txt compression project.ppt

8/21/2019 TXT COMPRESSION PROJECT.ppt

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TEXT COMPRESSION

SUBMITTED BY

MUKESH KUMAR TANWAR 11104EN067

VIKASH KUMAR 11104EN068

PARAS CHANDOLIA 11104EN070


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ABSTRACT

TextCompressionisthescienceandartofrepresentinginfoacompactform.Therearealotoftextcompressionalgorithareavailable tocompress filesofdifferent formats. In this

haveusedHuffmanalgorithmforthetextcompression.


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INTRODUCTIONTextcompressionreferstoreducingtheamountofspacenstore

the

text.

There

are

two

types

ofcompression-

LOSSLESS- Data compression can be lossless only if it is pexactlyreconstructtheoriginaldatafromthecompressedversilossless technique is used when the original data of a sourimportant that we cannot afford to lose any details. Examplesource data are medical images, text, some computer executable f

LOSSY- Another method of compression algorithms is called losalgorithms irreversibly remove some parts of data and approximation of the original data can be reconstructed. Datmultimedia images, video and audio are more easily compressecompression techniques


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Huffman codingTheHuffmancodingtransformstheoriginalcodeusedforthecharactersoftcodeon8bits).Codingthetext isjustreplacingeachsymbolby itsnewcoHuffman algorithm uses the notion of prefix code. A prefix code is a scontainingnowordthatisprefixofanotherwordoftheset.Theadvantageo

isthatthedecoding is immediate. Huffman Code assigns shorter encodingswith a high frequency. Elements with the highest frequency get assigned thlength code. The key to decompressing Huffman code is a Huffman tree.

Thestepsrequiredwhilecompressingthetextareasfollows- Countsthecharacterfrequencies. BuildstheHuffmancodingtree. Buildscharactercode-wordsfromthecodingtree. Storesthecodingtreeinthecompressedfile. Encodesthecharactersinthecompressedfile. CompletefunctionfortheHuffmanencoding. Rebuildsthetreefromheaderofthecompressedfile. Recoverstheoriginaltext. CompletefunctionfortheHuffmandecoding.


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ENCODING Thecodingalgorithmiscomposedofthreesteps:countofcharafrequencies,constructionoftheprefixcode,encodingofthetext

stepconsistsincountingthenumberofoccurrencesofeachchartheoriginaltext.

Algorithmtocountthefrequenciesofcharaters-intCOUNT(FILECODE*code) { intc;

while((c=getc(fin))!=EOF)code[c].freq++;code[END].freq=1; Thesecondstepofthealgorithmbuildsthetreeofaprefixcodecharacterfrequency freq(a)ofeachcharacterainthefollowingway


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Create a root-tree t for each character a with weight (t) =freq(a),

Repeat

Select the two least frequent trees t1 and t2,

Create a new tree t3 with left son t1, right son t2 and weight(t3)=weight(t1)+weight

Until it remains only one tree.

The tree is constructed by the algorithm, the only tree remaining at the end of thethe coding tree.

In the implementation of the above scheme, a priority queue is used to identifyfrequent trees. After the tree is built, it is possible to recover the code-words acharacters by a simple depth-first-search of the tree. In the third step, the oencoded. Since the code depends on the original text, in order to be able tcompressed text, the coding tree must be stored with the compressed text. Again t

a depth-first-search of the tree. Each time an internal node is encountered a 0 is pra leaf is encountered a 1 is produced followed by the ASCII code of the corresponon 9 bits (so that the end marker can be equal to 256 if all the ASCII characters aoriginal text). Bits are written 8 by 8 in the compressed file using both a buffer counter (bits2go). Each time the counter is equal to 8, the corresponding byte is writotal number of bits is not necessarily a multiple of 8, some bits may have to be prend. All the preceding steps are composed to give a complete encoding program.


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Building the Huffman coding tre

int BUILD_HEAP(CODE *code, PRIORITY_QUEUE queue) {

int i, size; NODE node; size=0;

for (i=0; i


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BUILDING THE CODING TRE NODE BUILD_TREE(PRIORITY_QUEUE queue, int size)

{

NODE root, leftnode, rightnode; while (size > 1) {

leftnode=HEAP_EXTRACT_MIN(queue);

rightnode=HEAP_EXTRACT_MIN(queue);

root=ALLOCATE_NODE();

PUT_WEIGHT(root, GET_WEIGHT(leftnode)+GET_WEIGHT(rightno

PUT_LEFT(root, leftnode); PUT_RIGHT(root, rightnode);

HEAP_INSERT(queue, root);

size--;

}

return(root);


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Builds the character codes by a dfirst-search of the coding tre

void BUILD_CODE(NODE root, CODE *code, int length) {

static char temp[ASIZE+1];

int c;

if (GET_LEFT(root) != NULL) {

temp[length]=0;

BUILD_CODE(GET_LEFT(root), code, length+1);

temp[length]=1;

BUILD_CODE(GET_RIGHT(root), code, length+1);

}


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else {

c=GET_LABEL(root);

code[c].codeword=(char *)malloc(length); code[c].lg=length;

strncpy(code[c].codeword, temp, length);

}

}


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MEMORIZES THE CODING TRETHE COMPRESSED FILE

void CODE_TREE(FILE *fout, NODE root){

if (GET_LEFT(root) != NULL){

SEND_BIT(fout, 0);

CODE_TREE(fout, GET_LEFT(root)); CODE_TREE(fout, GET_RIGH}

else { SEND_BIT(fout, 1); ASCII2BITS(fout, GET_LABEL(root));

