diamond-fallback.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// Find facet for function that is called and execute the
// function if a facet is found and return any value.
fallback() external payable {
  // get facet from function selector
  address facet = selectorTofacet[msg.sig];
  require(facet != address(0));
  // Execute external function from facet using delegatecall and return any value.
  assembly {
    // copy function selector and any arguments
    calldatacopy(0, 0, calldatasize())
    // execute function call using the facet
    let result := delegatecall(gas(), facet, 0, calldatasize(), 0, 0)
    // get any return value
    returndatacopy(0, 0, returndatasize())
    // return any return value or error back to the caller
    switch result
      case 0 {revert(0, returndatasize())}
      default {return (0, returndatasize())}
  }
}