Dictionary Techniques

Welcome back.

In many applications, such as the encoding of text,

the output of the source consists of getting patterns.

In such a case, a very reasonable approach is to encode such

sources by creating a list or a dictionary of the frequently recurring pattern.

Then the patterns are encoded by their address in the dictionary.

So this way the input is split into two classes.

The frequently occurring and the infrequently occurring patterns.

The former are encoded by the dictionary, and the latter by less efficient method.

We describe in this segment both static and adaptive dictionary techniques.

The static ones are rather straight forward to understand and

implement, but they have limited

applicability and local operation efficiency.

Most adaptive techniques have their roots in 2 papers by Ziv and Lempel.

The 1977 paper, LZ77, and the 1978 paper, LZ78.

We describe in this segment, both such papers and approaches.

So let's proceed with the details about these algorithms.

In many applications, like text, the output

of the source consists of recurring patterns.

In such a case, it's reasonable to encode such sources by

making a list or a dictionary of the frequently occurring patterns.

The patterns in the dictionary are encoded by their index, in the dictionary.

So in essence the input is split into two classes.

The frequently occurring patterns are included in the dictionary, and

the infrequently occurring patterns are encoded with less efficient methods.

For this method to be effective, the size of the

dictionary must be much smaller than the number of possible patterns.

There are two types of dictionary techniques, static and adaptive.

Most of the adaptive techniques are based on these two papers by Ziv and

Lempel, the 1977 paper, referred to as the LZ77 dictionary

technique, and the 1978 paper referred to as the LZ78 technique.

We're going to describe both of these techniques.

Static dictionary techniques are quite straightforward to explain.

They're suitable for specific applications, like for

example, encoding the student records at the university.

So in this toy example, this is the alphabet of the source,

it contains five symbols, and here is that static dictionary that was designed.

Fixed code word length are used, so we see that there are the code words for the five

symbols, and then there are code words for combinations,

of popular combinations of symbols, and these are called diagrams.

So with this dictionary, we want to encode the sequence here, abracadabra.

And in a straightforward way, we see

that the symbol ab's included in the dictionary.

This is the code word.

R is in the dictionary with this code world.

A,C is in the dictionary with this corresponding code word.

A, D this code word.

A, B, R and A.

So for this particular toy example, we

have 11 symbols in the sequence, and

without the diagrams here, we would need

times three 33 beats, while now we only

use seven times three or 21 beats.

In the LZ77 Approach, the dictionary is

simply a portion of the previously encoded sequence.

The encoder examines the input sequence, through a sliding window, as shown here.

The window consists of the search buffer, that

contains a portion of the recently coded sequence,

and the look ahead buffer, that contains the

next portion of the sequence to be encoded.

To recall the sequence, in the look ahead

buffer, the encoder moves a search pointer back

through the search buffer, until it encounters a

match to the first symbol the look ahead buffer.

So let's assume that the pointer finds a match at this point.

The distance of the pointer from the look ahead buffer is called the offset.

So,

here, this is the offset.

[SOUND] The encoder then examines the symbols following

the symbol at the pointer location, to see if they match

consecutive symbols in the look-ahead buffer.

So let's say these symbols here match these symbols here.

The number of consecutive symbols in the

search buffer that match consecutive symbols in

the look-ahead buffer, starting with the first

symbol, it's called the length of the match.

So, this is the length here.

Once the longest match is found, the encoder

then encodes the, this triplet, where o is

the offset, l is the length of the match.

And c is the code for next symbol.

The reason for sending this third element as

a triplet is to take care of the situation

where no match for the symbol in the look

ahead buffer can be found in the search buffer.

In this case both the offset and and match length values are set equal

to 0, and the third element is the code for the symbol it's self.

The bits needed to encode this triplet are

shown here, this denotes here a single operation.

So the offset can be as long as the length of the search buffer.

To encode the length of the match, we notice here that we don't

include log 2 of s, but log 2 of W, because the match can exceed

into the look-ahead buffer and we'll see this in the example.

And finally, to encode the code for the next symbol, this is the cardinality

of the alphabet, the number of elements in the alphabet and using fixed

length codes, we need log 2 of this cardinality code, the symbol.

Let us look at a specific example of the application of LZ77.

So we assume that the length of the search buffer is

7, and the length of the look-ahead buffer equal to 6.

Were given to encode this sequence here.

And we know encoding with LZ77 means the generation of this triplet.

Offset length of much and code word for next symbol.

So here's the sequence again, and here's the current situation.

We assume that we have encoded what's in the search buffer.

So we look at the first symbol here d, we look

into the search buffer, whether we have a match for d.

Since there is no d in the search buffer, then

clearly the code word generated this offset

0 length, and the code for the symbol d.

Then we move the search box over by one position, and

now we try to see whether a matches into the search buffer.

We have a march of a here, but it's just a single match.

We'll move further back, another match of a, single match as well.

Further back, we match a here as well, but now in

addition, we match b, we match r, and we match a.