}}


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Encodes the characters in the compressed file

voidCODE_TEXT(FILE *fin, FILE *fout)

{

int c, i;rewind(fin);

while ((c=getc(fin)) != EOF)

for (i=0; i < code[c].lg; i++)

SEND_BIT(fout, code[c].codeword[i]);

for (i=0; i < code[END].lg; i++)

SEND_BIT(fout, code[END].codeword[i]);

}


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Sends one bit in the compressed filevoid SEND_BIT(FILE *fout, int bit)

{

buffer>>=1;

if (bit) buffer|=0x80;

bits2go++;if (bits2go == 8)

{

putc(buffer, fout);

bits2go=0;

}

} Encodes n on 9 bits

Int ASCII2BITS(FILE *fout, int n)

{

int i;

for (i=8; i>= 0; i--) SEND_BIT(fout, (n>>i)&1);

}


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Outputs a final byte if necessary

Void SEND_LAST_BITS(FILE *fout)

{

if (bits2go) putc(buffer>>(8-bits2go), fout);

} Initializes the array code

Void INIT(CODE *code)

{

int i;

for (i=0; i < ASIZE; i++) {

code[i].freq=code[i].lg=0;

code[i].codeword=NULL;

}

}


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Complete function of Huffman cod#defineASIZE255

#defineEND(ASIZE+1)/*codeofEOF*/

typedefstruct

{

intfreq,lg;char*codeword;

}CODE;intbuffer;intbits2go;voidCODING(char*fichin,char*fichout){

FILE*fin,*fout;intsize;

PRIORITY_QUEUEqueue;NODEroot;CODEcode[ASIZE+1];if((fin=fopen(fichin,"r"))==NULL)exit(0);

if((fout=fopen(fichout,"w"))==NULL)exit(0);INIT(code);

COUNT(fin,code);


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size=BUILD_HEAP(code,queue);root=BUILD_TREE(queue,siz

BUILD_CODE(root,code,0);buffer=bits2go=0;CODE_TREE(fo

CODE_TEXT(fin,fout);SEND_LAST_BITS(fout);

fclose(fin);

fclose(fout);

}

Example:

y

=

CAGATAAGAGAA

Length:12*8=96bits(ifan8-bitcodeisassumed)


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character frequencies:

The required coding tree:Tree code:

0001binary(END,9)01binary(C,9)1binary(T,9)1binary(G,

A,9)

Length: 54

Text code: 0010 1 01 1 0011 1 1 01 1 01 1 1 000Length: 24

Total length: 78


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DECODING

Decoding a file containing a text compressed by Huffman algorithm is a mere program

First the coding tree is rebuild by the algorithm1 given below. It uses a function to dec

written on 9 bits. Then, the uncompressed text is recovered by parsing the compresse

text with the coding tree. The process begins at the root of the coding tree, andbranch when a 0 is read or a right branch when a 1 is read. When a leaf is enccorresponding character (in fact the original codeword of it) is produced and the resumes at the root of the tree. The parsing ends when the codeword of the end mThe decoding of the text is presented in Figure Algorithm3. Again in this algorithm thwith the use of a buffer

(buffer)

and a counter (bits in stock). A byte is read in the compressed file only if the counter izero Algorithm4.

The complete decoding program is given in Algorithm5. It calls the preceding function

running time of the decoding program is linear in the sum of the sizes of the texts it m


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Algorithm1:Rebuildsthetreereadfrom compressed file

voidREBUILD_TREE(FILE *fin, NODE root)

{

NODE leftnode, rightnode;

if (READ_BIT(fin) == 1) { /* leaf */

PUT_LEFT(root, NULL);

PUT_RIGHT(root, NULL);

PUT_LABEL(root, BITS2ASCII(fin));

}

else {

leftnode=ALLOCATE_NODE();

PUT_LEFT(root, leftnode);REBUILD_TREE(fin, leftnode);

rightnode=ALLOCATE_NODE();

PUT_RIGHT(root, rightnode);

REBUILD_TREE(fin, rightnode);

}

}

Algorithm2: Reads the next 9 bits in the compressed file and


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Algorithm2: Reads the next 9 bits in the compressed file and the corresponding value

Int BITS2ASCII(FILE *fin)

{

int i, value;value=0;

for (i=8; i >= 0; i--) value=value


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( ) {

if (GET_LEFT(node) == NULL)

if (GET_LABEL(node) != END)

{

putc(GET_LABEL(node), fout);

node=root;

}else break;

else if (READ_BIT(fin) == 1) node=GET_RIGHT(node);

else node=GET_LEFT(node);

}

}

Algorithm4: Reads the next bit from the compressed file

Int READ_BIT(FILE *fin){

int bit;

if (bits_in_stock == 0) {

buffer=getc(fin);

bits_in_stock=8;

}

bit b ffer&1


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bit=buffer&1;

buffer>>=1;

bits_in_stock--;

return(bit);

}

Algorithm5: Complete function for decoding

int buffer;

int bits_in_stock;

void DECODING(char *fichin, char *fichout)

{

FILE *fin, *fout;

NODE root;


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if ((fin=fopen(fichin, "r")) == NULL) exit(0);

if ((fout=fopen(fichout, "w")) == NULL) exit(0);

INIT(code);

buffer=bits_in_stock=0;root=ALLOCATE_NODE();

REBUILD_TREE(fin, root);

UNCOMPRESS(fin, fout, root);

fclose(fin);fclose(fout);

}

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