//! Implementation details of mock node.

use std::collections::HashMap;
use std::future::ready;
use std::sync::{Arc, Mutex};

use ethcontract::common::abi::{Function, StateMutability, Token};
use ethcontract::common::hash::H32;
use ethcontract::common::{Abi, FunctionExt};
use ethcontract::jsonrpc::serde::Serialize;
use ethcontract::jsonrpc::serde_json::to_value;
use ethcontract::jsonrpc::{Call, MethodCall, Params, Value};
use ethcontract::tokens::Tokenize;
use ethcontract::web3::types::{
    Bytes, CallRequest, TransactionReceipt, TransactionRequest, U256, U64,
};
use ethcontract::web3::{helpers, BatchTransport, Error, RequestId, Transport};
use ethcontract::{Address, BlockNumber, H160, H256};
use parse::Parser;
use sign::verify;

use crate::details::transaction::TransactionResult;
use crate::range::TimesRange;
use crate::CallContext;
use std::any::Any;

mod default;
mod parse;
mod sign;
mod transaction;

/// Mock transport.
#[derive(Clone)]
pub struct MockTransport {
    /// Mutable state.
    state: Arc<Mutex<MockTransportState>>,
}

/// Internal transport state, protected by a mutex.
struct MockTransportState {
    /// Chain ID.
    chain_id: u64,

    /// Current gas price.
    gas_price: u64,

    /// This counter is used to keep track of prepared calls.
    request_id: RequestId,

    /// This counter is used to keep track of mined blocks.
    block: u64,

    /// This counter is used to generate contract addresses.
    address: u64,

    /// Nonce for account.
    nonce: HashMap<Address, u64>,

    /// Deployed mocked contracts.
    contracts: HashMap<Address, Contract>,

    /// Receipts for already performed transactions.
    receipts: HashMap<H256, TransactionReceipt>,
}

#[allow(clippy::type_complexity)]
impl MockTransport {
    /// Creates a new transport.
    pub fn new(chain_id: u64) -> Self {
        MockTransport {
            state: Arc::new(Mutex::new(MockTransportState {
                chain_id,
                gas_price: 1,
                request_id: 0,
                block: 0,
                address: 0,
                nonce: HashMap::new(),
                contracts: HashMap::new(),
                receipts: HashMap::new(),
            })),
        }
    }

    /// Deploys a new contract with the given ABI.
    pub fn deploy(&self, abi: &Abi) -> Address {
        let mut state = self.state.lock().unwrap();

        state.address += 1;
        let address = H160::from_low_u64_be(state.address);

        state.contracts.insert(address, Contract::new(address, abi));

        address
    }

    /// Deploys a new contract with the given ABI and address
    pub fn deploy_with_address(&self, abi: &Abi, address: Address) {
        let mut state = self.state.lock().unwrap();

        assert!(
            state
                .contracts
                .insert(address, Contract::new(address, abi))
                .is_none(),
            "replacing contract at address {:?}",
            address
        );
    }

    pub fn update_gas_price(&self, gas_price: u64) {
        let mut state = self.state.lock().unwrap();
        state.gas_price = gas_price;
    }

    pub fn checkpoint(&self) {
        let mut state = self.state.lock().unwrap();
        let contracts = state.contracts.values_mut();
        for contract in contracts {
            contract.checkpoint();
        }
    }

    pub fn expect<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
    ) -> (usize, usize) {
        let mut state = self.state.lock().unwrap();
        let method = state.method(address, signature);
        method.expect::<P, R>()
    }

    pub fn contract_checkpoint(&self, address: Address) {
        let mut state = self.state.lock().unwrap();
        let contract = state.contract(address);
        contract.checkpoint();
    }

    pub fn times<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        times: TimesRange,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);

        if expectation.sequence.is_some() && !times.is_exact() {
            panic!("only expectations with an exact call count can be in a sequences")
        }
        if expectation.sequence.is_some() && times.lower_bound() == 0 {
            panic!("expectation in a sequences should be called at least once")
        }

        expectation.times = times;
    }

    pub fn in_sequence<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        sequence: &mut mockall::Sequence,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);

        if !expectation.times.is_exact() {
            panic!("only expectations with an exact call count can be in a sequences")
        }
        if expectation.times.lower_bound() == 0 {
            panic!("expectation in a sequences should be called at least once")
        }
        if expectation.sequence.is_some() {
            panic!("expectation can't be in multiple sequences")
        }

