# Chain

```markdown
// The chain is the source of truth for dark pool state and token ownership
Contract behavior is fully specified by the above descriptions. Writing the psuedocode here for completeness. The Solidity complements of the core three functions all involve 1) verifying the ZKP for proper commitent construction and balance attestation, 2) distributing balances, and 3) tracking the consumption of commitments. 

The bulk of gas expenditure is from the ZKP verifier (300k gas) and the ERC20 `transfer()` (50k gas per balance) calls. As a rough estimate on Mainnet, this is $22 to fill one order against 6 orders and $14 to place / cancel orders. The actual costs when we deploy will be a bit higher when the other operations are included. 

$place(\pi_{back}, \phi, \bar O, b, \sigma)$
1. Assert $V(pp_{back}.vkey, \pi_{back}, \bar O, b)$, where $pp_{back}.vkey$ is the verifying key for our $back$ circuit. 
2. Assert $V_{sig}(pk_{ellp}, \bar O, \sigma)$, where $V_{sig}(⋅)$ is the algorithm from our chosen signature scheme.
3. Call `transfer(msg.sender, this, b.0)` for $O.t.χ$ and `transfer(msg.sender, this, b.1)` for $O.t.d$. Assume appropriate `ERC20` permissions.
4. Add $\bar O$ with owner `msg.sender` to contract storage.

$fill(\pi_{fill}, \bar O_{own}, \{\bar O_i\}_{i=0}^n, b_{own}, \{b_i\}_{i=0}^n, O_n.p, \bar O_{cho}, \bar O_{chn}, \sigma_{cho}, \sigma_{chn})$
1. Assert $V(pp_{fill}.vkey, \pi_{fill}, \bar O_{own}, \{\bar O_i\}_{i=0}^n, b_{own}, \{b_i\}_{i=0}^n, O_n.p, \bar O_{cho}, \bar O_{chn})$, where $pp_{fill}.vkey$ is the verifying key for our $fill$ circuit. 
2. Assert $V_{sig}(pk_{ellp}, \bar O_{cho}, \sigma_{cho})$ and $V_{sig}(pk_{ellp}, \bar O_{chn}, \sigma_{chn})$, where $V_{sig}(⋅)$ is the algorithm from our chosen signature scheme.
3. Assert $\bar O_i \forall i \in [0, n)$ and $\bar O_{own}$ are active orders, then remove all of them from contract storage.
4. Assert `msg.sender` is the owner of $\bar O_{own}$.
5. Call `transfer(this, O_i_owner, b_i.0)` for $\bar O_i.t.χ$ and `transfer(this, b_i.1)` for $\bar O_i.t.d$ $\forall i \in [0, n)$. Assume appropriate `ERC20` permissions.
6. Call `transfer(this, msg.sender, b_own.0)` for $\bar O_{own}.t.χ$ and `transfer(this, b_own.1)` for $\bar O_{own}.t.d$. Assume appropriate `ERC20` permissions.
7. Add $\bar O_{cho}$ with owner `msg.sender` to contract storage.
8. Add $\bar O_{chn}$ with the owner of $O_n$ to contract storage. 

$cancel(\pi_{back}, \bar O, b)$
1. Assert $V(pp_{back}.vkey, \pi_{back}, \bar O, b)$, where $pp_{back}.vkey$ is the verifying key for our $back$ circuit. 
2. Assert `msg.sender` is the owner of $\bar O$
3. Call `transfer(this, msg.sender, b.0)` for $\bar O.t.χ$ and `transfer(this, msg.sender, b.1)` for $\bar O.t.d$. Assume appropriate `ERC20` permissions.
4. Remove $\bar O$ from contract storage.
```


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