// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of Tetsy Vapory.

// Tetsy Vapory 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.

// Tetsy Vapory 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 Tetsy Vapory.  If not, see <http://www.gnu.org/licenses/>.

//! Vvm interface.

use std::{ops, cmp, fmt};
use vapory_types::{U128, U256, U512};
use tetsy_vm::{Ext, Result, ReturnData, GasLeft, Error};

/// Finalization result. Gas Left: either it is a known value, or it needs to be computed by processing
/// a return instruction.
#[derive(Debug)]
pub struct FinalizationResult {
	/// Final amount of gas left.
	pub gas_left: U256,
	/// Apply execution state changes or revert them.
	pub apply_state: bool,
	/// Return data buffer.
	pub return_data: ReturnData,
}

/// Types that can be "finalized" using an VVM.
///
/// 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 call result.
	fn finalize<E: Ext>(self, ext: E) -> Result<FinalizationResult>;
}

impl Finalize for Result<GasLeft> {
	fn finalize<E: Ext>(self, ext: E) -> Result<FinalizationResult> {
		match self {
			Ok(GasLeft::Known(gas_left)) => {
				Ok(FinalizationResult {
					gas_left,
					apply_state: true,
					return_data: ReturnData::empty()
				})
			},
			Ok(GasLeft::NeedsReturn { gas_left, data, apply_state }) => {
				ext.ret(&gas_left, &data, apply_state).map(|gas_left|
					FinalizationResult { gas_left, apply_state, return_data: data }
				)
			},
			Err(err) => Err(err),
		}
	}
}

impl Finalize for Error {
	fn finalize<E: Ext>(self, _ext: E) -> Result<FinalizationResult> {
		Err(self)
	}
}

/// Cost calculation type. For low-gas usage we calculate costs using usize instead of U256
pub trait CostType: Sized + From<usize> + Copy + Send
	+ 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) {
		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 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)
	}
}

#[cfg(test)]
mod tests {
	use vapory_types::U256;
	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());
	}
}