        expectation.sequence = Some(sequence.next_handle());
    }

    pub fn confirmations<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        confirmations: u64,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.confirmations = confirmations;
    }

    pub fn predicate<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        pred: Box<dyn predicates::Predicate<P> + Send>,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.predicate = Predicate::Predicate(pred);
    }

    pub fn predicate_fn<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        pred: Box<dyn Fn(&P) -> bool + Send>,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.predicate = Predicate::Function(pred);
    }

    pub fn predicate_fn_ctx<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        pred: Box<dyn Fn(&CallContext, &P) -> bool + Send>,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.predicate = Predicate::TxFunction(pred);
    }

    pub fn allow_calls<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        allow_calls: bool,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.allow_calls = allow_calls;
    }

    pub fn allow_transactions<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        allow_transactions: bool,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.allow_transactions = allow_transactions;
    }

    pub fn returns<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        returns: R,
    ) {
        // Convert `R` into `Token` here because `Token` is `Clone` while `R` is not.
        // We need to clone result const if method is called multiple times.
        let token = returns.into_token();
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.returns = Returns::Const(token);
    }

    pub fn returns_fn<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        returns: Box<dyn Fn(P) -> Result<R, String> + Send>,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.returns = Returns::Function(returns);
    }

    pub fn returns_fn_ctx<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        returns: Box<dyn Fn(&CallContext, P) -> Result<R, String> + Send>,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.returns = Returns::TxFunction(returns);
    }

    pub fn returns_error<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
        error: String,
    ) {
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.returns = Returns::Error(error);
    }

    pub fn returns_default<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
    ) {
        // Convert `R` into `Token` here because `Token` is `Clone` while `R` is not.
        // We need to clone result const if method is called multiple times.
        let mut state = self.state.lock().unwrap();
        let expectation = state.expectation::<P, R>(address, signature, index, generation);
        expectation.returns = Returns::Default;
    }
}

impl MockTransportState {
    /// Returns contract at the given address, panics if contract does not exist.
    fn contract(&mut self, address: Address) -> &mut Contract {
        match self.contracts.get_mut(&address) {
            Some(contract) => contract,
            None => panic!("there is no mocked contract with address {:#x}", address),
        }
    }

    /// Returns contract's method.
    fn method(&mut self, address: Address, signature: H32) -> &mut Method {
        self.contract(address).method(signature)
    }

    /// Returns contract's expectation.
    fn expectation<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &mut self,
        address: Address,
        signature: H32,
        index: usize,
        generation: usize,
    ) -> &mut Expectation<P, R> {
        self.contract(address)
            .method(signature)
            .expectation(index, generation)
    }
}

impl Transport for MockTransport {
    type Out = std::future::Ready<Result<Value, Error>>;

    /// Prepares an RPC call for given method with parameters.
    ///
    /// We don't have to deal with network issues, so we are relaxed about
    /// request IDs, idempotency checks and so on.
    fn prepare(&self, method: &str, params: Vec<Value>) -> (RequestId, Call) {
        let mut state = self.state.lock().unwrap();

        let id = state.request_id;
        state.request_id += 1;

        let request = helpers::build_request(id, method, params);

        (id, request)
    }

    /// Executes a prepared RPC call.
    fn send(&self, _: RequestId, request: Call) -> Self::Out {
        ready(self.process_call(request))
    }
}

impl BatchTransport for MockTransport {
    type Batch = std::future::Ready<Result<Vec<Result<Value, Error>>, Error>>;

    fn send_batch<T>(&self, requests: T) -> Self::Batch
    where
        T: IntoIterator<Item = (RequestId, Call)>,
    {
        let mut results = Vec::new();
        for (_, call) in requests.into_iter() {
            results.push(self.process_call(call));
        }

        ready(Ok(results))
    }
}

impl MockTransport {
    fn process_call(&self, request: Call) -> Result<Value, Error> {
        let MethodCall { method, params, .. } = match request {
            Call::MethodCall(method_call) => method_call,
            Call::Notification(_) => panic!("rpc notifications are not supported"),
            _ => panic!("unknown or invalid rpc call type"),
        };

        let params = match params {
            Params::None => Vec::new(),
            Params::Array(array) => array,
            Params::Map(_) => panic!("passing arguments by map is not supported"),
        };

