Re: State transition table compression, and search in compression structure

From:
Maxim Yegorushkin <maxim.yegorushkin@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 27 Oct 2008 09:16:26 CST
Message-ID:
<e7581756-3754-4c1b-966d-2a6291edda7d@j22g2000hsf.googlegroups.com>
On Oct 24, 10:31 pm, Alexander Zezulinsky <palgol...@gmail.com> wrote:

I code state transition table with such structure:

     0 1 2 3 4 5 6 7.... //states
'a' 1 8 8 8 9 8 1 76 ....//next states
'b' 1
'c'
//'a','b','c',.. transition term

- It has 128 symbols (rows) and less then 65536 states(cols);
- 8 is initial transition ("error state" for this table);
- table can have various richness;
(50% table richness = if 65536/2 * 128 = 32768 * 128 transitions is
unerror transitions, we added it's to table after it's
initialization).

I implemented table as a array (128*65536) unsigned short.

There are two ideas to compress such structure:
1. RLE compress method (Run-length encoding);
(State, Length)

2. Edge list;
(CurState, NextState)
or
('symbol', NextState)

Adding states for edge structure without initial error (excess)
states.
For fast search we can segmentate 'a','b',... symbol segments or
0,1,... segments.

Task:
We must provide low memory costs with various table richness % and
fast search in compress table structure.
Questions:
How can i compress transition table structure effectively with low
memory costs and fast performance search time?


You can store the transitions in a sorted vector and use binary search
to find the transition from the current state upon a particular event.
This way undefined transitions do not use any memory.

Something like this:

    #include <vector>
    #include <algorithm>

    struct transition
    {
        unsigned short state_current, state_next;
        unsigned char event;

        // order by { state_current, event }
        bool operator<(transition const& b) const
        {
            return state_current == b.state_current
                ? event < b.event
                : state_current < b.state_current
                ;
        }
    };

    struct transition_table
    {
        typedef std::vector<transition> transitions;
        mutable transitions transitions_;
        mutable bool sorted_ : 1;

        transition_table()
            : sorted_(false)
        {}

        void transition_add(unsigned short from, unsigned char event,
unsigned short to)
        {
            transition t = { from, to, event };
            transitions_.push_back(t);
            sorted_ = false;
        }

        // return NULL if not found
        transition const* transition_find(unsigned short
state_current, unsigned char event) const
        {
            if(!sorted_) { // sort on first search
                std::sort(transitions_.begin(), transitions_.end());
                sorted_ = true;
            }
            transition needle = { state_current, 0, event };
            // use binary search
            transitions::const_iterator i =
std::lower_bound(transitions_.begin(), transitions_.end(), needle);
            return i != transitions_.end() && i->state_current ==
state_current && i->event == event
                ? &*i
                : NULL
                ;
        }
    };

--
Max

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