var history_max_len = 30;


function History( o )
{
    this.init( o );
}

History.prototype =
{
    curr_idx: 0,

    list: null,

    init: function ( o ) 
    {
        this.list = new Array();
        this.list.push( o );
        this.curr_idx = 0;
    },

    save: function (o) 
    {
        if (this.curr_idx + 1 < this.list.length) 
        {
            this.list[++this.curr_idx] = o;

            // erase forward history
            this.list.length = this.curr_idx + 1;
        }
        else if (this.list.length < history_max_len) 
        {   
            //alert(2)
            this.curr_idx = this.list.length;
            this.list.push(o);
        }
        else // shift array
        {
            for (var i = 1; i < this.list.length; ++i) 
            {
                this.list[i - 1] = this.list[i];
            }

            if (this.list.length) 
            {
                this.list[this.list.length - 1] = o;
            }
        }
    },
    
    can_undo: function()
    {
        return this.curr_idx > 0;
    },

    can_redo: function()
    {
        return this.curr_idx + 1 < this.list.length;
    },

    undo: function () 
    {
        if( this.can_undo() )
        {
            --this.curr_idx;
            return true;
        }
        
        return false;
    },

    redo: function () 
    {
        if( this.can_redo() )
        {
            ++this.curr_idx;
            return true;
        }
        
        return false;
    },

    curr: function () 
    {
        return this.list[ this.curr_idx ];
    }
};