        let result = match method.as_str() {
            "eth_blockNumber" => {
                let name = "eth_blockNumber";
                self.eth_block_number(Parser::new(name, params))
            }
            "eth_chainId" => {
                let name = "eth_chainId";
                self.eth_chain_id(Parser::new(name, params))
            }
            "eth_getTransactionCount" => {
                let name = "eth_getTransactionCount";
                self.eth_transaction_count(Parser::new(name, params))
            }
            "eth_gasPrice" => {
                let name = "eth_gasPrice";
                self.eth_gas_price(Parser::new(name, params))
            }
            "eth_estimateGas" => {
                let name = "eth_estimateGas";
                self.eth_estimate_gas(Parser::new(name, params))
            }
            "eth_call" => {
                let name = "eth_call";
                self.eth_call(Parser::new(name, params))
            }
            "eth_sendTransaction" => {
                let name = "eth_sendTransaction";
                self.eth_send_transaction(Parser::new(name, params))
            }
            "eth_sendRawTransaction" => {
                let name = "eth_sendRawTransaction";
                self.eth_send_raw_transaction(Parser::new(name, params))
            }
            "eth_getTransactionReceipt" => {
                let name = "eth_getTransactionReceipt";
                self.eth_get_transaction_receipt(Parser::new(name, params))
            }
            unsupported => panic!("mock node does not support rpc method {:?}", unsupported),
        };

        result
    }

    fn eth_block_number(&self, args: Parser) -> Result<Value, Error> {
        args.done();

        let state = self.state.lock().unwrap();
        Self::ok(U64::from(state.block))
    }

    fn eth_chain_id(&self, args: Parser) -> Result<Value, Error> {
        args.done();

        let state = self.state.lock().unwrap();
        Self::ok(U256::from(state.chain_id))
    }

    fn eth_transaction_count(&self, mut args: Parser) -> Result<Value, Error> {
        let address: Address = args.arg();
        let block: Option<BlockNumber> = args.block_number_opt();
        args.done();

        let block = block.unwrap_or(BlockNumber::Pending);
        let state = self.state.lock().unwrap();
        let transaction_count = match block {
            BlockNumber::Earliest => 0,
            BlockNumber::Number(n) if n == 0.into() => 0,
            BlockNumber::Number(n) if n != state.block.into() => {
                panic!("mock node does not support returning transaction count for specific block number");
            }
            _ => state.nonce.get(&address).copied().unwrap_or(0),
        };
        Self::ok(U256::from(transaction_count))
    }

    fn eth_gas_price(&self, args: Parser) -> Result<Value, Error> {
        args.done();

        let state = self.state.lock().unwrap();
        Self::ok(U256::from(state.gas_price))
    }

    fn eth_estimate_gas(&self, mut args: Parser) -> Result<Value, Error> {
        let request: CallRequest = args.arg();
        let block: Option<BlockNumber> = args.block_number_opt();
        args.done();

        let state = self.state.lock().unwrap();

        let block = block.unwrap_or(BlockNumber::Pending);
        match block {
            BlockNumber::Earliest => {
                panic!("mock node does not support executing methods on earliest block");
            }
            BlockNumber::Number(n) if n != state.block.into() => {
                panic!("mock node does not support executing methods on non-last block");
            }
            _ => (),
        }

        match request.to {
            None => panic!("call's 'to' field is empty"),
            Some(to) => to,
        };

        // TODO:
        //
        // We could look up contract's method, match an expectation,
        // and see if the expectation defines gas price.
        //
        // So, for example, this code:
        //
        // ```
        // contract
        //     .expect_method(signature)
        //     .with(matcher)
        //     .gas(100);
        // ```
        //
        // Indicates that call to the method with the given signature
        // requires 100 gas.
        //
        // When estimating gas, we'll check all expectation as if we're
        // executing a method, but we won't mark any expectation as fulfilled.

        Self::ok(U256::from(1))
    }

    fn eth_call(&self, mut args: Parser) -> Result<Value, Error> {
        let request: CallRequest = args.arg();
        let block: Option<BlockNumber> = args.block_number_opt();

        let mut state = self.state.lock().unwrap();

        let block = block.unwrap_or(BlockNumber::Pending);
        match block {
            BlockNumber::Earliest => {
                panic!("mock node does not support executing methods on earliest block");
            }
            BlockNumber::Number(n) if n != state.block.into() => {
                panic!("mock node does not support executing methods on non-last block");
            }
            _ => (),
        }

        let from = request.from.unwrap_or_default();
        let to = match request.to {
            None => panic!("call's 'to' field is empty"),
            Some(to) => to,
        };

        let nonce = state.nonce.get(&from).copied().unwrap_or(0);