So in this case, the offset is equal to 7.

So encode 7 for the offset.

The match equals 4 symbols, and we need the code for the

next symbol which is this symbol here, the code for r.

We'll move now the buffer over by five positions, and this is the

situation, so we try to see if r can match in the previous segment of the

search buffer, so we have a match of r here, but it's a single match.

I move further back, and I see can match r here as well.

And now, in addition, this a matches with this a.

This r matches with this r.

But I can keep going, since this r matches this r, and this a matches this a.

So, this is the case I referred to earlier,

whereby, the match which exceeds into the look-ahead buffer.

So in this case, the offset is three,

but the length of the match is five symbols.

Therefore the code word is 3 for the offset, 5 from the length of

the match, and the next symbol I need to encode is D, over here.

Let us proceed with a decoding now of the stream.

So, we assume that this part of the sequence has been decoded.

And then, we receive the first code

word, which is 00 and the code for D.

So, 0, offset 0 length of the match, and therefore, I need to add D to my sequence.

Then I receive the second code word 74 and the code for R, so

we need to move back 7 positions, so I move the pointer here,

and 4 symbols are matched, so these 4 symbols are matched.

And, therefore the symbol so long with the code for

r need to be added to the already decoded sequence.

So this is what, now the decoded sequence looks like.

Then I receive the next code word which is 3, 5, and the code for d.

So I move the pointer back three positions, so the pointer

is now at this location, but they have a module five symbols.

I don't have five, I have only three but I will generate the three first.

So, I generate the rar and now, looking at this point of location,

I can see the five symbols, that they have been matched.

That is, one, two, three, four, five.

So, the first three have already been generated,

so I need to add these two, and of course the

last symbol is d because I have the code for d.

So we see that through decoding I can

generate again the original image with no error.

An implicit assumption with LZ77 is that light particles will occur close together.

Any particle that occurs over a period longer than

what's covered by the coded window will not be captured.

Actually, the worst case scenario is when we have a sequence

to encode that is periodic, with period longer than the search buffer.

So there are cases where this finite view of the past is indeed a drawback.

And in LZ78 the reliance on the search buffer is dropped,

and instead an explicit dictionary is generated.

Since the dictionary needs to be built.

Both the encoder and decoder, synchronization is

very important so that the dictionaries are identical.

So, with LZ78 what's encoded is the index here in the dictionary.

And when we have a new entry, then the index is equal to 0.

And since the code for the symbol, after the

matched part, similarly to what we had in LZ77.

So let us look at a simple example now.

So here is a sequence we are asked to encode using LZ78.

So we start with the first symbol, this is a new

symbol, actually there's nothing in the dictionary so it's a new entry.

i equals 0 is the first entry of the

dictionary, and I need the code word for b.

I look the next symbol, a does not exist in

the dictionary, it's also a new entry, i equals 0.

That's the second entry in my dictionary.

Look at w, similarly a new entry, the fifth element in the dictionary.

I look at this w, well w exists in the

dictionary, so I look at the next symbol as well.

So, w exists in the dictionary is, the index is 3 for

w, and then this is the code word for the next symbol A.

I move on, this is lets say the symbol for space, it's

a new symbol I equals 0 is the fifth, enter the dictionary,

then B exists in the dictionary.

So

it's actually the first element in the dictionary, the code word for a.

And now ba is my 6th entry in the dictionary.

Similarly, w exists, so here ww becomes a new entry in the dictionary.

A exists so A-space is my new entry in the dictionary.

Over here b exists, ba actually exist in the dictionary.

So therefore this becomes a new entry, baw,

w exists, wa exists in the dictionary so.

W a space is my new entry.

This

is, the

index of that is ten.

Then I proceed here, b exists, b a exists, b a w exists in the dictionary.

And therefore b-a-w-w is my new entry.

Right?

So the index of b-a-w is 9.

This is the ninth entry in the dictionary, that's why i

equals 9 here, and I need a code word for w.

So a exists, a space.

This exists, so a space w is my, I'm sorry,

a space b is my new entry in the dictionary.

O a new entry.

O space becomes a new entry, and finally b o is the final entry.

Into the dictionary and the index is 15.

So it's a rather straightforward procedure.

Simple I would say than LZ77.

And as the encoding proceeds, we're going to see larger and

larger lengthwise entries entering in the dictionary.

I'm adding some final comments regarding dictionary techniques here.

There have many variations of LZ77.

Some of which are that the

variable lengths to efficiently encode the triplets.

There are versions where the sizes of both buffers can vary,

and also versions where the third element in the triplet

is eliminated by adding a flag when a single symbol

is encountered.

A variation of LZ78 is the LZW version, the Lempel-Ziv-Welch

version, according to which the encoder only sends the index.

Now, there are numerous applications of the family of LZ variants.

It's been used in the Unix compress command.

The command to compress data

files or text files.

It's also used in the GIF, the Graphics Interchange Format for images.

And finally, is used as part of the standard

V 42 bits, that is used for compression over modems.

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