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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity is free software: you can redistribute it and/or modify
    // it under the terms of the GNU General Public License as published by
    // the Free Software Foundation, either version 3 of the License, or
    // (at your option) any later version.

    // Parity is distributed in the hope that it will be useful,
    // but WITHOUT ANY WARRANTY; without even the implied warranty of
    // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    // GNU General Public License for more details.

    // You should have received a copy of the GNU General Public License
    // along with Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Evm interface.

    use std::{ops, cmp, fmt};
    use util::{U128, U256, U512, Uint};
    use action_params::ActionParams;
    use evm::Ext;

    /// Evm errors.
    #[derive(Debug, Clone, Copy, PartialEq)]
    pub enum Error {
    	/// `OutOfGas` is returned when transaction execution runs out of gas.
    	/// The state should be reverted to the state from before the
    	/// transaction execution. But it does not mean that transaction
    	/// was invalid. Balance still should be transfered and nonce
    	/// should be increased.
    	OutOfGas,
    	/// `BadJumpDestination` is returned when execution tried to move
    	/// to position that wasn't marked with JUMPDEST instruction
    	BadJumpDestination {
    		/// Position the code tried to jump to.
    		destination: usize
    	},
    	/// `BadInstructions` is returned when given instruction is not supported
    	BadInstruction {
    		/// Unrecognized opcode
    		instruction: u8,
    	},
    	/// `StackUnderflow` when there is not enough stack elements to execute instruction
    	StackUnderflow {
    		/// Invoked instruction
    		instruction: &'static str,
    		/// How many stack elements was requested by instruction
    		wanted: usize,
    		/// How many elements were on stack
    		on_stack: usize
    	},
    	/// When execution would exceed defined Stack Limit
    	OutOfStack {
    		/// Invoked instruction
    		instruction: &'static str,
    		/// How many stack elements instruction wanted to push
    		wanted: usize,
    		/// What was the stack limit
    		limit: usize
    	},
    	/// Returned on evm internal error. Should never be ignored during development.
    	/// Likely to cause consensus issues.
    	#[allow(dead_code)] // created only by jit
    	Internal,
    }

    impl fmt::Display for Error {
    	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    		use self::Error::*;
    		let message = match *self {
    			OutOfGas => "Out of gas",
    			BadJumpDestination { .. } => "Bad jump destination",
    			BadInstruction { .. } => "Bad instruction",
    			StackUnderflow { .. } => "Stack underflow",
    			OutOfStack { .. } => "Out of stack",
    			Internal => "Internal error",
    		};
    		message.fmt(f)
    	}
    }

    /// A specialized version of Result over EVM errors.
    pub type Result<T> = ::std::result::Result<T, Error>;

    /// Gas Left: either it is a known value, or it needs to be computed by processing
    /// a return instruction.
    #[derive(Debug, PartialEq, Eq, PartialOrd, Ord)]
    pub enum GasLeft<'a> {
    	/// Known gas left
    	Known(U256),
    	/// Return instruction must be processed.
    	NeedsReturn(U256, &'a [u8]),
    }

    /// Types that can be "finalized" using an EVM.
    ///
    /// In practice, this is just used to define an inherent impl on
    /// `Reult<GasLeft<'a>>`.
    pub trait Finalize {
    	/// Consume the externalities, call return if necessary, and produce a final amount of gas left.
    	fn finalize<E: Ext>(self, ext: E) -> Result<U256>;
    }

    impl<'a> Finalize for Result<GasLeft<'a>> {
    	fn finalize<E: Ext>(self, ext: E) -> Result<U256> {
    		match self {
    			Ok(GasLeft::Known(gas)) => Ok(gas),
    			Ok(GasLeft::NeedsReturn(gas, ret_code)) => ext.ret(&gas, ret_code),
    			Err(err) => Err(err),
    		}
    	}
    }

