Add example for fork + ref_transact impl (#296)

* Add example for fork + ref_transact impl

* Update example to ethersdb

* Update examples/fork_ref_transact.rs

examples/fork_ref_transact.rs

@@ -0,0 +1,107 @@
+use std::{str::FromStr, sync::Arc};
+use anyhow::{Ok, Result};
+use bytes::Bytes;
+use ethers::{
+    abi::parse_abi,
+    prelude::BaseContract,
+    providers::{Http, Provider},
+};
+use revm::{
+    db::{CacheDB, EmptyDB, EthersDB},
+    Database, TransactOut, TransactTo, B160, EVM, U256 as rU256,
+};
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    // create ethers client and wrap it in Arc<M>
+    let client = Provider::<Http>::try_from(
+        "https://mainnet.infura.io/v3/c60b0bb42f8a4c6481ecd229eddaca27",
+    )?;
+    let client = Arc::new(client);
+
+    // ----------------------------------------------------------- //
+    //             Storage slots of UniV2Pair contract             //
+    // =========================================================== //
+    // storage[5] = factory: address                               //
+    // storage[6] = token0: address                                //
+    // storage[7] = token1: address                                //
+    // storage[8] = (res0, res1, ts): (uint112, uint112, uint32)   //
+    // storage[9] = price0CumulativeLast: uint256                  //
+    // storage[10] = price1CumulativeLast: uint256                 //
+    // storage[11] = kLast: uint256                                //
+    // =========================================================== //
+
+    // choose slot of storage that you would like to transact with
+    let slot = rU256::from(8);
+
+    // ETH/USDT pair on Uniswap V2
+    let pool_address = B160::from_str("0x0d4a11d5EEaaC28EC3F61d100daF4d40471f1852")?;
+
+    // generate abi for the calldata from the human readable interface
+    let abi = BaseContract::from(
+        parse_abi(&[
+            "function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast)",
+        ])?
+    );
+
+    // encode abi into Bytes
+    let encoded = abi.encode("getReserves", ())?;
+
+    // initialize new EthersDB
+    let mut ethersdb = EthersDB::new(Arc::clone(&client), None).unwrap();
+
+    // query basic properties of an account incl bytecode
+    let acc_info = ethersdb.basic(pool_address).unwrap().unwrap();
+
+    // query value of storage slot at account address
+    let value = ethersdb.storage(pool_address, slot).unwrap();
+
+    // initialise empty in-memory-db
+    let mut cache_db = CacheDB::new(EmptyDB::default());
+
+    // insert basic account info which was generated via Web3DB with the corresponding address
+    cache_db.insert_account_info(pool_address, acc_info);
+
+    // insert our pre-loaded storage slot to the corresponding contract key (address) in the DB
+    cache_db
+        .insert_account_storage(pool_address, slot, value)
+        .unwrap();
+
+    // initialise an empty (default) EVM
+    let mut evm = EVM::new();
+
+    // insert pre-built database from above
+    evm.database(cache_db);
+
+    // fill in missing bits of env struc
+    // change that to whatever caller you want to be
+    evm.env.tx.caller = B160::from_str("0x0000000000000000000000000000000000000000")?;
+    // account you want to transact with
+    evm.env.tx.transact_to = TransactTo::Call(pool_address);
+    // calldata formed via abigen
+    evm.env.tx.data = Bytes::from(hex::decode(hex::encode(&encoded))?);
+    // transaction value in wei
+    evm.env.tx.value = rU256::try_from(0)?;
+
+    // execute transaction without writing to the DB
+    let ref_tx = evm.transact_ref();
+    // select ExecutionResult struct
+    let result = ref_tx.0;
+
+    // unpack output call enum into raw bytes
+    let value = match result.out {
+        TransactOut::Call(value) => Some(value),
+        _ => None,
+    };
+
+    // decode bytes to reserves + ts via ethers-rs's abi decode
+    let (reserve0, reserve1, ts): (u128, u128, u32) =
+        abi.decode_output("getReserves", value.unwrap())?;
+
+    // Print emualted getReserves() call output
+    println!("Reserve0: {:#?}", reserve0);
+    println!("Reserve1: {:#?}", reserve1);
+    println!("Timestamp: {:#?}", ts);
+
+    Ok(())
+}