////////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   StateStack.cs
//
//  Note: File is part of Sing# runtime files and copied into Singularity tree
//        whenever a new version of Sing# is dropped.
//

using System;
using Microsoft.Contracts;

namespace Microsoft.Singularity.Channels {

    public pointerfree struct StateStack {
        public const int STATEBITS = 8;
        public const int STATEMASK = (1 << STATEBITS) - 1;

        private uint stack; // 4 x 8 bits for state of machine of this endpoint. Lowest bits = top of stack

        public StateStack(int initialState) {
          stack = (uint)initialState & STATEMASK;
        }

        public void Push(int i) {
            stack = stack << STATEBITS | ((uint)i & STATEMASK);
        }

        public void Pop() {
            stack = stack >> STATEBITS;
        }

        public void Set(int i) {
            stack = (stack & (uint)~(uint)STATEMASK) | ((uint)i & STATEMASK);
        }

        [Pure]
        public int Top() {
            return (int)(stack & STATEMASK);
        }
    }
}