        let gas_price = state.gas_price;

        let contract = state.contract(to);

        let context = CallContext {
            is_view_call: true,
            from: request.from.unwrap_or_default(),
            to,
            nonce: U256::from(nonce),
            gas: request.gas.unwrap_or_else(|| U256::from(1)),
            gas_price: request.gas.unwrap_or_else(|| U256::from(gas_price)),
            value: request.value.unwrap_or_default(),
        };

        let data = request.data.unwrap_or_default();

        let result = contract.process_tx(context, &data.0);

        match result.result {
            Ok(data) => Self::ok(Bytes(data)),
            Err(err) => Err(Error::Rpc(ethcontract::jsonrpc::Error {
                code: ethcontract::jsonrpc::ErrorCode::ServerError(0),
                message: format!("execution reverted: {}", err),
                data: None,
            })),
        }
    }

    fn eth_send_transaction(&self, mut args: Parser) -> Result<Value, Error> {
        let _request: TransactionRequest = args.arg();
        args.done();

        // TODO:
        //
        // We could support signing if user adds accounts with their private
        // keys during mock setup.

        panic!("mock node can't sign transactions, use offline signing with private key");
    }

    fn eth_send_raw_transaction(&self, mut args: Parser) -> Result<Value, Error> {
        let raw_tx: Bytes = args.arg();
        args.done();

        let mut state = self.state.lock().unwrap();

        let tx = verify(&raw_tx.0, state.chain_id);

        let nonce = state.nonce.entry(tx.from).or_insert(0);
        assert!(
            *nonce == tx.nonce.as_u64(),
            "nonce mismatch for account {:#x}: expected {}, actual {}",
            tx.from,
            tx.nonce.as_u64(),
            nonce
        );
        *nonce += 1;

        let contract = state.contract(tx.to);

        let context = CallContext {
            is_view_call: false,
            from: tx.from,
            to: tx.to,
            nonce: tx.nonce,
            gas: tx.gas,
            gas_price: tx.gas_price,
            value: tx.value,
        };

        let result = contract.process_tx(context, &tx.data);

        state.block += 1;

        let receipt = TransactionReceipt {
            transaction_hash: tx.hash,
            transaction_index: U64::from(0),
            block_hash: None,
            block_number: Some(U64::from(state.block)),
            from: tx.from,
            to: Some(tx.to),
            cumulative_gas_used: U256::from(1),
            gas_used: None,
            contract_address: None,
            logs: vec![],
            status: Some(U64::from(result.result.is_ok() as u64)),
            root: None,
            logs_bloom: Default::default(),
            transaction_type: None,
            effective_gas_price: Some(tx.gas_price),
        };

        state.receipts.insert(tx.hash, receipt);

        state.block += result.confirmations;

        Self::ok(tx.hash)
    }

    fn eth_get_transaction_receipt(&self, mut args: Parser) -> Result<Value, Error> {
        let transaction: H256 = args.arg();
        args.done();

        let state = self.state.lock().unwrap();

        Self::ok(state.receipts.get(&transaction).unwrap_or_else(|| {
            panic!("there is no transaction with hash {:#x}", transaction);
        }))
    }

    fn ok<T: Serialize>(t: T) -> Result<Value, Error> {
        Ok(to_value(t).unwrap())
    }
}

impl std::fmt::Debug for MockTransport {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str("MockTransport")
    }
}

/// A mocked contract instance.
struct Contract {
    address: Address,
    methods: HashMap<H32, Method>,
}

impl Contract {
    fn new(address: Address, abi: &Abi) -> Self {
        let mut methods = HashMap::new();

        for functions in abi.functions.values() {
            for function in functions {
                methods.insert(function.selector(), Method::new(address, function.clone()));
            }
        }

        Contract { address, methods }
    }

    fn method(&mut self, signature: H32) -> &mut Method {
        match self.methods.get_mut(&signature) {
            Some(method) => method,
            None => panic!(
                "contract {:#x} doesn't have method with signature 0x{}",
                self.address,
                hex::encode(signature)
            ),
        }
    }

    fn process_tx(&mut self, tx: CallContext, data: &[u8]) -> TransactionResult {
        // TODO:
        //
        // We could support receive/fallback functions if data is empty.

