# Router

The router is the single entry point for all user-facing protocol operations. It uses a diamond-like proxy pattern where each function selector maps to an action facet contract via `delegatecall`.

{% hint style="info" %}
Since all facets share storage via `delegatecall`, `RouterStorage` uses ERC-7201 storage namespacing to avoid collisions.
{% endhint %}

## FiraRouterV4

Extends OpenZeppelin's `Proxy` and overrides `_implementation()` to look up `CoreStorage.selectorToFacet[msg.sig]`. If no facet is registered, it reverts with `INVALID_SELECTOR`.

The constructor hard-wires the `setSelectorToFacets` and `selectorToFacet` selectors to the `ActionStorageV4` facet, allowing the owner to configure all other mappings post-deployment.

## Facet contracts

* [ActionBorrow](https://docs.fira.money/developers/protocol-contracts/router/action-borrow) — Fixed-rate borrow and repay
* [ActionSwapBT](https://docs.fira.money/developers/protocol-contracts/router/action-swap-bt) — BT swap operations
* [ActionSwapCT](https://docs.fira.money/developers/protocol-contracts/router/action-swap-ct) — Synthesized CT swaps
* [ActionAddRemoveLiq](https://docs.fira.money/developers/protocol-contracts/router/action-add-remove-liq) — Liquidity operations
* [ActionMisc](https://docs.fira.money/developers/protocol-contracts/router/action-misc) — FW/BC minting, exits, multicall
* [ActionSimple](https://docs.fira.money/developers/protocol-contracts/router/action-simple) — Gas-optimized on-chain approximation

## Shared base contracts

All facets inherit from `ActionBase`, which provides:

* FW mint/redeem orchestration
* BT/CT swap orchestration with limit order support
* Flash-callback encoding/decoding via `CallbackHelper`
* Delegation to `ActionSimple` when on-chain approximation suffices


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