Page 256 - Programming Microcontrollers in C

Data Compression 241

The listing shown below is a simple Huffman-encoded routine to
print out to the screen several aircraft-oriented terms that might be
used in an on-board avionics system. The first part of the function is
a list of the several words or phrases.

#include <stdio.h>
/* The messages to send out */
static unsigned char M1[]={0x9f,0x36,0xef,0xdc,0x0a};
static unsigned char M2[]={0xfd,0x26,0x0e,0x7c,0xa0};
static unsigned char M3[]={0xc1,0xee,0xe6,0x7f,0xc6,
0x3a,0x39,0x1c,0xb9,0xf2,0x80 };
static unsigned char M4[]={0xfc,0xf4,0xf9,0x8e,0x8e,
0x47,0x2e,0x7c,0xa0};
static unsigned char M5[]={0x8e,0xb1,0xef,0xf0,0x61,
0x40};
static unsigned char M6[]={0x73,0x8f,0xef,0xdf,0x75,
0xda};
static unsigned char M7[]={0xdb,0xc2,0x80};
static unsigned char M8[]={0x3b,0xb7,0x93,0x66,0x18,
0xed,0xe1,0x8f,0xdf,0xcc,0x66,
0xb4,0xff,0xe7,0xca};
static unsigned char M9[]={0xf3,0x5f,0x7d,0xce,0x18,
0xf7,0xf8,0x30,0xa0};
/* The Huffman tree */
const static char
Node[]={4, 2, ‘E’|0X80, ‘I’|0X80, 4, 2,‘A’|0X80,
‘\n’|0X80, 10, 6, 4, 2,‘D’|0X80, ‘ ‘|0X80,
‘N’|0X80, 2, ‘R’|0X80, ‘T’|0X80, 4, 2, ‘M’|0X80,
‘S’|0X80, 4, 2, ‘G’|0X80, ‘U’|0X80, 4, 2,
‘L’|0X80, ‘O’|0X80, 4, 2, ‘F’|0X80, ‘H’|0X80, 2,
‘P’|0X80, 2, ‘C’|0X80, ‘V’|0X80};

void decode(unsigned char *M)
{
unsigned char mask = 0x80;
char i=0,j=0,k=0,l=0; /* j is the node pointer,
i is the byte pointer, M
is the message pointer */

while(k !=’\n’)