AIS Parser: /Users/brianlane/projs/aisparser/c/src/sixbit.c File Reference

The state of the sixbit machine is held in the sixbit structure so that multiple streams can be processed by maintaining separate instances of sixbit. init_6bit() should be called with a pointer to the sixbit instance, it will setup the structure for parsing. The 6-bit data should then be copied into the sixbit.bits buffer. A maximum of 255 characters are allowed (this is changed by SIXBIT_LEN in sixbit.h)

The size of the packet can be calculated with strlen(state->bits) * 6 but note that due to padding at the end of the string the data may be 1-6 bits longer than the the expected length for the message id.

It is up to the calling function to keep track of how many bits have been fetched. When it reaches the end of the 6-bit string get_6bit() will return 0's.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "portable.h"
#include "sixbit.h"


Functions
unsigned int __stdcall	sixbit_length (sixbit *state)
	Calculate the number of bits remaining in the six_state.
char __stdcall	binfrom6bit (char ascii)
	Convert an ascii value to a 6-bit binary value.
char __stdcall	binto6bit (char value)
	Convert a binary value to a 6-bit ASCII value.
int __stdcall	init_6bit (sixbit *state)
	Initialize a 6-bit datastream structure.
unsigned long __stdcall	get_6bit (sixbit *state, short numbits)
	Return 0-32 bits from a 6-bit ASCII stream.
Variables
const unsigned char	pow2_mask [] = { 0x00, 0x01, 0x03, 0x07, 0x0F, 0x1F, 0x3F }

Function Documentation

char __stdcall binfrom6bit ( char ascii )

Convert an ascii value to a 6-bit binary value.

This function checks the ASCII value to make sure it can be coverted. If not, it returns a -1. Otherwise it returns the 6-bit binary value.

Parameters:

ascii

character to convert

returns:

-1 if it fails
6-bit value (0x00-0x3F)

This is used to convert the packed 6-bit value to a binary value. It is not used to convert data from fields such as the name and destination -- Use ais2ascii() instead.

char __stdcall binto6bit ( char value )

Convert a binary value to a 6-bit ASCII value.

This function checks the binary value to make sure it can be coverted. If not, it returns a -1. Otherwise it returns the 6-bit ASCII value.

Parameters:

value

to convert

returns:

-1 if it fails
6-bit ASCII

This is used to convert a 6 bit binary value to the packed 6-bit value It is not used to convert data from fields such as the name and destination -- Use ais2ascii() instead.

unsigned long __stdcall get_6bit	(	sixbit *	state,
		short	numbits
	)

Return 0-32 bits from a 6-bit ASCII stream.

Parameters:

	state	pointer to a sixbit state structure
	numbits	number of bits to return

This function returns the requested number of bits to the calling function. It pulls the bits from the raw 6-bit ASCII as they are needed.

The full string can be addressed by pointing to state->bits and the length can be calculated by strlen(state->bits) * 6, but note that the string also includes any final padding, so when checking lengths take into account that it will be a multiples of 6 bits.

Example:

    sixbit  state;
    unsigned char i;

    init_6bit( &state );
    strcpy( state.bits, "5678901234" );
    i = get_6bit( &state, 6 );

    i == 5

int __stdcall init_6bit ( sixbit * state )

Initialize a 6-bit datastream structure.

Parameters:

state

state of 6-bit parser

This function initializes the state of the sixbit parser variables and 0 terminates the 6-bit string.

Example:

    sixbit state;
    
    init_6bit( &state );

unsigned int __stdcall sixbit_length ( sixbit * state )

Calculate the number of bits remaining in the six_state.

Parameters:

state

returns:

Number of bits remaining

/Users/brianlane/projs/aisparser/c/src/sixbit.c File Reference

Detailed Description

Functions

Variables

Function Documentation

Variable Documentation