        assert!(data.len() >= 4, "transaction has invalid call data");

        let signature = H32::try_from(&data[0..4]).unwrap();
        let method = self.method(signature);

        method.process_tx(tx, data)
    }

    fn checkpoint(&mut self) {
        for method in self.methods.values_mut() {
            method.checkpoint();
        }
    }
}

impl Drop for Contract {
    fn drop(&mut self) {
        if !std::thread::panicking() {
            self.checkpoint();
        }
    }
}

struct Method {
    /// Description for this method.
    description: String,

    /// ABI of this method.
    function: Function,

    /// Incremented whenever `expectations` vector is cleared to invalidate
    /// expectations API handle.
    generation: usize,

    /// Expectation for this method.
    expectations: Vec<Box<dyn ExpectationApi>>,
}

impl Method {
    /// Creates new method.
    fn new(address: Address, function: Function) -> Self {
        let description = format!("{:?} on contract {:#x}", function.abi_signature(), address);

        Method {
            description,
            function,
            generation: 0,
            expectations: Vec::new(),
        }
    }

    /// Adds new expectation.
    fn expect<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &mut self,
    ) -> (usize, usize) {
        let index = self.expectations.len();
        self.expectations.push(Box::new(Expectation::<P, R>::new()));
        (index, self.generation)
    }

    /// Returns an expectation.
    fn expectation<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static>(
        &mut self,
        index: usize,
        generation: usize,
    ) -> &mut Expectation<P, R> {
        assert!(
            generation == self.generation,
            "old expectations are not valid after checkpoint"
        );

        let expectation: &mut Expectation<P, R> = self
            .expectations
            .get_mut(index)
            .unwrap()
            .as_any()
            .downcast_mut()
            .unwrap();

        if expectation.checked {
            panic!(
                "can't modify expectation for {} because it was already in use",
                self.description
            )
        }

        expectation
    }

    /// Executes a transaction or a call.
    fn process_tx(&mut self, tx: CallContext, data: &[u8]) -> TransactionResult {
        if !tx.value.is_zero() && self.function.state_mutability != StateMutability::Payable {
            panic!(
                "call to non-payable {} with non-zero value {}",
                self.description, tx.value,
            )
        }

        let params = self
            .function
            .decode_input(&data[4..])
            .unwrap_or_else(|e| panic!("unable to decode input for {}: {:?}", self.description, e));

        for expectation in self.expectations.iter_mut() {
            if expectation.is_active() {
                // We clone `params` for each expectation, which could potentially
                // be inefficient. We assume, however, that in most cases there
                // are only a few expectations for a method, and they are likely
                // to be filtered out by `is_active`.
                if let Some(result) =
                    expectation.process_tx(&tx, &self.description, &self.function, params.clone())
                {
                    return result;
                }
            }
        }

        panic!("unexpected call to {}", self.description)
    }

    fn checkpoint(&mut self) {
        for expectation in self.expectations.iter_mut() {
            expectation.verify(&self.description);
        }
        self.generation += 1;
        self.expectations.clear();
    }
}

trait ExpectationApi: Send {
    /// Convert this expectation to `Any` for downcast.
    fn as_any(&mut self) -> &mut dyn Any;

    /// Checks if this expectation is active, i.e., still can be called.
    fn is_active(&self) -> bool;

    /// Matches and processes this transaction.
    ///
    /// If transaction matches this expectation, processes it and returns
    /// its result. Otherwise, returns `None`.
    fn process_tx(
        &mut self,
        tx: &CallContext,
        description: &str,
        function: &Function,
        params: Vec<Token>,
    ) -> Option<TransactionResult>;

    /// Verifies that this expectation is satisfied.
    fn verify(&self, description: &str);
}

struct Expectation<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static> {
    /// How many times should this expectation be called.
    times: TimesRange,

    /// How many times was it actually called.
    used: usize,

    /// Indicates that predicate for this expectation has been called at least
    /// once. Expectations shouldn't be changed after that happened.
    checked: bool,

    /// How many blocks should node skip for confirmation to be successful.
    confirmations: u64,

    /// Only consider this expectation if predicate returns `true`.
    predicate: Predicate<P>,

