ();
}

on_bounce function is defined here:

in_msg_body~skip_bits(32); ;; 0xFFFFFFFF: we do this because the body of the bounced message will contain 32 bit 0xffffffff followed by 256 bit from original message.
we load the data with load_data().
the original op is retrieved by int op = in_msg_body~load_uint(32);. Recall that the structure of the message body is 32 bits of operation followed by 64 bits of query id.
throw_unless(709, (op == op::internal_transfer()) | (op == op::burn_notification())); throws if the bounced operation isn't internal transfer or burn notification. The error code is static invalid_op = 709.
int query_id = in_msg_body~load_uint(64); is loaded to get past the 64 bits.
int jetton_amount = in_msg_body~load_coins(); loads the amount that was sent in the message body.
The amount is credited again back to balance and written to the storage. This prevents failed messages from falsely deducting the balance.
```
balance += jetton_amount;
save_data(balance, owner_address, jetton_master_address, jetton_wallet_code);
```

We will understand this function better once we look at send_tokens and burn_tokens functions.

op::transfer

Next, we prepare arguments to use. Recall the int_msg_info$0 constructor and its scheme. We are fast forwarding up to the end of fwd_fee:Grams in src:MsgAddressInt dest:MsgAddressInt value:CurrencyCollection ihr_fee:Grams fwd_fee:Grams:

muldiv(cs~load_coins(), 3, 2) means floor(cs~load_coins() * 3 / 2), meaning it wants to get 1.5 times the original message's fwd_fee. muldiv is a multiple-then-divide operation. The intermediate result is stored in 513-bit integer, so it won't overflow if the actual result fits into a 257-bit integer. ¹

The forward fee is for outgoing messages (any messages that go out of a contract).

Now when the opcode is op::transfer, we call send_tokens:

Let's break it down.

Note the TL-B schemes above send_tokens declaration:

transfer constructor is the message coming from the user's master wallet into this jetton wallet. internal_transfer constructor is the message going out of this jetton wallet.
int query_id = in_msg_body~load_uint(64);. Remember we already loaded the opcode, so the next up is 64-bits long query_id.
The rest of in_msg_body is customizable. We can find the rest of the body at wrappers/JettonWallet.ts:

After query_id, jetton_amount, specifying the amount to send, is stored by int jetton_amount = in_msg_body~load_coins();. Same for to_owner_address. The rest of the cell structure is pretty self-explanatory. Just refer to this while reading send_tokens.
force_chain(to_owner_address);

The reason for calling this function can be traced back to the comment at the top of jetton-wallet.fc file:

The TON Blockchain consists of one masterchain and up to $2^32$ workchains. Each workchain is a separate chain with its rules. Each workchain can further split into 260 shardchains, or sub-shards, containing a fraction of the workchain's state. Currently, only one workchain is operating on TON - Basechain ². The Basechain has an workchain_id of 0.3 So the default mode is always transferring within the same workchain; and without changing this line, the jetton will always be forced to be sent to the same workchain.

parse_std_addr returns the workchain id and account id. int workchain() asm "0 PUSHINT"; just means declaring a zero integer variable. Why zero? because we know that there is only one chain right now, which is the Basechain. And its workchain id is 0. So if to_owner_address is from another workchain whose workchain id isn't 0, throw_unless(333, wc == workchain()); will throw.
(int balance, slice owner_address, slice jetton_master_address, cell jetton_wallet_code) = load_data();. Load the data.
balance -= jetton_amount;. Deduct the amount being transferred.
throw_unless(705, equal_slices(owner_address, sender_address));. Throw if the message didn't come from the owner's wallet.
throw_unless(706, balance >= 0);. Throw if the balance is insufficient.
cell state_init = calculate_jetton_wallet_state_init(to_owner_address, jetton_master_address, jetton_wallet_code);.

Don't know why the cell structure looks like this? Back to the TL-B scheme.
1. store_uint(0, 2). Two bits of zeroes mean there's nothing in split_depth and special. So we go past them.
2. store_dict(jetton_wallet_code). You might be wondering why store_dict instead of something like store_ref, because it's a cell? Well, store_dict actually looks like builder store_dict(builder b, cell c) asm(c b) "STDICT"; and stores dictionary D represented by cell c or null into builder b. In other words, stores 1-bit and a reference to c if c is not null and 0-bit otherwise. ³ So it's a perfect use case for short-circuiting Maybe ^Cell at once, because Maybe requires 1 to be prefixed if there is something in it. For that reason, store_maybe_ref is equivalent to store_dict`.
3. .store_dict(pack_jetton_wallet_data(0, owner_address, jetton_master_address, jetton_wallet_code))
4. .store_uint(0, 1). This is library:(Maybe ^Cell).
slice to_wallet_address = calculate_jetton_wallet_address(state_init);.

calculate_jetton_wallet_address takes the result of calculate_jetton_wallet_state_init and returns a wallet address. to_wallet_address will be dest:MsgAddressInt in int_msg_info$0, so we will need to refer to MsgAddressInt when looking into this.
1. store_uint(4, 3). It stores 0b100. The first 0b10 is for addr_std$10 prefix. The next 0 is for Maybe, to say that there is none.
2. .store_int(workchain(), 8). Store a 8-bit long workchain id, which is 0b00000000. The workchain id is a signed 32-bit integer, but addr_std dictates that workchain_id needs to be int8 in this specific type.
3. cell_hash returns 256-bit uint hash of a cell. state_init is composed of jetton_wallet_code, owner_address, and jetton_master_address, so every jetton wallet address is practically unique, because if it were a different jetton, owner, or a jetton creator, the hash would be different. This is address:bits256.
slice response_address = in_msg_body~load_msg_addr(); This is response_destination:MsgAddress of transfer constructor.
cell custom_payload = in_msg_body~load_dict(); is custom_payload:(Maybe ^Cell) of transfer constructor. We use load_dict() here to load Maybe ^Cell, because it can be used for values of arbitrary Maybe ^Y types or a dictionary.
int forward_ton_amount = in_msg_body~load_coins(); is loading forward_ton_amount:(VarUInteger 16). Note that VarUInteger 16 means $128 - 8 = 120$ bits of uint.
throw_unless(708, slice_bits(in_msg_body) >= 1);. slice_bits(in_msg_body) will check if there is any remaining data if slice_bits(in_msg_body) >= 1 is not true. This can happen in case of a malformed forward payload. For example, if forward_payload:(Either Cell ^Cell) is not included in in_msg_body at all, this would throw because slice_bits(in_msg_body) == 0.
Now that we checked that there's a remaining part of the message, we safely call slice either_forward_payload = in_msg_body;.