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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/>.

    use std::fmt;
    use evm::instructions;

    /// Stack trait with VM-friendly API
    pub trait Stack<T> {
    	/// Returns `Stack[len(Stack) - no_from_top]`
    	fn peek(&self, no_from_top: usize) -> &T;
    	/// Swaps Stack[len(Stack)] and Stack[len(Stack) - no_from_top]
    	fn swap_with_top(&mut self, no_from_top: usize);
    	/// Returns true if Stack has at least `no_of_elems` elements
    	fn has(&self, no_of_elems: usize) -> bool;
    	/// Get element from top and remove it from Stack. Panics if stack is empty.
    	fn pop_back(&mut self) -> T;
    	/// Get (up to `instructions::MAX_NO_OF_TOPICS`) elements from top and remove them from Stack. Panics if stack is empty.
    	fn pop_n(&mut self, no_of_elems: usize) -> &[T];
    	/// Add element on top of the Stack
    	fn push(&mut self, elem: T);
    	/// Get number of elements on Stack
    	fn size(&self) -> usize;
    	/// Returns all data on stack.
    	fn peek_top(&self, no_of_elems: usize) -> &[T];
    }

    pub struct VecStack<S> {
    	stack: Vec<S>,
    	logs: [S; instructions::MAX_NO_OF_TOPICS]
    }

    impl<S : Copy> VecStack<S> {
    	pub fn with_capacity(capacity: usize, zero: S) -> Self {
    		VecStack {
    			stack: Vec::with_capacity(capacity),
    			logs: [zero; instructions::MAX_NO_OF_TOPICS]
    		}
    	}
    }

    impl<S : fmt::Display> Stack<S> for VecStack<S> {
    	fn peek(&self, no_from_top: usize) -> &S {
    		&self.stack[self.stack.len() - no_from_top - 1]
    	}

    	fn swap_with_top(&mut self, no_from_top: usize) {
    		let len = self.stack.len();
    		self.stack.swap(len - no_from_top - 1, len - 1);
    	}

    	fn has(&self, no_of_elems: usize) -> bool {
    		self.stack.len() >= no_of_elems
    	}

    	fn pop_back(&mut self) -> S {
    		let val = self.stack.pop();
    		match val {
    			Some(x) => x,
    			None => panic!("Tried to pop from empty stack.")
    		}
    	}

    	fn pop_n(&mut self, no_of_elems: usize) -> &[S] {
    		assert!(no_of_elems <= instructions::MAX_NO_OF_TOPICS);

    		for i in 0..no_of_elems {
    			self.logs[i] = self.pop_back();
    		}
    		&self.logs[0..no_of_elems]
    	}

    	fn push(&mut self, elem: S) {
    		self.stack.push(elem);
    	}

    	fn size(&self) -> usize {
    		self.stack.len()
    	}

    	fn peek_top(&self, no_from_top: usize) -> &[S] {
    		assert!(self.stack.len() >= no_from_top, "peek_top asked for more items than exist.");
    		&self.stack[self.stack.len() - no_from_top .. self.stack.len()]
    	}
    }

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