    /// Should this expectation match view calls?
    allow_calls: bool,

    /// Should this expectation match transactions?
    allow_transactions: bool,

    /// Function to generate method's return value.
    returns: Returns<P, R>,

    /// Handle for when this expectation belongs to a sequence.
    sequence: Option<mockall::SeqHandle>,
}

impl<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static> Expectation<P, R> {
    fn new() -> Self {
        Expectation {
            times: TimesRange::default(),
            used: 0,
            checked: false,
            confirmations: 0,
            predicate: Predicate::None,
            allow_calls: true,
            allow_transactions: true,
            returns: Returns::Default,
            sequence: None,
        }
    }
}

impl<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static> ExpectationApi
    for Expectation<P, R>
{
    fn as_any(&mut self) -> &mut dyn Any {
        self
    }

    fn is_active(&self) -> bool {
        self.times.can_call(self.used)
    }

    fn process_tx(
        &mut self,
        tx: &CallContext,
        description: &str,
        function: &Function,
        params: Vec<Token>,
    ) -> Option<TransactionResult> {
        self.checked = true;

        if tx.is_view_call && !self.allow_calls || !tx.is_view_call && !self.allow_transactions {
            return None;
        }

        if !self.times.can_call(self.used) {
            return None;
        }

        let param = P::from_token(Token::Tuple(params))
            .unwrap_or_else(|e| panic!("unable to decode input for {}: {:?}", description, e));

        if !self.predicate.can_call(tx, &param) {
            return None;
        }

        self.used += 1;
        if let Some(sequence) = &self.sequence {
            sequence.verify(description);

            if self.used == self.times.lower_bound() {
                sequence.satisfy();
            }
        }

        let result = self
            .returns
            .process_tx(function, tx, param)
            .map(|result| ethcontract::common::abi::encode(&result));

        Some(TransactionResult {
            result,
            confirmations: self.confirmations,
        })
    }

    fn verify(&self, description: &str) {
        if !self.times.contains(self.used) {
            panic!(
                "{} was called {} {}, but it was expected to be called {} {} {}",
                description,
                self.used,
                if self.used == 1 { "time" } else { "times" },
                if self.times.is_exact() {
                    "exactly"
                } else {
                    "at least"
                },
                self.times.lower_bound(),
                if self.times.lower_bound() == 1 {
                    "time"
                } else {
                    "times"
                }
            )
        }
    }
}

#[allow(clippy::enum_variant_names, clippy::type_complexity)]
enum Predicate<P: Tokenize + Send + 'static> {
    None,
    Predicate(Box<dyn predicates::Predicate<P> + Send>),
    Function(Box<dyn Fn(&P) -> bool + Send>),
    TxFunction(Box<dyn Fn(&CallContext, &P) -> bool + Send>),
}

impl<P: Tokenize + Send + 'static> Predicate<P> {
    fn can_call(&self, tx: &CallContext, param: &P) -> bool {
        match self {
            Predicate::None => true,
            Predicate::Predicate(p) => p.eval(param),
            Predicate::Function(f) => f(param),
            Predicate::TxFunction(f) => f(tx, param),
        }
    }
}

#[allow(clippy::type_complexity)]
enum Returns<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static> {
    Default,
    Error(String),
    Const(Token),
    Function(Box<dyn Fn(P) -> Result<R, String> + Send>),
    TxFunction(Box<dyn Fn(&CallContext, P) -> Result<R, String> + Send>),
}

impl<P: Tokenize + Send + 'static, R: Tokenize + Send + 'static> Returns<P, R> {
    fn process_tx(
        &self,
        function: &Function,
        tx: &CallContext,
        param: P,
    ) -> Result<Vec<Token>, String> {
        match self {
            Returns::Default => Ok(function
                .outputs
                .iter()
                .map(|i| default::default(&i.kind))
                .collect()),
            Returns::Error(error) => Err(error.clone()),
            Returns::Const(token) => Ok(Self::convert_result(token.clone(), function)),
            Returns::Function(f) => {
                f(param).map(|x| Self::convert_result(x.into_token(), function))
            }
            Returns::TxFunction(f) => {
                f(tx, param).map(|x| Self::convert_result(x.into_token(), function))
            }
        }
    }

    fn convert_result(token: Token, function: &Function) -> Vec<Token> {
        // When ethcontract determines appropriate rust type
        // for function output, it has a special case
        // for functions that return a single element.
        // Normally, function return type would always be a tuple.
        // With a single return value, the tuple is unwrapped
        // to make code more ergonomic:
        //
        // - for `function x()` output type is `()`,
        // - for `function x() returns (T)` output type is `T` (not `(T,)`),
        // - for `function x() returns (A, B, ...)` output type is `(A, B, ...)`.
        //
        // We need to account for this conversion
        // and wrap output of any function that returns a single value
        // into an additional tuple.
        if function.outputs.len() == 1 {
            vec![token]
        } else {
            match token {
                Token::Tuple(tuple) => tuple,
                _ => unreachable!(),
            }
        }
    }
}