    /// Cost calculation type. For low-gas usage we calculate costs using usize instead of U256
    pub trait CostType: Sized + From<usize> + Copy
    	+ ops::Mul<Output=Self> + ops::Div<Output=Self> + ops::Add<Output=Self> +ops::Sub<Output=Self>
    	+ ops::Shr<usize, Output=Self> + ops::Shl<usize, Output=Self>
    	+ cmp::Ord + fmt::Debug {
    	/// Converts this cost into `U256`
    	fn as_u256(&self) -> U256;
    	/// Tries to fit `U256` into this `Cost` type
    	fn from_u256(val: U256) -> Result<Self>;
    	/// Convert to usize (may panic)
    	fn as_usize(&self) -> usize;
    	/// Add with overflow
    	fn overflow_add(self, other: Self) -> (Self, bool);
    	/// Multiple with overflow
    	fn overflow_mul(self, other: Self) -> (Self, bool);
    	/// Single-step full multiplication and shift: `(self*other) >> shr`
    	/// Should not overflow on intermediate steps
    	fn overflow_mul_shr(self, other: Self, shr: usize) -> (Self, bool);
    }

    impl CostType for U256 {
    	fn as_u256(&self) -> U256 {
    		*self
    	}

    	fn from_u256(val: U256) -> Result<Self> {
    		Ok(val)
    	}

    	fn as_usize(&self) -> usize {
    		self.as_u64() as usize
    	}

    	fn overflow_add(self, other: Self) -> (Self, bool) {
    		Uint::overflowing_add(self, other)
    	}

    	fn overflow_mul(self, other: Self) -> (Self, bool) {
    		Uint::overflowing_mul(self, other)
    	}

    	fn overflow_mul_shr(self, other: Self, shr: usize) -> (Self, bool) {
    		let x = self.full_mul(other);
    		let U512(parts) = x;
    		let overflow = (parts[4] | parts[5] | parts[6] | parts[7]) > 0;
    		let U512(parts) = x >> shr;
    		(
    			U256([parts[0], parts[1], parts[2], parts[3]]),
    			overflow
    		)
    	}
    }

    impl CostType for usize {
    	fn as_u256(&self) -> U256 {
    		U256::from(*self)
    	}

    	fn from_u256(val: U256) -> Result<Self> {
    		let res = val.low_u64() as usize;

    		// validate if value fits into usize
    		if U256::from(res) != val {
    			return Err(Error::OutOfGas);
    		}

    		Ok(res)
    	}

    	fn as_usize(&self) -> usize {
    		*self
    	}

    	fn overflow_add(self, other: Self) -> (Self, bool) {
    		self.overflowing_add(other)
    	}

    	fn overflow_mul(self, other: Self) -> (Self, bool) {
    		self.overflowing_mul(other)
    	}

    	fn overflow_mul_shr(self, other: Self, shr: usize) -> (Self, bool) {
    		let (c, o) = U128::from(self).overflowing_mul(U128::from(other));
    		let U128(parts) = c;
    		let overflow = o | (parts[1] > 0);
    		let U128(parts) = c >> shr;
    		let result = parts[0] as usize;
    		let overflow = overflow | (parts[0] > result as u64);
    		(result, overflow)
    	}
    }

    /// Evm interface
    pub trait Evm {
    	/// This function should be used to execute transaction.
    	///
    	/// It returns either an error, a known amount of gas left, or parameters to be used
    	/// to compute the final gas left.
    	fn exec(&mut self, params: ActionParams, ext: &mut Ext) -> Result<GasLeft>;
    }

    #[cfg(test)]
    mod tests {
    	use util::{U256, Uint};
    	use super::CostType;

    	#[test]
    	fn should_calculate_overflow_mul_shr_without_overflow() {
    		// given
    		let num = 1048576;

    		// when
    		let (res1, o1) = U256::from(num).overflow_mul_shr(U256::from(num), 20);
    		let (res2, o2) = num.overflow_mul_shr(num, 20);

    		// then
    		assert_eq!(res1, U256::from(num));
    		assert!(!o1);
    		assert_eq!(res2, num);
    		assert!(!o2);
    	}

    	#[test]
    	fn should_calculate_overflow_mul_shr_with_overflow() {
    		// given
    		let max = u64::max_value();
    		let num1 = U256([max, max, max, max]);
    		let num2 = usize::max_value();

    		// when
    		let (res1, o1) = num1.overflow_mul_shr(num1, 256);
    		let (res2, o2) = num2.overflow_mul_shr(num2, 64);

    		// then
    		assert_eq!(res2, num2 - 1);
    		assert!(o2);

    		assert_eq!(res1, !U256::zero() - U256::one());
    		assert!(o1);
    	}

    	#[test]
    	fn should_validate_u256_to_usize_conversion() {
    		// given
    		let v = U256::from(usize::max_value()) + U256::from(1);

    		// when
    		let res = usize::from_u256(v);

    		// then
    		assert!(res.is_err());
    	}
    }

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